US2304739A - Electrical prospecting - Google Patents

Description

Deb. 8, 1942.

1. P. MINTQNr ELECTRICAL PROSPECTING Filed Feb. 5, 1941 2 Sheets-Sheet 1 Dec. s, 1942. q. P. MINTON 2,304,739

ELECTRICAL PROSPECTING Filed Feb. 5, 1941 2 Sheets-Sheet 2 Patented Dec. s, 1942 ELECTRICAL PROSPECTING John P. Minton, Dallas, Tex., asslgnor, by mesne assignments, to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York Application February 5, 1941, Serial/No. 377,457

Claims. (Cl. F15- 182) This invention relates generally to the art of geophysical prospecting and more-particularly to electrical methods and apparatus for determining the presence of minerals, oil and other deposits in the substrata. This is an improvement over the methods disclosed by L. W. Blau in Patent No. 1,911,137 and J. J. Jakosky in Patent No. 2,211,125.

Blaus method, commonly known in the art as the Eltran method, relies upon information gained by measuring the build-up time of an electrical transient which has been introduced into the surface of the.earth, that is, the time required for an impulse whose wave form has a square top, to reach its maximum value after having been introduced into the earths surface by suitable electrodes. Jakoskys method diiers from Blaus in that after a transient has been introduced into the earths surface and the ini- -tial impulse is over, the maximum amplitude of the steady state current is measured. Jakosky uses these maxima to compute apparent electrical resistances for ,various spread distances. This is a direct current method. The Eltran method is a relatively high frequency method and as a yresult does not permit of deep exploration.

The instant invention allows `exploration at intermediate frequencies and in' addition gives data on both the Eitran and Jakosky methods.

Therefore, lthe primary object of this method resides in the provision of a method and apparatus whereby data can be obtained from which a profile of the substrata can be made.

Another object of this invention resides inthe provision of a method and apparatus whereby not only the data obtainable Aby the Eltran and J akosky methods can be obtained but additional valuable and more reliable data can also be recorded.

Other objects and advantagesof this invention trical wave or impulse;

Figure 3 is a curve which has been plotted lwith frequency as abscissae and amplitude as ordinates;

Figure 4 isl another curve which has been plotted in a similar manner to that disclosed in Figure 3, except that the value of fo has been changed;

, leads from the electrode il to a source of current I4. Another conductor i5 leads from the electrodeA l2 through the switch I6 to the source of current i4. Current induced into the surface of the earth is detected by electrodes I1 and i8 which yare placed between, but in line with electrodes Il and I2. The spacing of electrodes l'l and I8 may be varied to suit any required conditions. In fact, as disclosed by Jakosky, electrode l 2 can be made to move along the surface while the data is being obtained. Currents detected by the electrodes l1 or I8 are passed by means of the conductors I9 to a recording means 20, such as a cathode ray oscilloscope. These currents are recorded .in coordination with time.

The general practice in electrical surveying has been heretofore to introduce an electric current into the earths surface. This current passes between a pair of spaced energizing electrodes, the electrodes being electrically connected to the earth. This current passing between the electrodes creates, at a position on the earths surface having no special relationship with respect to these energizing electrodes, a quantity which is dependent on the path of flow of the current and the electrical characteristics of the earth traversed by the current. Data are taken between potential electrodes for each of a plurality of spacings of the energizing electrodes and it is customary to change .the spacing between the building-up portion of the impulse applied at a Itime T1 and the dying down portion applied at a time T1 (see Figure 2). The Fourier integral method of analysis is similar to that given in Sheas book entitled Transmission Networks and Wave Filters. This wave, as disclosed by Figure 2, is composed of one wave changing from to +gat a time t=T1 and another that changes from at a time t=Tz.

Therefore this combined wave contains no direct current value. The total pulse is repre- But v sin :c-sin y=2 sin :ig-y cos For an origin at the wave would therefore be symmetrical about the voltage axis and composed of cosine terms. The coeincients of the components are altered by the factor 2sinw Cali T11-T1 the half period, and let its frequency be fo, then The curve for this factor is as shown in Figure 3. Multiplying this factor by then Equation 3 coefficients become 1 w 2 wu and this is a curve like that shown in Figure 4. Additionally,

E L(2 sin So transmission of a direct current pulse requires transmission of a continuous band of Ai'requencies. If fo is taken as the fundamental frequency, it is observed that the important energy regions are at harmonic frequencies which are important in the Fourier series of a rectangular wave. So the Fourier integrals degenerate into something like a Fourier series zero coefficients at Now fn may be varied at will and the system can be built with means to vary fo to suit the exploration needs. Pictures could be observed and taken on a cathode ray screen at each value of'fo.

By making sure that fo covers a Wide range of frequencies from zero up, one could make sure that the resultant pictures covered from deep to shallow depths', i. e. fu=0, means deep explorations, and fo high, means shallow exploration.

From the above it is apparent that data as illustrated by the curve of Figure 5 is obtainable. The Eltran method, as illustrated in this figure, deals with the period of time To to tn The Jakosky method relies upon the current amplitude as illustrated by the ordinate A in this same gure. The additional data obtainable by the instant invention is represented by the total area B under the curve and is taken over the period of time from To to T1. These results could not be obtained by using either the Eltran or Jakosky methods alone for no consideration is given to the period of time throughout which the current dies down. As shown in Figure 6, the Eltran method is only interested in the build-up times as illustrated by ta, tb, tc for the three curves illustrated and the J akosky method is only interested in the maximum obtainable steady state amplitude as represented by the ordinate line A. In this illustration, current values are used which regardless of the rate at which they build up, reach an equal maximum value.

The curves of Figure 7 illustrate the manner in which the build-up time can vary simultaneously with the maximum amplitude obtainable. As illustrated by Figure 5, it is not only possible with.

the instant invention to obtain the data illustrated in Figures 6 and 7, but additional valuable information represented by the entire area underneath the curve. The instant invention is speciflcally directed to this phase of electrical prospecting.

Referring to Figure 8, it is seen that by varying the rate of application of the current impulses into the earths surface, curves of widely varying areas can be obtained. For example, 'the curve A1 represents the time from T1 to To. Curve A2 represents the time from T2 to To, and curve A3 represents the time from Ta to To. Since, as illustrated above, fo can be varied at the will of the operator, the rate of application that is lmost suitable from a penetrating point of view can be selected for the particular area being explored.

I claim:

1. In a method of geophysical prospecting in which electrical current pulses are introduced into the earths surface and caused .to flow be :en spaced electrodes, the improvement which comprises varying the rate of application of the current to the earths surface until the rate which gives the penetration desired is found, causing the current pulses to flow between the electrodes, detecting the current thus caused to flow through the earths surface between a second pair of spaced electrodes as a function of time and recording parameters of the current pulses from W"=ch anomalies in the subsurface strata can be located.

2. In a method of geophysical prospecting .in which electrical current pulses are introduced intof .the earths surface and caused to flow between` spaced electrodes, the. improvement whichcomprises varying the rate of application of the cur- 2fo, 4in, etc. and large coefficients at fo, 3io, etc. 75

rent to the earths surface until the rate which gives the penetration desired is found, causingthe current pulses of this frequency to flow between the electrodes, detecting the current thus caused to ow through the earths surface between a second pair of spaced electrodes and simultaneously recording the time required for a square top wave to reach its maximum, the maximum steady state value of the current after the passage of the build-up transient, and the dying out time from which anomalies in the `subsurface strata can be located 3. In a method of geophysical prospecting in which electrical current pulses are introduced into the earths surface and caused to flow between spaced electrodes, the improvement which comprises varying the rate of application of the current to the earths surface until the rate which gives the penetration desired is found, causing the current'pulses to flow between the electrodes,

detecting the current thus caused to ilow through A 3 through the'surface of the earth for a predetermined time, the improvement which comprises which electric current pulses are caused to flow 25 the steps of detecting the current pulses between spaced electrodes along the surface of the earth, indicating a function of the-detected pulses by means of an oscilloscope, photographing the indicated function and integrating the photographed function to determine the area beneath the curve forming the function from which anomalies in the subsurface strata can be 1ocated.

5. In a method of geophysical prospecting in Y which electrical current pulses are caused to flow through the surface of the earth, the improvement which comprises the steps of selecting a rate of application of current pulses to the earth that will give the desired penetration into the substrata thereof, .detecting the current between spaced electrodes along the surface Vof the earth and indicating the detected pulses by means of an oscilloscope and determining the area beneath the curve forming a single impulse from which irregularities in the subsurface strata can be located.

JOHN P. MINTON.

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