US2015401A - Method for determining underground structure - Google Patents

Description

J. .1. JAKOSKY 2,015,401

METHOD FOR DETERMINING UNDERGROUND STRUCTURE Sept. 24, 1935.

Filed May 15, 1 93s- 5 Sheets-Sheet 1 Sept. 24, 1935. J. .1. JAKOSKY METHOD FOR DETERMINING UNDERGROUND STRUCTURE Filed May 15, 1933 5 Sheets-Sheet 2 HOR/ZONTA L COMPO NEH T Fla. 6

Sept. 24, 1935. J. J. JAKOSKY METHOD FOR DETERMINING UNDERGROUND STRUCTURE Filed May 15, 1935 5 Sheets-Sheet 3 Sept.- 24, 1935 J JAKQSKY 7 2,015,401

METHOD FOR DETERMINING UNDERGROUND STRUCTURE Filed May 15, 1933 5 Sheets-Sheet 4' Sept. 24, 1935. J J JAKQSKY 2,015,401

METHOD FOR DETERMINING UNDERGROUND STRUCTURE Filed May 15, 1935 PR/MRY FIELD f 5 Sheets-Sheet, 5'

Patented Sept. 24, 1935 METHOD FOR DETERMINING UNDER- GROUND STRUCTURE John J. Jakosky, Los Angeles, Calif.

Application May 15, 1933, Serial No. 671,099

29 Claims.

This invention relates to a method and field procedure for the study of subsurface structure, by means of electrical potential and magnetic measurements made at the surface of the earth. A portion of the method and procedure described herein is in amplification of my co-pending United States Patent application Serial No.

The method and procedure described herein are particularly suitable for determination of inhomogeneities -in' the electrical conductivity of the subsurface, with special reference to such inhomogeneities as may be caused by the presence of mineralized bodies or the like having relatively high conductivity as compared to the surrounding country rock, or for determination of the boundaries or contacts of different materials as for instance in determining the thickness of and 'depth to bedrock in a'region where the bedrock is covered with a layer of gravel or other fill material.

In present methods of electrical prospecting, a flow of direct or alternating current is produced in the subsurface, either conductively by means of two spaced electrodes, or inductively by means of an energizing loop, and this flow of current is continuously maintained while'balancingor nullpoint measurements are being made, of the potential or magnetic field created by such current flow. These measurements usually require a period of thirty seconds or more, which necessitates that the flow of current be continuous and be held practically constant while the measurements are being made.

Field tests have shown that the effects of nearsurface inhomogeneities having no geological significance as regards the deeper subsurface conditions, and the instrumental errors caused by stake resistance, induction and capacity effects, and high sensitivity, can be greatly reduced and minimized by employing much greater flows of current in the energizing circuit. In other words, it is advantageous as regards accuracy of field work and interpretation, to employ heavy current flows for energizing the earth,

' and medium sensitive detecting or measuring instruments for studying this flow and distribu-v tion of current in the subsurface. Heretofore, however, heavy current flows could only be obtained with heavy and bulky power equipment which was entirely unsuited for rapid work.

As distinguished from these previous methods, the subject of this invention comprises a method of subsurface surveying utilizing the creation of an electric disturbance of high intensity but short duration in the earth so as to produce in the region of said disturbance a sudden short duration electromagnetic field impulse having electrical and magnetic components, and determining the effect of said field impulse, by measure- 5 whereby errors due to surface and near-surface inhomogeneities are minimized and whereby simultaneous observations can be madeat a number of points using only .one source of current 0 with a resultant increase in field or operating efficiency.

According to this invention, measurements of the magnetic field or potential values associated with the flow of current in the subsurface are or- 2 dinarily made at a plurality of points on the earths surface, the effective value of the current impulse being kept substantially constant for all of such measurements, or else measured and calculated to an equivalent or constant jvalue, so that the relative intensity and direction of the magnetic or electrical component of the impulse at the several ponts whereat measurements are made, affords an indication of the path of underground current flow; that is, any electrical in- 35 homogeneities in-the subsurface will be indicated. by anomalies or variations in the actual values obtained, as compared to the theoretical values for a homogeneous subsurface.

An important feature of the present invention is the method of employing a high current flow during a short period of time. This may be better illustrated by comparing it to another wellknown geophysical method. In the seismic method of prospecting, a low-frequency sound wave is created near the surface of the ground and measurements made at various points away from the point where the wave was first initiated. Knowing the time for the reflected or refracted waves to reach the recording instruments, it is possible to calculate the effective speed and path of the wave, and thereby predict the nature of the subsurface. In the seismic method of prospecting a very high pressure or disturbance is created for a very short interval of time by use the electrical or magnetic power applied to the subsurface is continuous over a period of time to permit steady-state conditions to be reached and as a result, very large generating apparatus must be employed in order to obtain measurable values on moderately sensitive recording instruments,

-when working to appreciable'depths, or else the 'recording or indicating instruments must be of such great sensitivity that they are influenced by many other factors, with resultant decrease in accuracy. Since electrical energy is a function of both intensity of the current and the time of flow, it can be seen that for a given quantity of energy a very high power can be obtained by employing a high current flow for a short period of time.

The energy expended in heating a conductor, for instance, is proportional to the average of the squared instantaneous currents:

' wherein W=energy, in watts, R=resistance, in ohms,

T=time, in seconds, i=instantaneous current, in amperes.

The direct current that would produce the same heating effect in a-circuit having the same resistance would be equivalent to the square root of the mean square of the instantaneous currents. This value is termed the effective alternating current, or the root mean square, value, and is represented by I in the above equation.

It can therefore be seen that a very high cur- -rent fiow can be obtained from a relatively small quantity of electrical energy, by employing a very short period of.time during which the energy is expended. In this respect, the present invention is comparable to the seismic method wherein .very high mechanical force is employed for a very short interval of time, in order to create the lowfrequency mechanical disturbance necessary for the study of the underground structure.

According to the present invention I create an electrical disturbance or surge of relatively short duration, within the subsurface, so as to create in the region of said disturbance a sudden short duration electro-magnetic field impulse having electrical and magnetic components, and measure one of said components of said field impulse.v

The energy pulse used for this purpose may be obtained, for example, by storing electrical energy at relatively high *voltage in a condenser of relatively large capacity, and then suddenly discharging the stored energy. The electrical disturbance in the subsurface may be obtained either (a) 'by direct conduction, by driving two electrodes in the earths surface and connecting the high voltage stored energy supply to the electrodes, thereby causing the energy pulse to pass through the earth, or (b) inductively, by passing the energy pulse through a large coil or loop, thereby creating an intense magnetic field which radiates from the coil and induces a corresponding energy pulse in the earth. The measurement of the electrical or magnetic component of the impulse thus produced. in the earth may be done by (a) determining the electrical impulse created between two electrodes driven into the earths surface, "or (b) determining the electrical impulse induced in a coil or loop on theearth's surface, or (c) by direct magnetometric measurement of the magnetic component of the impulse.

Any one of the methods for measuring the effect of the electrical disturbance may be employed 10 in conjunction with either of the above-described methods of creating the electrical disturbance.

The electromagnetic field impulse produced by the electrical disturbance in the earth, may be either a unidirectional impulse or an alternating 15 impulse, and in the latter case measurement may be made of either the initial or final half-wave or of the full-wave, that is, the positive and negative portions thereof, and in the appended claims all references to the measurement of the electrical g0 ormagnetic component of the impulse will be understood to include either the measurement of a unidirectional impulse, or the measurement of the initial half-wave, or.measurement of the final half-wave, or measurement of the full-waveof 25 an alternating impulse.

As an illustration of a method for carrying out the above, one method of the present invention may advantageously comprise the passage of a unidirectional, high-intensity, short-duration 10 pulse through the earth between. two separated electrodes making contact with the earth and measurement of the electrical impulse setup between two other points having a known or fixed spacial relationship with the first two points.

Another method of the present invention may advantageously comprise passage of a unidirectional, high-intensity, short-duration ,pulse through .the earth between the two separated electrodes making contact with the earth, and 0 the direct measurement of the horizontal component of the magnetic field impulse created by this'fiow of current, by means of an undamped or ballistic type magnetometer.

Another method of the present invention may 45 advantageously comprise passage of a unidirectional, high-intensity, short-duration current pulse through the earth, and measurement of the horizontal component of the magnetic field impulse created at various stations, as by means of 50 a suitable pickup coil and ballistic galvanometer system.

Another method contemplated by the present invention may comprise passage of a unidirectional, highintensity, short-duration current 55 pulse through a coil or loop placed on the surface of the ground; and measurement of the horizontal and vertical'components of the resultant electromagnetic field impulse created by 'the current flowing through the coil or loop and the induced w currents flowing in a subsurface conductive medium, or measurement of the electrical impulse 1 set up between two spaced points. Another object of the present invention is th measurement of theeffects produced by the ini- 5 tial and final portions of the current pulse, and thereby obtaining information relative to current distribution at different frequencies, by employing a current pulse whose rate of increase is materially different from'the rate of decrease. In any of theabove cases, the measurement of the electrical or magnetic component of the field impulse produced by the energizing current pulse is preferably conducted in such manner as to measure or indicate the time-integrated eflect 01.

' measurement of the electrical impulse between two ance with my invention. Referring to the drawpoints is to be made, this may be done by means of a ballistic galvanometer whose natural period is relatively long as compared to the duration of the impulse, said galvanometer being connected be.- tween said two points so as to measure the timeintegrated efiect of the current impulse produced by said electrical impulse. If the magnetic component of the impulse is to bemeasured, the use of a ballistic magnetometer whose natural period is relatively long as compared to the duration of the impulse will also permit a measure of the timeintegrated effect of the impulse to be obtained. In case the electromagnetic field impulse is to be measured by means of a pickup coil, the timethe earths surface; showing. a sohematic layout of the direct energizing'apparatus which maybe used according to this invention, together with a schematic representation of the magnetic fields produced upon the passage of current between the two energy electrodes, the instantaneous field from the current. induced in a subsurface conductive body, and the distribution of potential; that is, equipotential lines, on the surface of the ground between the two electrodes, due to theinitial fiow of current.-

Figure 3 is a section thereof on'line 3-3 in Figure 2,'showing the magnetic field associated with the flow of current.

Figure 4 is a schematic" view of one form of the potential detecting circuit.

Figures 5 and 5a are schematic views of two modifications of electromagnetic field detecting circuit which may be -used according to this invention, for measuring a unidirectional component of an induced current.

Figure 6 shows the relative directions, but not necessarily the vector relationships, of the primary field produced by current fiow.in the energizing coil and the secondary field from the induced current fiow in the orebody; together with typical values for effective horizontal and vertical component voltages obtained when readings are made across a traverse perpendicular to a long conducting body.

Figure I illustrates a switching and current-reversing device which may be advantageously employed in use of the-present invention.

Figure 8 is a view of one form of ballistic or undamped magnetometer which'may be used for measuring the strength of the magnetic field.

' Figures 9, 9a, 9b, 9c, Qdand 9e illustrate general circuit diagrams which may advantageously be employed where it is desired'to separate the effects of the initial and the final portions of the current pulse, or for measuring both coinponents of an alternating pulse. 5

Figure 10 is an isogonic section of a portion of the earths surface showing a schematic layout of the inductive energizing apparatus, and the electromagnetic pickup apparatus for measuring the magnetic field produced upon the passage of- 1 the current through the energizing coil.

Figure 11 illustrates a type of power supply which may be advantageously employed for direct excitation of the subsurface, or for charging the condenser-type ofenergy supply as illustrated-15 diagrammatically in Figure 1. A

Figure 12 illustrates the type of wave-fronts or form of the current discharge curve desired for the potential and the electromagnetic pickup systems, and the dotted curves illustrate the general type of waves reaching the pickup system.

Figure 1 illustrates schematically a preferred form of energizing apparatus foruse with the system contemplated by this invention. A large capacity, high-voltage condenser 8 is connected in series with a variable inductance H and variable resistor l2. A quick-throw, single-pole, double-throw connection switch 9 is provided so that the condenser-reactor-resistor combination may be connected to the battery 5 by placing the 80 switch on contact 9b. The energy stored in the condenser may now be discharged into the output circuit PP' by throwing the switch to the contact 91:. A. current-limiting resistor ID and a milliammeter l3 are connected in series with the 85 charging battery- 5. Two auxiliary single-pole, double-throw switches 9d and 9e are provided for connecting the output terminaIS-P -P' either, to

. the condenser-reactor-resistor combination, .or to an. auxiliary measuring' circuit. The auxiliary 40 measuring circuit contains the necessary instruments for measuring the direct-current resistance and the alternatingecurrent impedance of the output or earth circuit. The alternating current measuring portion consists essentially of an alternating-current milliammeter a, a 10 to IOOO-cycle generator 15, and an alternating-current voltmeter Ba. The direct current measuring circuit consists essentially of a direct-current milliammeter Mb, a direct-current voltmeter lfib, and a battery 5b. A switch 9r is provided for connecting either the alternating-current or the direct-current measuring instruments with the control switches 9d and 9e. This combination is used for measuring the total impedance resistance and inductance of the total circuit connected to PP', by throwing the switches 9.1 and 9e so as to connect the measuring equipment to the output terminals. After the measurements are made, the switches 9d and 9e are again'thrown so as to connect the condenser-reactor-resistor system to the line. The variable inductor I I and. the variable resistor l2 are calibrated preferably to read millihenrys and ohms, respectively. During operation, when a series of readings are being made, the impedance of the total circuit is to measure time-integrated effects of the impulse..

In actual operation therefore, it has been found that detailed measurements need not always be made of the impedance," inductance and resistance of the external circuit, since the readings from the measuring circuits 30 and 30a give comparable values for the various electrode set-ups.

One method of carrying out my invention is illustrated in Figure '2. The numeral I indicates a section of the earths surface where the work is to be conducted. The numerals 2 and 3 indicate partially imbedded energizing electrodes having electric connections to a timing and reversing mechanism and source of instantaneous electric energy .4, and its power supply 5, through an insulated surface level conductor 6-6. The source of instantaneous electrical energy, shown at to, may be of the type shown in Figure 1, including condenser 8, reactor I l and resistance l2, and may also be provided with an auxiliary measuring circuit such as shown in Figure l, and with measuring instruments 30 and 30a. The timing and reversing mechanism, shown at 41), may for example, be of the type shown in Figure '7, as .hereinafter described. With such an arrangement, magnetic field measurements may be taken in a region adjacent to the imaginary line joining the electrodes 2 and 3 as shown in Figure 2, or preferably in that portion of the area lying between traverse lines A and B which are spaced centrally, approximately one-third of the total distance between electrodes 2 and 3.

The surface conductor is preferably comprised tween electrodes 2 and '3 should be relatively.

great in comparison with the depth to which the survey is intended to penetrate into the earth, for example, approximately three or more times such depth, and the length of the longitudinal conductor portions 6a and 6b should be as great as convenient, preferably equal to the depth to which it is desired to operate. In the event that the surface conductor configuration can not be placed so as to occupy a relatively horizontal plane, then calculations are to be made to eliminate from the final readings the effects of these conductors. The method of calculating and correcting for the effect of a conductor carrying a current is well known, and need not be described here. However, it may be pointed out that the greater the distance of the conductor 6-6 and its longitudinal extension 6a and (it from the points of a point of observation at which magnetic measurements are to be made, said point being preferably in the same plane of the surface conductors 66, 6a and 6b, and being shown in this case as located on a line Joining the two elecsurface of the ground. Inasmuch as the magtrodes 2 and 3 and midway between the two electrodes.

In Figures 2 and 3, the circle V represents the magnetic field through the point 0 set up by the passage of current through the portions 8-8 of the surface conductor which is parallel to the line joining the electrodes, and the circle H represents the magnetic field through the point 0, set up by the passage of current through the earth along the dotted line 0-0, through a zone OB of relatively high electrical conductivity as compared to the surrounding structure, as for instance, a sulphide ore body having a small diameter and great length, situated at a distance it below the netic field set up in the subsurface due to the flow of current therethrough isa summation of the fields set up around each individual current path, an average path such as is represented by the dotted line CC' may be taken as the path which serves as a center for the magnetic field induced by the flow of current in the earth. The horizontal component of this field is represented by the vector Fh in Figures 2 and 3. If the surface level conductor 6-4; including the lateral conductor portions 6a and 6b is substantially at the level of point 0, the magnetic field set up by the passage'of current through said conductor will have substantially no horizontal component at point 0, or at any other point at the same level at which observations are being taken, and therefore the magnetic field from the conductor 6-6 will have substantially only a vertical component. This vertical component is shown in Figures 2 and 3 as Fv.

In case the underground material is of fairly uniform conductivity, similar theoretical conditions will prevail and the differential current paths can be represented by a mean effective path of current flow. The magnetic field set up in 40 the earth by a passage of current under such conditions will have practically no vertical component at the observation point 0, but will consist chiefiy of a horizontal component as previously indicated. Since the horizontal and vertical components of the current flowing the subsurface and in the surface conductor can be differentiated easily due to their geometric configuration, measurements can be madeat point 0 to determine the intensity and direction of the horizontal component Fn created by' the underground current fiow. Any deviation from these conditions will indicate an inhomogeneity in the subsurface conductivity and thereby give indications which may be utilized todetermine the the nature and location of such inhomogeneity. Studies can further be made to determine the depth of the interface or contact between an overlying and an underlying layer or the depth at which a highly conductive zone lies below the surface by making readings atthe point 0 for various distances of separation between the electrodes 2 and 3, since a general relationship exists between the depth of the mean effective current flow and, the distance between the electrodes 2 and 3. When employing certain types of detecting equipment, an important factor in the successful operation of the method is proper control of the shape of the current-time curve. For one type of system, where measurements are being made which are dependent upon direct potentials caused by current flow between two electrodes, a different type of wave-front usually is desired than when inductive energizing or pickup 5Y5;

tems are being employed. Control of the wave shape is obtained by varying the resistor I2 and the inductance I l in order to keep a substantially constant impedance in the circuit to compensate for various lengths of cable 66, various electrode resistances, variations in resistance-of the sub-, surface between the electrodes, and variations in inductance of the earth at different electrode spacings. The inductance of the earth and the skin effect vary with the spacing between the power electrodes, shape of the current wave, the effective conductivity of the earth, structural features and topography. In practice, the impedance of the circuit may be measured by means of the apparatus shown in Figure 1. Switches 9d," 9e and 9 are thrown in the upward position. To determine the total impedance in the circuit, it is merely necessary to operate the generator and substituted therefor. Readings are again taken, A

with meters Mb and "b to determine the directcurrent resistance of the circuit. From these readings simple and well-known calculations can be made-to determine the inductance in the circuit. I.

In order to obtain the characteristics of the circuit at a frequency comparable to the wavefront being used, it is of course desirable to have the generator frequency approximately of that value. A few moments time are required to obtain the readings as described and the inductance ii and resistor ii are then set to such a value" as to keep a predetermined constant inductance and resistance in the circuit. The magnitude of these values depends upon the wave-front desired. Generally, the chief point is to maintain a constant wave-front throughout any series of readings in order to allow the results to be comparable. In some cases it is possible to make the value of inductance It and resistor l2 quite large so that variations in the inductance of the lead wires 66', inductance of the earth or contact resistances at the electrodes 2 and 3 may not greatly affect the total impedance, and therefore have a negligible effect on the final interpretation. One large variable in the circuit is the contact resist ances encountered at the electrodes 2 and 3, and this can often be minimized by using a plurality of stakes for each electrode, thereby obtaining a lower contact resistance. The factors governing the shape of the current-time curve may be summarized as follows:-For the discharge of a given quantity of energy stored in the. condenser, the effective value of this current will be governed largely by the resistance in the circuit, that is, the greater the total resistance, the lower the current; while the steepness of the wave-front will be governed largely by the inductance in the circuit, that is. the greater the total inductance in the circuit, the less abrupt the wave-front or the longer the time required for the current to reach the maximum value governed by the resistance in the circuit. Although the shape of the current-time curve may affect the results being obtained, simple measurements can be made which allow direct computation of results, without repetition of the complete measurements of the characteristics of the earth circuit for each and every reading. These simple measurements are made on the circuit 30 and 30a, Figure 1. The readings from 30, connected to the transformer 29, give a value which may be termed the inductive effect, whilethe readings from 30a, con nected to the resistor, give a value which may be termed the in-phase or potential efiect. When using those methods of detecting which employ a direct measurement of the electrical impulse or a measurement of the magnetic field impulse by use of a ballistic magnetometer, the reading on circuit 30a is most indicative; and

when using the methods which employ an energizing coil and, or pickup coil and ballistic galvanometer', then the reading from circuit 3|] is used.- This will be described in more detail in connection with the use of the potential and inductive pickup systems, to be described later.

The electro-inagnetic field set up by the flow of current in the, subsurface may be measured in a number of ways, based on direct measurement of either the magnetic or the electric component of said field.

A method for determining the effective earth resistance and the general distribution of subsurface current flow consists in measuring the electrical impulse set up between two (measuring) electrodes while passing the current pulse into the subsurface between two other (energizing) electrodes. For simplicity, the measuring electrodes usually have a fixed spacial relationship with the energizing electrodes, as indicated for example, at ll and I? in Figure 2, or else the energizing electrodes are kept in a fixed position and by moving the two measuring electrodes along traverses perpendicular to a line joining the two energizing electrodes, calculations canbe made showing any anomaly of current flow. The measuring electrodes are com nected by a conductor M in which is connected a device 85 for measuring the time-integrated effect of the electrical impulse. A device which may be used for making such measurement is schematically shown in Figure 4 and consists essentially of the two electrodes lland H, a high resistance ballistic galvanometer i8, and damping resistance l9, if desired, and a shunt condenser 29. In areas where extreme values of earth currents are encountered it is sometimes desirable to use a blocking condenser 20,. in order to prevent erroneous readings on the galvanometer.

An auxiliary earth potential neutralizing system is of advantage. This consists of a voltage divider 32, having a battery 33; and a reversing switch 34. In operation, the switch 34 and the slider 32' are adjusted to balance out any earth potentials which may exist between the electrodes I1 and II. This condition is indicated when the galvanometer It! reads zero, when condenser 20 is out of the circuit.

The voltage induced by the magnetic field impulse in the lead lines -84 connecting the electrodes l1 and H1, will have only a minor effect on the reading, due to the shape of the currenttime curve, as is describedzlater, and the long period of the galvanometer which eliminates the alternating inductive component.

Another method which may be employed for determining the distribution of the subsurface current flow, consists of measuring inductively the magnetic field set up by this current flow The electromagnetic detecting or pickup 's'ysdiagrammatically in Figure A, and consists of a pickup coil :sa, rectifier 31a and a very low leakage condenser 38a. The'condenser is charged to a certain value which depends upon its calli pacityand the magnitude of the potential induced in the coil- 35a. The switch 39a is then closed and a reading made on the galvanometer a. It will be noted that the galvanometer in this case is damped by a resistor of suitable capacity to allow storage of practically all or a major portion of the energy picked up by the coil- 350 during the initial half wave. of the induced current.

26 As can be seen, this same system of measuring the charge of a condenser is applicable to the other systems described in this invention.

Due. to the fact that the pickup circuit employing a coil and ballistic galvanometer is es- 30 sentially an aperiodic circuit, slight phase shifts in'the magnetic fields will not have the detrimental eifects that exist in those circuitsemploying a continuous flow of alternating currentfor energizing the ground, with their, tuned pickup systems.

When it is desired to measure the magnetic field, the coil 35 is preferably so placed that its plane includes, or is parallel to, the line joining the electrodes 2 and 3, and perpendicular to the 40 Y plane formed by the lead wires 6a and 6b. In other words, the coil usually is so placed that it will pick up only the horizontal component F11 and not the vertical component Fv, as shown in Figures 2 and 3. The current pulse is then 45 passed into the ground and the deflection on the measuring circuit recorded.

An alternative method of energizing the subsurface by electromagnetic induction is' illustrated in Figure 10, which shows a large loop or energizing coil 21, connected to the terminals P-P' of the power supply 4. The magnetic field impulse created by the flow of current through this coil travels outward in all directions and the downward portion of the wave induces aplan location of the conductor being indicated 5 by the maximum value obtained when directly over the conductor. In operation, the energizing I coiland the pickup coilare oriented so that they lie in the same plane. When making observation at point A along the traverse shown in Fig- 7 we 10, the energizing coil-is oriented along line A'A', and when making a reading at point B, I

the energizing coil is oriented along line BB',

' etckgh preferred form of measuring circuit is illustrated in' FIgure 9c; employing a vacuum 75 thermocouple 95. ,The heater" wire 96 is convalue.' The condenser 38a must be of sufficient nected to the terminals of the pickup coil 98. The thermo-junction 51, making contact withthe heater wire or to a heater insulator head,

' is connected to a galvanometer 36.

When direct current ispassed into the ground; 5

two effects operate to hinder the continued flow of current. These two major effects are known as polarization and electrolysis. The effects of polarization. usually are negligible. Electrolysis, however, becomes of increasing importance with 10 high current values and some means 'must be providedfor minimizing or eliminating its effect. The effects of electrolysis are not instantaneous but are accumulative through a definite time interval. This means that a few very large pulses of current can be passed without undue resistance being offered the fiow of current. Where a series of pulses are to be sent into the ground, some provision must be made 'to minimize the effects of electrolysis. inating the effects of electrolysis for electrical prospecting is to employ a reversing mechanism so that the direction of current passed into the ground is reversed between each discharge. 'In

other words, the current is passed in one direction and then reversed in polarity and passed into the ground in the opposite direction. Reversal of the\ current also furnishes a check on the measuring instrument and allows corrections to be made for non-symmetrical deflection of the measuring instrument. Final calculations are preferably made by obtaining the arithmetical means of two or more reversed current pulses.

Another important function of an automatic reversing mechanism lies in its allowing a series of observations to be made by one operator for any given setup of the energizing system. This is done by arranging the timing mechanism.to make contact with. a 'circuit on, '-say," five-minute periods. The operator sets up his measuring ap- 40 paratus :and obtains a. reading for two consecutive pulses, which alternately pass through the earth in opposite directions. The operator then moves on to the next station and takes two more readings at the proper five-minute intervals. A discharging and reversing mechanism which may be used for this purpose is illustrated at lb in Figure 7 and consists'essentially of a special quick-throw type commutator 2| so arranged that the contact bars 22 and 22 make sudden or instantaneous contact with the conductor bars 23 and 23'. The conductor segments 23 and 23 are connected to two slip rings 24 and 24'. Low resistance brushes 25 and 25' bear on the contact rings and are connected to the condensers 8; Connected in series with the condensers is the milliammeter I 3, a highvoltage battery supply 5, and a current-limiting resistor Ill. The function of the current-limiting resistor is to prevent a. sudden or excessive a flow of current at the instant the direct-current power supply -5 is connected to the condenser system, and also to prevent an excessive flow of current when contact is made to the energizing electrodes by the commutator system. , Suitable o5 terminals 26 and 26 are provided on the contact arms 22 and 22' to allow connection to the energizing electrodes, or the energizing coil. The circuit may also include variable reactor ii, variable resistor l2, and means such as 29, ll, 28 and 20a for measuring the relative inductive and potential values of the energizing pulse, in the same manner as in Figure 1. If desired, the mechanism may also be provided with switch means for disconnecting the battery 76 The best method of elim- 2O ror or telescope forobs'erving the angular de-,

fiectio'n of the magnetic system. Figure 8 illustrates a form of bar-magnet magnetometer which is adapted to be used as a horizontalcomponent measuring means when the current pulse is passed through the earth between two energizing electrodes, and referring thereto;

numeral 40 indicates a cylindrical instrument case or housing, preferably formed ofaluminum or other non-magnetic material, provided with a glass window 4| for the purpose of allowing light from any source to fall on the interior of the case. An external reflecting mirror 42 is provided for directing more light into the window 4!. A bar magnet 43 is supported to swing in a horizontal plane by means of a vertical torsion wire 44, secured to the magnet support 45.

The line of suspension of the bar magnet by the torsion wire 45 constitutes the axis of rotation of said magnet. The upperend of the torsion wire is fastened to a movable support 46, whereby the magnet may be raised or lowered in position. The collar tLprovided with clamp screw 48 holds the movable support 66 in position. The collar 4'! is fastened to the head by means of a suitable bearingto allow rotation of the collar 41, with respect to thehead M. A pointer '58 is attached to the collar, and a suitable scale Si is engraved on the upper portion of the head. A suitable index mark is providcl on the end of the pointer in. order to allow readings to be made showing the degree of rotation of the pointer, with respect to the housing or head. The housing 40 is rotatably mounted on the base 52, provided with the conventional leveling screws 53 supported by the tripod head 56%.

A suitable telescope, having a magnification of 5 to 100 times, is provided for viewing the scale 56 fastened to the magnet system t3. 1

The lower portion of the housing 48 is provided with graduation or markings 58, whereby the rotation of the instrument case with respect to an index mark on the base 52, may be determined.

A centrally located pointer 51 is provided for proper leveling of the instrument. A similar pointer 51'' is fastened to or made integral with the magnet support 45.

A suitable heat-insulating material, such as cork, is provided as a lining 59, to protect the magnetic system from rapid changes in temper-' ature. A thermometer 68 is placed in the hous- 1 ing, for indicating temperature of the magnetic system and thereby allowing the proper corrections to be made for "changes in magnetic strength of the magnet. The thermometer is read through the window 6|. Windows 4| and SI are also used for observing the relative posi-. tions of pointers 51 ands? for proper leveling of the instrument.

\ A- non-magnetic .and non-conductive support clamped against the lower side of disc '81, by

means of the disc 88, when the instrument is to be transported. When setting up the instrument for making a reading, the knob 89 is rotated slightly to move the disc 88 about I millimeter below 5 the free rotation position of the magnet 43. In this position, the disc 88 acts as a damping plate, by the eddy currents generated by "movement of. the magnet. After the magnet has been brought to rest, and proper leveling and magnetic orienta- 10 tion of the instrument completed, the torque head 58, and its pointer 50 are rotated until the magnet 43 swings-to a mean position parallel to the line joining electrodes 2 and 3, of Figure 2. From the rotation of thetorque head 48, and the orien- 15 tation of the instrument, the angular rotation of the suspension fibre is obtained, and the strength of the earth's field determined. This gives the scale value of the instrument; that is, the field strength in gammas per degree deflection at that 20 particular station. If the magnet system oscillates or swings, it is brought to rest by means of the damping disc 88 and the scale reading noted, and then the plate 88 is lowered to a position about 28 millimeters below the magnet, at which 25' position it has no appreciable damping eifect on the magnet. The current is now passed'into the 'ground and the deflection of the magnet scale noted. The field strength due to the current flow can now be calculated. For further details of the methods of measuring and calculating field strength, reference is made to Directions for Magnetic Measurements by D. L. Hazard, U. S. Department of Commerce, Coast and Geodetic Survey, Serial No. 166, or to any advanced text 86 dealing with magnetic measurements of the earth's field.

The magnetometer used in the above manner for measuring the effects of a current pulse is comparable in its action to the familiar ballistic do galvan'ometer and, as is well known, the kick or throw imparted to any one instrument will be proportional (for small deflections) to -the strength of the field. Magnetometer measurements made along the surface of the ground 45 therefore give comparable results and serve as a means for determining the path of greatest subsurface current flow. An instrument of the type described measures only thehorizontal component of the magnetic field. 'The vertical com- 50 ponent of the field has no effect on the instrument.

When measuring the electrical impulse between two measuring electrodes or when measuring the magnetic field impulse with the ballistic magnet- 55 ometer method. it is desirable to have a currenttime wave of the. general type shown byCurve I1 in Figure'll, wherein the rise of current, as evidenced by the time 751 is small compared to the total time of discharge T1. The main current 0 flow should take place within a short 'period of time, varying for example from to 30 seconds time and preferably less than about ten seconds. Furthermore, this period should be materially less than the natural peri'odof the ballistic in- 65 strument used for measurement. When making measurements of the electrical impulse between measuring electrodes, the initial rise of current from zero to the peak or maximum value reached, as governed by the resistance in the circuit, should 7 be in less than of the time for the total current flow. Under such conditions,. the effect of changes in the steepness of the wave-front due to variations in the inductance of the lead lines and reels at different setups will be minimized. 7

tiveeil'ects will practically neutralize each other on a long-period ballistic galvanometer or magnetometer when measurements are being made 01 the electrical impulse or of the magnetic field impulse associated with the pulse of current flow. For comparative measurements, these phase differences and polarity relationships should be constant throughout any series of measurements. When passing a current pulse, of the shape shown by Curve I, in Figure 12, between two energizing electrodes, and measuring the magnetic field by inductive methods, the use of 'rectifiers offers a means of eliminating the inductive effects of either the ascending or the decaying por-' tion of the current wave. By use of half-wave rectification, it is of course possible to select for measurement either the current induced by the first half of the wave, or the current induced by the last half of the wave. Use of half -wave rectifiers also allows the use of a more symmetrical wave-form, wherein the time tafor the current to reach its maximum value is about one-half of the total time T3. This form of current discharge burve is shown by Is in Figure 12, and the induced current in the pickup coil is shown by Curve Is in that figure. Schematic circuit diagrams for rectifying systems are shown in Figures5, 5a, 9, 9a., 9b and 9c, wcemploying crystal and vacuum tube rectifiers. Referring to Figure 9, the numeral 90 indicates a coil having a center terminal or tap. The ends of the coil are connected to two rectifiers Ma and Slb. During the increasingportion of the enmergy pulse, the current induced in coil 90 will flow through one of the rectifiers, say 9 Ia, into the measuring galvanometer G, while during the opif posite portion of the pulse, the current will flow through the rectifier Gib into the galvanomete'r.

fiince both of these rectified currents new through the galvanometer in the same direction, their effects are additive and measurement can be made of the time-integrated effect of the full wave at the points of measurement. .55, An alternative form of apparatus is shown in Figure 9a, employing four rectifiers, 92a, 92b, 92c and 92d, so connected as to allow full rectification of the current from the coil 93. When using the four rectifiers as shown, the coil 93 need not have a center tap.

When vacuum tube rectification is employed, some means must be used to balance out or block the current flow due to electronic emission. With some equipment, blocking condensers can .-=be employed in series with the galvanometer. A

preferred method of eliminating the electron current is illustrated in Figure 9b. The galvanometer coil G is provided with two symmetrical windings 04 and 09. In series with winding 94 is a pickup coil 93 and a rectifier tube I00. In

series with the other winding is a similar rectifler tube I00; The windings 04 and 99 on the galvanometer system areconnected so that the flux caused by current flow in one circuit is neu- 15 ,tralized by the flux from the other circuit. The

electron flow in the two windings therefore balance each other, and the galvanometer is deflected only by the rectified current from the pickup coil 93. Suitable battery power supply IM and IN, and control resistances I02 and I02 are 5 provided for lighting the filaments, and balancing the electron current flow from the two tubes. If desired, tube I00 may be replaced with a suitable battery and resistor for controlling the current flow, and this current used to balance out the elecl0 tron current from tube I00.

Figure 11 illustrates an alternative form of energy supply which is especially adapted for the methods requiring a fiat-topped current-time wave form; such as the use of two energizing l5 electrodes for passing the current into the ground, as shown in Figure 2, and measuring the electrical impulse between two auxiliary or measuring electrodes, as shown in Figure 4, or measuring the magnetic field impulse by use of a ballistic mag- 20 netometer as illustrated in Figure 8.

A direct-current generator 62 is suitably mounted on an extension or base II, fastened to the base I I. On the shaft of the generator is mounted a pinion 13 and a flywheel 03. Engageable 25 65. A cam 14 causes the shaft 65 to move to the right, and the gear 64 to the position shown bythe dotted lines, when the crank is rotated. This arrangement allows the gear. and its as- 35 sembly to be automatically engaged with the generator and its drive pinion I3, and automativ cally disengaged when rotation of the crank'is stopped. The generator is provided with ball bearings and every precaution taken to decrease 40 its friction and windage losses to a minimum.

In operation, the crank is rotated until sumcient speed has been obtained by the generator. The handle is then released, the gear automatically disengages the pinion, allowing the genera- 46 tor to be rotated at a high speed by the kinetic energy stored in the flywheel 03. The switch 18 is now closed, thereby completing the electrical circuit, and allowing a heavy current of short time duration to flow in the circuit consisting of the 60 generator 02, the resistor 10, the lead wires II and 82, the electrodes 10 and 80, and the earth within the sphere of influence of the particular electrode spacing being used.

The voltmeter I! is connected across the gen- 55 erator and serves to indicate proper speed of rotation. The resistance 18 and galvanometer II are provided for measuring the energizing current, as described in connection with Figure 1.

' This same energy supply can also be used for 00 charging the condenser, in place of the battery power supply illustrated in Figure 1.

The generator type of energy supply is advantageous for producing an energizing impulse hav- 'inga wave form 'of the type shown by Curve 65 I1 in Figure 12, while the condenser type of energy supply is advantageous for producing an energizing impulse having a wave form of the type shown by I: and Is in the same figure. It will be seen that when the generator is used merely for supplying the charging current for the condenser, the wave form of the energizing impulse will be governed by the condenser and circuit constants;, the generator merely being a source of energy. II

/ scribed in this invention lies in making suitable I distance between the power electrodes.

measurements to obtain the depth of average current flow. This is of importance when it is desired to ascertain the depth of the inter-face between two horizontal layers or strata, or the depth of a highly conduting ore'body imbedded in a higher resistant country rock. In work of this type advantage is taken of the fact that the effective depth of penetration isa function of the distance between the electrodes, the relative conductivities of the upper and lower layers or strata and the shape of the current-time curve. For ordinary conditions, the effective depth of current penetration is from 30 to 50 per cent of the The field procedure for the magnetometer or inductive type pickup equipment consists in setting up the impulse measuring apparatus at a point, in a line and preferably midway between the two en'- ergizing electrodes. A current pulse is now passed through the ground and a reading made. energizing electrodes are now moved a greater distance apart, and a second reading taken. This procedure is repeated at increasingly greater distances between the energizing electrodes, and av curve obtained which shows the relationship between field strength and electrode distance. Variations between this curve and one for a uniform area indicate an anomalous condition. Multiplying the electrode separation by a factor or percentage, as previously described, gives the indicated depth at which the subsurface conditions change. i

The field procedure for the potential type of pickup equipment employing two measuring electrodes consists in making a series of readings,

' wherein a fixed ratio and configuration is maintained between the, energizing and measuring electrodes; and the distance between-the energizing electrodes and the measuring electrodes correspondingly increased for each reading. From such a series of readings a curve can be plotted showing the relationship between energizing electrode separation and the effect of the electrical impulse at the surface of' the ground.

It has been found that the earths resistance,

decreases with increasing frequencies. It is believed that this decrease with frequency is due to a number of factors, chief of which may be mentioned; the electrolytic capacity at the surface of the electrodes; capacities existing between structural features separated by fault zones, gouges, bedding planes, et cetera; capacities existing between disseminated mineralized particles; and various polarization phenomena. In addition, the inductance of the earth is of appreciable magnitude and influences the subsurface distribution of current at different frequencies. Topographic and skin effect phenomena', together with relative conductivities of the component layers comprising the subsurface also The 9 influence the distribution of current. Many of these factors can be evaluated and given their relative importance when final interpretation of the data is made,'by studying their effects on current pulses of different time duration or dif- 5 and when the current induced in a coil is meas- By means of rectifier 9m and galvanometer Ma,

together with rectifier Mb and galvanometer Mb, the two half-wave portions of the alternating-current wave may be separated and their relative magnitudes determined from the read= ings on the two galvanometers. Qomparison may thus be made of the penetrating effects of the impulse at the different time intervals of current flow represented by the respective half-waves. For work of this type, the current pulse passed into the ground should have the general char acteristics of Curve I1 shown in Figure 12.,

In cases where an automatic reversal mechanism as illustrated in Figure 7 is not employed, the effects of electrolysis, when passing exces= sively heavy current pulses into the ground, may be overcome by passing these current pulses through-the primary winding of a transformer and then allowing the alternating-current pulse to go into the ground. The energizing equipment so for this work would be that illustrated in Figure 9d. Using, for instance, the energy supply apparatus illustrated in Figure l, the output termi= nals P-P' of this apparatus are connected to the primary of a transformer N33. The secondary of as the transformer is connected to ground electrodes 2 and 3, by means of connecting wires t and 6'.

It will be understood that measurement of the impulse in any pickup circuit or apparatus refers not necessarily to the impulse available in the entire subsurface, but rather to the proportion of the impulse available at the point or line of measurement, as compared to the total energizing pulse transferred to the ground. Interpretation of the data implies always the relative proportion of the impulse in the pickup circuit or ap paratus as compared to the energizing pulse transferred to the ground. For instance, when. using the energizing system shown in Figure l and the pickup system shown in Figure 4, interpretation is'based on the relationship between the meter readings from circuit 30a as compared to the readings of the circuit shown in Figure 4. For measurements with the electromagnetic systems; that is, when employing a pickup coil and galvanometer as shown in Figures 9 to 9e inclusive the interpretation is based on the relationship between thereadings obtained on the pickup circuit as compared to the readings from the circuit 30 connected to transformer 29. In otherwords, all readings are based on a relationship between the energizing circuit and the pickup circuit, and final interpretation can best,be made by bringing all readings to some predetermined value for the energizing circuit.

I claim:

1. The method of determining the nature of the sub-surface which comprises creating an electrical disturbance in the earth, of short duration and high intensity, so as to produce in the region of said disturbance a sudden electromagnetic field impulse, and determining the time-integrated effect of said impulse at a known position.

2. The method of determining the nature of the sub-surface which comprises creating an electrical disturbance in the earth, of short duration and high intensity, so as to produce in the region of said disturbance a sudden field impulse having electrical and magnetic components, and determining the time-integrated efiect of said field impulse at a known position by measuring the time-integrated effect of one of said components at said position.

3. The method of determining the nature of the subsurface which comprises creating an electrical disturbance in the earth, of short duration and'high intensity, so as to produce in the region of said disturbance a sudden field impulse including an initial unidirectional impulse having electrical and magnetic components, and determining the time-integrated effect of said unidirectional impulse at a known position by measuring the time-integrated effect of one of said components at said position.

4. The method of determining the nature of the sub-surface which comprises creating a unidirectional electrical disturbance in the earth, of highintensity and having a duration of less than thirty seconds, so as to produce in the region surrounding said disturbance a sudden field impulse having electrical and magnetic components, and determining the time-integrated efiect oi. said field impulse by measuring the time-integrated eflect of one of said components over the entire duration of said impulse.

5. The method of determining the nature-of the sub-surface which comprises creating an electrical disturbance in the earth, of high intensity and having a duration of less than one second, so

as to produce in the region surrounding said disturbance a sudden field impulse having electrical and magnetic components, and determining the time-integrated effect of said field impulse at a known position by measuring the timeintegrated eflect or one of said components at said position.

6. The method of determining the nature of the sub-surface which comprises passing an electric current impulse of short duration and high in- 1 tensity through the earth between two spaced trical current impulseoi' short duration and high.

intensity through the earth between two spaced electrodes, so as to produce in the region adjacent the path of said current impulse a sudden electromagnetic field impulse includin an initial unidirectional impulse having electrical and magnetic components, and determining the timeintegrated effect of said unidirectional impulse by measuring the time-integrated effect of one 01' said components.

8. The method of determining the nature of the subsurface which comprises passing a unidirectional current of extremely short duration and high intensity into the ground through two spaced electrodes so as to produce a sudden unidirectional magnetic field impulse in the region 5 surrounding the path of such current, and measuring said magnetic field impulse.

9. The method of determining the, nature of the subsurface which comprises passing-a unidirectional current of extremely short duration 10 and high intensity into the ground through two spaced electrodes so as to produce a sudden electrical field impulse in the region adjacent said electrodes, and measuring the total time-integrated eiiect of the electrical field impulse thus produced.

10. The method of determining the nature of the subsurface which comprises passing a unidirectional current impulse of extremely short duration and high intensity into the ground through two spaced energizing electrodes so as to produce a. sudden unidirectional electrical impulse in the region through which such current is passed, and measuring the total time-integrated efiect of the electrical impulse'thus produced between two pickup electrodes having a' known spacial relationship with respect to the energizing electrodes.

11. The method of determining the nature of the subsurface which comprises passing a unidirectional current impulse of extremely short duration and high intensity through the ground between two spaced electrodes so as to produce a sudden unidirectional magnetic field impulse in the region surrounding the path of such current, measuring the magnetic field impulse so produced, and repeating the passage of said current impulse and the measurement'of said magnetic field impulse so as to obtain a series of such measurements, with said two electrodes placed at dinerent distances apart for the respective measurements of said series.

12. The method of determining the nature of the sub-surface which comprises passing a uni- 45 directional electric current impulse of short duration and high intensity through the earth between two spaced energizing electrodes so as to produce a sudden unidirectional electrical impulse in the region adjacent said electrodes, measuring the total time-integrated effect of the electrical impulse thus produced between two measuring electrodes disposed within said region and having a known space relationship with respect to the energizing electrodes, and repeating the passage 55 T of such a current impulse and the measurement oi the total time-integrated eflect oi the resulting electrical impulse between said measuring electrodes so as to obtain a series of such measurements, with. the energizing electrodes placed 00 at diii'erent distances apart for the respective measurements of said series.

13. The method of determining the nature of the subsurface which comprises creating an electric current impulse of short duration and high 65 intensity within the earth, so as to produce a sudden magnetic field impulse in the region surrounding the path of such current, and measuring the horizontal component of said'magnetic field impulse. o

14. The method of determining the nature of the subsurface which comprises creating an electric current impulse through the earth, said impulse being of high intensity and short duration and reaching its maximum value in less than one-half second, so as to produce a sudden magnetic field impulse in the region surrounding the path of said current impulse, and electromagnetically measuring said magnetic field impulse.

15. The method of determining the nature of e the subsurface which comprises creating an elec- 16. The method of determining the nature of the sub-surface which comprises producing unidirectional lectrical energy, storing said energy in a condenser, suddenly discharging said stored energy in such manner as to create a sudden electrical disturbance in the earth of high intensity and short duration and thus produce, in the region of said disturbance, a sudden field impulse having electrical and magnetic components, and measuring said field impulse.

17. 'The method of determining the nature of the subsurface which comprises producing unidirectional electrical energy, storing said energy in a condenser, suddenly discharging said energy through the earth between two spaced electrodes so as to create a sudden unidirectional current impulse of high intensity through the earth and thus produce, in the region adjacent the path of such current impulse, a sudden field impulse having electrical and magnetic components, andmeasuring said field impulse,

18. The method of determining the nature of the sub-surface which comprises producing 1midirectional electric energy, storing said energy in a condenser, suddenly discharging said energy through an energizing loop so as to induce.

an alternating current impulse within the earth and thus produce, in the region adjacent the path of said induced current impulse. a sudden alternating field impulse having electrical and mag! netic components,'and measuring said field impulse. r I

19. The method of determining the nature of the sub-surface which comprises creating an alternating current impulse within the earth and thus producing in the region adjacent the path of such current impulse a sudden alternating field impulse having electrical and magnetic components, and measuring the time-integrated efiect of at least a half-wave portion of said fieldimpulse.

20. The method of determining the nature of Y the subsurface which comprises creating an alternating current impulse within the earth, and thus producing in the region adjacent the path of said current impulse a sudden alternating field impulse having electrical and magnetic ,components, and measuring the time-lntegrateddfiect of said field impulse.

21, The method of determining the nature of the sub-surface which comprises creating a nonsymmetrical alternating current impulse within the earth and thus producing, in the region adjacent the path of said current impulse, a sudden non-symmetrical alternating field impulse having electrical and magnetic components, and measure ing the time-integrated efiect of each halfrwave portion of said field impulse, in order to determine the relative magnitudes of'the field impulse for different time-intervals of current fiow.

22. The method of determining the nature of the sub-surface which comprises creating an electric current impulse of extremely short duration and high intensity in an energizing circuit including a coil having its plane in a vertical position, so as to induce an alternating current impulse within the earth and thus produce, in the "5 the earth between two electrodes, so as to create in the region adjacent the path of said current 15 impulse a single non-symmetrical alternating field impulse having electrical and magnetic components, and measuring the time-integrated effect of the respective half-wave portions of said field impulse in order to determine the relative 2o magnitudes of the field impulse for difierent time intervals of current flow.

24. The method of determining the nature of the sub-suriacewhich comprises creating a plurality of successive non-symmetrical alternating 25 current impulses within the earth, so as to create in the region surrounding" the path of said current impulses a plurality of successive non-symmetrical alternating field impulses having electrical and magnetic components, and measuring so the time-integrated effect of the initial halfwave portionof one of said field impulses and the time-integrated effect of the final half-wave portion of another of said field impulses, in order to determine relative magnitudes oi the field im- 35 pulse for difierent time intervals of current flow. 25. The method of determining the nature of the sub-surface which comprises creating an electric current impulse within the earth, of short duration and high intensity, so as to produce in 4a known point within said region, repeating the 5 v creation and measurement of said current im- 7 pulse and the measurement of said field impulse at known points of different relative position with respect to the path of said current impulse, to obtain a plurality of sets of measurements at said 50 different relative positions, and calculating the relation between the current impulse and the field I impulse for each of said relative positions.

26. The method of determining the nature of the sub-surface which comprises successively cre- 55 ating within the earth a plura1ity of current impulses of, extremely short duration and high intensity and difiering from one another in rate of change ofcurr'ent, and thus producing, in the region adjacent the path of said current im- 60 pulses, a plurality of successive field impulses -having electrical and magnetic components, and differing from one another in rate of change of intensity, and measuring each of said field imrent impulse between two spaced points on the m e earth's surface, repeating the creation and measurement of said current impulse and the measurement of the resulting electric field impulse to obtain a plurality of sets of measurements at different relative positions of said two points with respect to the path of said current impulse, and

calculating the relation between the effective value of the current impulse and the resulting electric field impulse, as measured for each of such relative positions. 7

28. The method of determining the nature of the sub-surface which comprises creating an electric current impulse within the earth, of short duration and high intensity, so as to produce a sudden impulse in the magnetic field in the region of said current impulse, measuring the effective value of said current impulse, measuring the magnetic field impulse produced at a point on the earths surface, repeating the creation and measurement of said current impulse and the measurement of the magnetic field impulse produced to obtain a plurality of sets of such measurements for different relative positions of said point of measurement with respect to the path oi said current impulse, and calculating the relation between the effective value of the current impulse and the resulting magnetic field, for each of such positions.

29. The method of determining subsuriace structure which comprisescreating an electric current impulse of short duration and high intensity within the earth, so as to produce a sudden impulse in the electromagnetic field in the region and calculating the 'relation between the induc- 20 tive value of said current impulse and the current induced 'in said coil, for each of such positions.

JOHN J. JAKOSKY.

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