Title:
Geophysical prospecting
United States Patent 2345608


Abstract:
The invention disclosed herein may be used by or for the Government of the United States without payment of any royalty therefor. This invention relates to geophysical prospecting or surveying and aims generally to improve the same. In particular this invention enables the determination of...



Inventors:
Lee, Frederick W.
Application Number:
US33375240A
Publication Date:
04/04/1944
Filing Date:
05/07/1940
Assignee:
Lee, Frederick W.
Primary Class:
International Classes:
G01V3/02
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Description:

The invention disclosed herein may be used by or for the Government of the United States without payment of any royalty therefor.

This invention relates to geophysical prospecting or surveying and aims generally to improve the same.

In particular this invention enables the determination of the contour, shape, position and/or composition of geologic bodies, hereafter referred to as geologic structure, and more particularly provides a method of and means for making such determinations by electrical resistivity, electrical dielectric and magnetic permeability measurements obtained at the surface of the ground and other accessible points, such as shafts, stopes, drifts and drill holes. The invention differentiates ground which may be electrically isotropic or anisotropic in regard to its resistivity, dielectric constant, and magnetic permeability, hereafter referred to as electrical properties. By anisotropic properties of geological structure is meant that the material has different resistivities and electrical properties in different directions. This invention concerns itself in defining geologic structure by separating and using the directional resistivities and electrical properties as further to be described.

In the accompanying drawings, exemplifying the principles and application of the present invention: Fig. 1 is a perspective view showing a suitable ground configuration of current electrodes CI-C2, preferably equally spaced at opposite sides of a station point Po, and showing a suitable location of potential electrodes relative thereto; Fig. 2 is a diagrammatic representation of the current flow and transverse potential values under isotropic conditions or under anisotropic conditions symmetrical longitudinally and transversely of the line of centers of current electrodes; Fig. 3 is a similar diagram, for anisotropic conditions symmetrical only about the plane transverse to the center line of current electrodes; Figs. 4, 5 and 6 are similar diagrams for different conditions; Fig. 7 is a circuit layout for application to the ground configuration; and Fig. 8 is a three dimensional diagrammatic illustration showing subterranean anisotropic conditions. A simple case where an isotropic geologic body overlies an anisotropic one is found in glaciated districts where the glacial till covers the hard rock outcrops as for example in northern Michigan. Such hard-rock outcrops are often banded 55 ( in composition and tilted, which makes the ground anisotropic in character. The ability to deflne-these differentially conducting bands constitutes the nature of this invention, even when covered with other isotropic or anisotropic material.

It is well known in the art that by applying a potential to spaced connections on the ground an indication may be obtained which relates to or is a function of the electrical resistivity and other electrical constants of the ground. See F.

W. Lee, U. S. Patent No. 1,951,760, granted March 20, 1934.

In explaining the present invention, it is desirable to first consider the electrical resistivity which for convenience may be divided into two component parts, one being called the normal resistivity as -would exist in an isotropic medium, the other the transverse resistivity as would be caused by the non-isotropic portion of the medium. For simplicity of measurement and mathematical analysis the two resistivity components are separated in space by a right-angle relationship,-such that p=pn+ipt where p=apparent ground resistivity; pn=normal component of the apparent ground resistivity; pt=transverse component of the apparent resistivity; i=V-1 (i is the operator "square'root of minus one"). Thus the absolute value of p=Vpn2+pt2 and the direction is arc tan pt P. While it is possible to compute pn and pt between spaced potential and current contacts on the ground, this invention discloses a method for differentiating the two in such a way that each may be measured and interpreted separately.

Furthermore it. divides the ground in such a manner that the change of symmetry in the transverse resistivities discloses the changes of ground structure. In the above mentioned case where the ground is covered with glacial till the sand and gravel is of an isotropic character for which pn would have a value and pt the transverse resistivity would be zero.

Referring to Fig. 1, showing a suitable ground configuration, current of suitable nature is applied through the ground electrodes CiC2, preferably equally spaced at opposite sides of the central or station electrode Po, as shown. The lotted line electrodes PIP2, which are considered as located on a line with CIC2, may be used to determine the normal resistivity component, as explained in my above mentioned patent and my copending application entitled "Electrical resistivity or impedisivity measuring," Serial No. 200,948, filed April 8, 1938 patented March 31, 1942, Patent No. 2,277,707. To determine the transverse resistivity particularly cntemplated in the present invention, and newly coordinated thereby with the normal resistivity to give complex data as set forth, the new coordination of transversely positioned electrodes P3, P4, preferably equally spaced on opposite sides of the station point Ps and at right angles to the line of centers of the current electrodes CICt, is used.

This arrangement divides the ground by two partitioning planes, passing vertically into the ground through CICa and through P3P, respectively. Measurements on both sides of the vetical plane CiC2, by means, for example, of potential electrodes P3P4, in accordance with the present invention, enables determination of the anisotropic character changes on each side of this plane, as indicated by the transverse resistivity of the geologic structure.

Now referring to ig. 2, representing Isotropic conditions, it will be seen that with the electrode configuration shown the transverse current for small values of Po--P3 and Po--P4 flowing in the direction P3--P4 would be zero, since the field would be symmetrical about the P3-P4 axis as well as about the PI-P2 axis. With measurements of increased separations of C1--C2, Po--P, Po-P2, PO-P3, Po-P4 appreciable parts of the current will flow deeper and deeper in the terrain and current will eventually begin to flow through the underlying bed rock. If the bed rock is anisotropic, then potential will begin to appear between Po--P3 and Po--P4 and by noting the separation when this occurs it is possible to compute the depth of the isotropic overburden, In practicing the present invention the point Po, Fig. 1, is called a station point at which measurements are made at successive depths by increasing the distances between the other points of the configuration while maintaining the relative symmetry. The station points are preferably located at spaced distances along a line of traverse. Such spaced distances are preferably chosen so as to not omit any portion of ground and preferably so as preliminarily to eliminate the infrequently found positions at which the transverse observations become zero. Such positions may be found when the material AB, which has anisotropic electrical properties, passes perpendicularly between C--C2 at Po of Fig. 1 or when it passes parallel to CIC2 as shown in Fig.

3. The more frequently found and preliminarily more useful positions are shown in Figs. 4, 5 Su and 6.

In Fig. 4 the transverse components of potential are equal but in opposite direction. In Fig. 5 the trapsverse components of potential are equal and in the same direction and in Fig. 6 they are unequal depending upon the poistion of AB in reference to symmetry about the line CiC2 and the distance from Po. After a conducting material AB has been identified as for example in Fig. 6, or Fig. 4, the point Po is adjusted until an arrangement similar to Figure 5 is found, which PoP3 and PoP4 are.nearly equal in value indicating that Po is substantially electrically centered over the vein (AB). By then varying the azimuth of CICa relative to AB, the two positions of zero indication when AB lies parallel normal to Ct-C2 (giving a diagram simelar to Fig. 2) serve to further delineate the structure.

Fig. 8 shows a hypothetical cross-section of the earth in which the material AB sought for is dipping to the left. The isotropic overburden D covers this material. Adjacent to AB is material C which differs materially from the conductivity of AB. The stream lines of current E are indicated on the top of the ground on which measurements are made at the spaced electrical configuration of distances a and a'.

Referring again to Figs. 4, 5 and 6: from Fig; 4 it will be observed that the direction of vectors will reverse if A--B lies on the other side of Po, or if the polarities of C1C2 are reversed; from Fig. 5, it will be apparent that if the position of strike is displaced upwardly toward P3, parallel to the direction shown, the vector directions of P3, P4 will remain the same, but their values will become unequal; and from Figs. 4, 5 and 6 it is apparent that when, and only when both P3 and P4 are on the same side of the position of strike, will the vector directions be opposed.

Furthermore, if the configuration, relative to anomaly AB, Fig. 5, is turned about Po as a center, the values between PoP3 and PoP4 will gradually decrease, being zero when AB coincides with either P3P4 or CIC2, and the vector directions shown will reverse from quadrant to quadrant, the vector values being equal to one another so long as Po Is electrically centered over AB.

From the foregoing analysis it will be appreciated that the similarity, or opposition, of the vector directions relative to each other, their relative values and their absolute directions and values, in various combinations, will evidence definite position of strike and dip.

In the foregoing only the ohmic resistivity or direct current impedisivity was considered; in the alternating current case as explained in my above mentioned copending application, the impedisivity involves not only a real part corresponding to ohmic resistivity but also an imaginary part composed of a time vector operator which is a function of frequency, dielectric constant, and permeability of the material. A similar analysis is made in the alternating current case, using transverse impedisivity instead of resistivity, the phase-shift, or time vector operator, due to the dielectric and magnetic permeability constants of the structure, being determined by means of a phase shifter as explained in my said copending application.

In the method of making ground observations exemplified in ig. 7, both direct current and alternating current may be applied to the same ground contacts and the alternating characteristics of the ground compared to the direct current permitting differentiations of asymmetric 60 material based on direct measurements between material based on direct measurements between PoPi a nd P and transverse measurements between P oP3 and PoP4.

Thus the vector magnitude, in either the direct 65 current or fluctuating current case, indicates and enables determination of the position of the underlying strata and aids in identifying the composition of the material; and the phase shift, in the fluctuating current case, gives further eviin 70 dence of the composition of the materials of the structure. It will be appreciated by those skilled in the art that a fluctuating current may be an alternating current of simple sinusoidal or other continuous or discontinuous form, alone, or suor 75 perposed on a direct current component, as may best suit the conditions of use, although for most fluctuating current cases a generally sinusoidally varied alternating current is preferred.

To obtain a complete identification of isotropic and anisotropic material the potential electrodes Pi and P2, preferably with electrode Po-giving data as to the normal impedisivity, in accordance with my Patent No. 1,951,760-are employed as well as the transversely positioned electrodes PF-P---P4, the symmetry of which entirely eliminates normal impedisivity, thus giving evidence of the transverse resistivity uninfluenced by the isotropic factors.

By using both Pi-P2 and P3-P4 with the same Ci-Po--C2 configuration, it is possible for the first time to obtain complete geophysical evidence giving knowledge of the character and depth of isotropic overburden and contact with and position of anisotropic stratified and banded underlying material.

To summarize, by establishing a configuration exemplified in Figs. 1 and 8; (1) with relatively short distances values a, a', conditions at relatively shallow depths can be determined. These will usually be isotropic, as where a homogeneous overburden overlies the rock formations, zero values being determined for the transverse impedisivities between Po and P3 and between Po and P4, the character of Isotropic material being indicated by values appearing between Pi and Po and P2 and Po.

(2) By increasing the distances a and a , greater depth of survey is accomplished, and when repetitions result in the appearance of values between Po and P3 or between Po and P4, or both, then it is indicated that contact has been established with anisotropic material at the depth of survey funce tionally corresponding to the a, a' distances in use.

(3) By then varying the azimuthal direction of 4 line C--C2 the direction of strike may be determined as above explained.

(4) At this same time the relative values appearing between Po and Pi, and between Po and P2 evidence the direction and angle of dip, if 4 gentle enough to lie within the range of both observations, without repeating the set-up of the configuration at another location, as would be necessary if only Po, P3, P4 were used.

(5) For steep dips, not within the range of 6( Po--Pj and Po-P2 observations, an accurate indication can be obtained by moving the configuration longitudinally in the direction of dip indicated by item 3, above, and repeating items 1 and 2 in the new location to determine the depth 6. of the anisotropic structure thereat.

(6) During items 1 and 2, and before the orientation of item 3 is reached, the phase-shift factor of impedisivity determined by applying fluctuating current to the configuration, serves to give evidence of the character of the anisotropic material.

(7) Also, at any time evidence of the asymmetric locations of sink-holes, faults, dikes, etc., with reference to the partitioning plane along 05 Ci-C2 may be determined by the directions and values of the potentials appearing between Po and P3 or Po and P4 or both; and at the same time evidence of the locations thereof relative to the plane along P3-P4 may be determined by the relative values of potentials appearing between Po and Pi, or Po and P2. Thus the anomalies are seggregated into quadrants with only a single set up of the configuration.

(8) By repeated set-ups and measuring of the change in values and direction of the potentials of the transverse system at various values of a and al, it is possible to determine the change of strike through geological unconformities. Thus if one system of bedding having a definite-strike and dip overlies another system of bedding having a different strike and dip, or stratified in another direction, this may be determined.

While it will be apparent from the foregoing description that various arrangements may be used for applying and measuring the configuration currents and potentials, Fig. 7 shows for purposes of illustration, the manner of employing the arrangement disclosed in my copending apPlication for "Electrical resistivity or Impedisivity measuring," Ser. No. 200,948, filed April 8, 1938, for this purpose.

As shown in Fig. 7, the resistivity or impedisivity measuring device comprises three units, corresponding to the three units of my said application disclosure. The upper, or current-input unit is connected to apply a predetermined direct or fluctuating current to the current electrodes CICa.

The lower, or potential measuring unit is arranged to be connected to measure either the normal Impedisivity between PiPo and P2Po, or the transverse impedisivity between P3Po and PPo, preferably by suitable switching means, as the triple-pole double throw switch shown; and, as Sfully discussed in my above-mentioned application, preferably makes these measurements by balancing out the picked up potentials to prevent flow of current through the potential electrodes, thus avoiding the disturbances of the electric field 3 distribution in the earth which are always produced when currents are drawn from the potential electrodes, and enabling the ground resistivities or impedisivities to be measured by the relative magnitudes and signs of the balancing 0 potentials. The center unit, as in my copending application, is the calibrating unit enabling field calibration of the potential measuring unit directly from the current input unit.

This device, as more fully explained in said co. S Pending application, where its novelty is claimed per se, enables determination of the impedisivities both in the normal and in the transverse directions as regards quantity and sign, which is evidenced as direction in the direct current case, 0 and as phase shift in the fluctuating current case, and is to be so interpreted in the appended claims.

Furthermore, where certain of the claims, for brevity, employ the term "Lee configuration" this term designates the configuration shown in Pigs.

i 1 and 8 of the drawings; where they employ the term "direct impedisivity," this designates the values appearing between Po--pi and between Po-P2; while the term "transverse impedisivity" designates the values appearing between Po-P3 and Po--P4; the term "depth determining dimensions" designates the size of the configuration as determined by the dimensions a, al; and the term "point of origin" or "origin" designates the station point Po.

While I have described preferred embodiments of the several cooperating features of my invention, it is to be understood that these embodiments are but illustrative, and not restrictive of my invention.

I claim as my invention: 1. A method in geophysical surveying, comprising the steps of (a) establishing a flow of electric current through the earth between two Points, (b) measuring potentials from a third point intermediate the first two points to other points on a line transverse to the line connecting the first two points, (c) determining the relative magnitude and sign of the transversely measured potentials which reflect and thus indicate the presence and location of strike of subterranean strata, and (d) repeating steps a, b and c in various orientations of current flow near the position at which a subterranean strike is indicated, until equality of magnitude, with opposite sign if magnitude is other than zero, establishes accurately the direction of strike.

2. A method comprising steps a, b and c of cla:m 1, in which the relation of current to electrode spacing is maintained in a predetermined ratio throughout a series of determinations.

3. A method comprising steps a, b and c ox claim 1, which includes further determining the slope of the underlying strata by taking further potential measurements from the intermediate point to further points on the line of the first two points, and in which the relation of current to electrode spacing is maintained in a predetermined ratio throughout a series of determinations.

4. In geophysical surveying, the step of determining factors of position of an underlying structure by applying current to the terrain at the ends of a base line and measuring the impedisivity of the terrain transverse to the approximate center pf said base line.

5. In geophysical surveying, the step of determining factors of position of an underlying structure by applying current to the terrain at the ends of a base line and measuring the impedisivity of the terrain symmetrically transverse to the approximate center of said base line.

6. In geophysical surveying, the step.of determining factors of position of an underlying struec ture by applying current to the terrain at th< ends of a base line and measuring the impedisiv ity of the terrain at right angles to the approxi mate center of said base line.

7. The method of determining the presenc and azimuthal direction of strike of a subter ranean strata in geophysical surveying whic] comprises selecting a point of origin for the sur vey, causing a current to flow through the eart from points on a substantially straight line pass ing through the point of origin, and measurin the transverse impedisivity between the origi and points spaced transversely on opposite side thereof, and determining from the said measure impedisivities the approximate position and d rection of strike of the subterranean strata.. 8. The method of determining the presen and azimuthal direction of strike of a subte. ranean strata in geophysical surveying whic comprises selecting a point of origin for the su, vey, causing a current to flow through the earl from points on a substantially straight line pas ing through the point of origin and equal spaced therefrom, and measuring the transver impedisivity between the origin and points equal spaced transversely on opposite sides thereof 4 a line at right angles to the first straight lin and determining from the said measured i pedisivities the approximate position and dire tion of strike of the subterranean strata.

9. A method according to claim 8, which ft ther includes determining accurately the sE azimuthal direction of strike by repeating t steps of claim 8 at locations on lines substantia normal to the approximate direction of str: until substantially equal and opposite transve impedisivity measurements are obtained.

10. The method of determining, with a sevenelectrode configuration applied to the terrain, the depth of anisotropic material in geophysical surveying; which comprises employing one electrode as a point of origin, locating four other electrodes along a base line passing through said point of origin, connecting two of said other electrodes to apply electrical energy to said terrain and the other two to pick up potentials from said terrain which reflect and thus indicate the impedisivity of the terrain embraced in said configuration, locating two electrodes at opposite sides of said base line along a line transverse to said base line and connecting said last two electrodes to pick up potentials from said terrain which reflect and thus indicate the transverse impedisivity of the terrain embraced in said configuration, and repeatedly performing said steps with different depth determining dimensions of the configuration applied to the terrain to determine the depth at which indications of transverse impedisivity appear.

11. The method of determining, with a fiveelectrode configuration applied to the terrain, the direction of strike of anisotropic material in geophysical surveying; which comprises employing one electrode as a point of origin, locating two other electrodes along a base line passing through said point of origin, connecting said other eleco0 trodes to apply electrical energy to said terrain, locating two electrodes at opposite sides of said base line along a line transverse to said base line and connecting said last two electrodes to pick up potentials from said terrain which reflect and thus indicate the transverse impedisivity of the terrain embraced in said configuration, and varying the azimuthal orientation of said configuration about its point of origin while maintaining e depth determining dimensions of said configura- 40 tion such that indications of transverse impedisivity are obtained.

12. The method of determining the direction S and dip of gently sloping anisotropic material in geophysical surveying; which comprises estabS lishing by the method of claim 11 the azimuthal 5 orientation of the configuration therein defined h corresponding to symmetry with respect to the S direction of strike, and picking up potentials g aligned with the base line of said configuration n which reflect and determine the direct impedie 50 sivities of the configuration so oriented as an :d indication of the direction and angle of dip.

d- 13. The improved configuration for geophysical surveying comprising a point of origin Po, current electrodes CiC2 positioned on a first line e 55 passing through said point of origin and on oprh posite sides of and substantially equidistant from r- said point of origin, and means for determining rh transverse impedisivities comprising potential S60 electrodes P3P4 positioned at opposite sides of the ly 60 point of origin on a line passing through said se point of origin and transverse to said first line.

ly 14. The improved configuration for geophysical ny surveying comprising a point of origin Po, curie, 5 rent electrodes C1C2 positioned on a first line a- passing through said point of origin, and means !c- for determining direct and transverse impedisivities comprising potential electrodes PiPa posi.r- tioned at opposite sides of the point of origin on aid 70 said first line and other potential electrodes P3P4 he positioned at opposite sides of the point of origin ly on a line passing through said point of origin ike and transverse to said first line. rse 15. A method in geophysical surveying, which s. comprises establishing a flow of electric current through the earth between two points, picking up potentials from a third point intermediate the first two to other points on a line transverse to the line connecting the first two points, balancing out the picked up potentials to prevent dis- t turbance of the electric field established in the earth by the flow of current between said first two points, and determining the relative magnitude and sign of the balancing potentials which reflect and thus indicate the presence and loca- lR tion of strike of subterranean strata.

16. A method in geophysical surveying, which comprises establishing a flow of alternating current through the earth between two points, picking up potentials from a third point intermediate the first two to other points on a line transverse to the line connecting the first two points, balancing out the picked up potentials to prevent disturbance of the electric field established in the earth by the flow of current between said first two points, and determining the relative magnitude and phase of the balancing potentials which reflect and thus indicate the presence and location of strike of subterranean strata.

17. A method in geophysical surveying, which comprises establishing a flow of direct current through the earth between two points, picking up potentials from a .third point intermediate the first two to other points on a line transverse to the line connecting the first two points, balancing out the picked up potentials to prevent disturbance ,f the electric Ileld established in the earth by the flow of current between said first two points, and determining the relative magnitude and direction of the balancing potentials which reflect and thus indicate the presence and location of strike of subterranean strata.

18. In geophysical surveying, the method which comprises applying to the terrain a seven-electrode configuration comprising a station electrode Po, two current electrodes C1 C2 equidistant from the station electrode Po and lying on a base line passing through the station electrode Po, two normal-potential electrodes P1 and P2 equidistant from the station electrode Po and also lying on the said base line, and two transverse potential electrodes P3 p4 lying on a line at right angles to said base line and Passing through said station electrode, orienting said configuration in such direction that no potential difference exists between the transverse potentials obtained at the transverse potential measuring electrodes P3 and P4, thereby positioning the current base line CIC2 of the configuration in alignment with the direction of slope of underlying strata, and measuring the potential differences between the normalpotential measuring electrode pairs PoPi and PoP2, which reflect and thus indicate, in such orientation, the true direction of slope of the underlying strata.

FREDERIcK W. LEE.