SwRl Proposal No. 14-3306
August 9, 1985

A Proposal for

Investigation of the Self-Potential Measured
For Isolated Rock Samples

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Submitted to:

The Townsend Brown Foundation
P.O. Box 1565
Avalon, CA 90704

Prepared by:

Bob M. Duff
Institute Scientist


Thomas E. Owen, Director
Department of Geosciences

Southwest Research Institute
San Antonio, Houston

Copyrighted © by The Townsend Brown family. All rights reserved.

Table of Contents

I. Introduction (1)

II. Technical Discussion (2)

A. The Previous Experiment (2)
B. Electric Potentials and Fields (2)

III. Proposed Program (4)

A. Task A - Review of Literature and Past Experiments (4)
B. Task B - Final Design of the Experiments (4)
C. Task C - Experiments (4)
D. Task D - Final Report (6)

IV. Project Staffing and Task/Time Schedule (6)

V. Southwest Research Institute Qualifications and Related Experience (6)

A. General (6)
B. Electronic Systems Division (8)
C. Department of Geosciences (8)

I. Introduction:

Southwest Research Institute has been contracted by the Townsend Brown Foundation in regard to replicating and extending experiments associated with the measurement and long-term recording of the self-potential of isolated basaltic rocks. Of particular interest in relation to this experimental study are the observable variations in the measured data, which exhibit diurnal, weekly, seasonal, semiannual, and annual periodicities. In response to the request for proposal, a project is described which is aimed at conducting a thorough and critical examination of the reported self-potential phenomena.

Self or spontaneous potentials within the earth are well known and have been utilized for mineral exploration for over 150 years. One of the earliest applications of self potential measurements was reported by R.W. Fox(l) in 1830 in which he utilized self potential measurements along the walls of mines to locate sulphide deposits. These potentials within the earth originate most often from electrochemical reactions, particularly reductionoxidation (redox) processes. The self potentials produced by a sulfide body are normally produced by redox reactions. Water from the surface containing dessolved oxygen produces an oxidizing region over a part of the sulfide body while other parts of the surface are in a reducing environment. The resultant difference of potential between different parts of the body produces a current flow and an associated potential gradient which is measurable at the surface of the earth. Other electrochemical reactions can occur at the interface between different types of materials. For example, self-potentials are high at contacts between shale and sandstone. If this region is penetrated by a borehole, currents can flow in the borehole fluid between the two layers thus producing a potential gradient which may be measured in the borehole. Selective diffusion or absorption of ionic charges can also produce electric potentials between one type of geologic material and another. Electrokinetic charge separation resulting from the flow of ground water or borehole fluids through the porous medium can also produce spontaneous potentials. Thermoelectric generation of currents and, hence, natural potential gradients has also been observed, particularly in geothermal regions.

A careful review of the various processes which produce self potentials within the earth has failed to identify a process which is likely to produce similar electric potentials across an isolated rock sample. However, the experiment conducted by T.T. Brown has shown a measurable potential. The purpose of the proposed project is therefore to design and carry out experiments which will lead to definite identification of the source of the potentials previously observed. There are a variety of well known physical phenomena which could have produced the observed deflections of the strip chart recorder in the previous experiments. Since there are many possible sources and potentially valid explanations of the observations, a systematic and comprehensive investigation must be planned in order to assure that definitive results are obtained. Within the scope of the proposed project, it should be possible to answer the questions concerning the origin or cause of the previous observations to the satisfaction of all concerned, assuming that observations similar to those obtained by T.T. Brown can be repeated.

(1) Fox, R.W. (1830) "On the Electromagnetic Properties of Metalliferous Veins in the Mines of Cornwall" Philosophical Transactions of the Royal Society, 130,399.

II. Technical Discussion:

A. The Previous Experiment

The description of the experiment conducted by T.T. Brown described in Attachment A of the request for proposal is summarized as follows: "An oval-shaped basaltic rock, approximately 3 in. x 5 in. x 8 in., from the rim of the ancient Koolau volcano on Ohau, Hawaii, is thoroughly oven-dried and copper electrodes are painted on opposite sides. The rock is then covered with an insulating plastic sheath and enclosed in several layers of aluminum foil which is grounded. A 5-megohm wire-wound precision resistor (Shallcross) is attached to provide an electrical load. The load causes the freely-floating self-potential of the rock to drop from approximately 300 mV to a mean value of approximately 50 mV. The rock and resistor are encased in an electrically shielded constant-temperature box controlled to 0.1oC. BNC shielded cable connects the rock to a double-throw one-hour timing switch, thence to a strip-chart recorder as shown in the diagram. When the switch is in Position I for one minute, the recorder is directly connected to the rock. When in Position 2, for the remaining 59 minutes, the recorder is connected to a fixed 50 mV constant (battery-fed) source."

As a part of the proposed project, this experiment will be duplicated as nearly as possible including the use of the same type of strip chart recorder and wiring arrangement. In addition, other arrangements of this experiment will be conducted which will be designed to test or identify the possible sources of the observed potential and variations with time.

B. Electric Potentials and Fields

Static and quasistatic electric fields, or potential gradients, arise from chemical, thermal, or mechanical charge separation mechanisms or from induction by a time varying magnetic field. With respect to isolated rock samples, the only self-contained mechanisms which would produce a potential gradient within the rock are electrochemcial or thermoelectric. Since the experiment was conducted with the rock in a temperature-controlled environment, thermoelectric effects within the rock itself are not likely to be the source of the observed potential. Electrochemical processes probably do occur within the rock and would be dependent upon moisture content. However, if the rock is macroscopically homogeneous in its chemical and physical properties, then no net potential difference across the surface of the rock would be expected. For a potential difference to be produced by electrochemical reactions, some nonuniformity and anisotropy must exist which determines a unique direction within the rock. This requirement for a unique directional property within the rock becomes clear with regard to the question of which electrode on the rock will be positive and which will be negative. It is possible that the electrodes used to contact the rock develop a polarization and, hence, a potential difference between the metal electrode and the rock. This electrode polarization will balance out to zero if the two electrodes are of the same material and of equal area.

From these arguments it seems unlikely that the observed potentials originate from any physical or chemical reactions which are associated with the composition or structure of the rock. If these conclusions are correct, then the rock could be replaced by an equivalent electrical circuit without affecting the measured potential.

There may be an explanation for the observed potentials other than energy generation or energy conversion within the rock. Although thermoelectric phenomena within the rock are improbable because of the temperature-controlled environment, the strip-chart recorder and other parts of the electric circuit do not appear to have been in the controlled temperature enclosure. Thus, it is possible that some thermoelectric effect occurred between the components within the enclosure and those exposed to ambient temperature. Another possible origin of the observed data could be electrostatic fields which affect the strip chart recorder or other parts of the circuit which were not located within the shielded enclosure. For example, the normal fair weather electric field near the surface of the earth is of the order of 140 volts per meter although it may reach much higher values during thunderstorm activity. This fair weather field has a mean diurnal variation of 40 to 50 volts/meter measured over the ocean. (2) When using sufficiently high impedance instruments, it is possible for this field to produce measurable potentials within the circuit.

Magnetic induction would not at first appear to be a likely source of the observed potentials since time varying fields are required and the induced voltages would also be time varying. Even if the period of the alternating field were much longer than the one minute measurement time it would not necessarily be synchronized with the measurement cycle and would thus produce both positive and negative potential readings. On the other hand, if some element in the circuit should act to partially rectify the alternating currents, then even ambient radio frequency fields could become a possible source of the observed potential.

There are thus many possible sources of the observed recorder deflections. In any experimental measurement of this type, great care must be exercised to identify and eliminate all extraneous sources. With careful planning of the experiments and a thorough scientific approach to the investigation, it should be possible to determine the source of any potentials, which cannot be eliminated, and to explain any changes in these potentials.

(2) Pierce, E.T., "Some Topics in Atmospheric Electricity," in Recent Advances in Atmospheric Electricity-Proceedings of the Second Conference on Atmospheric Electricity, 1958, Pergamon Press.

III. Proposed Vacuum:

A. Task A - Review of Literature and Past Experiments

The first task of the proposed program is an in-depth literature search and review of related past experiments. This task is expected to require two months although additional literature findings will be reviewed throughout the project as new material is identified. The initial literature search will utilize Southwest Research Institute's access to several computerbased information retrieval services. The initial concentration of-the search will be on published observations of self potential produced by isolated rock samples. In addition, however, the search will be expanded to include publications on all related phenomena and other long-term measurements of small electric potentials. All identified papers will be reviewed to establish their relevance to the project. Those papers which appear to contain relevant information will be studied in detail and an interim report summarizing the results of the literature search will be prepared and submitted to the Townsend Brown Foundation.

During the time period of Task A, a review of the previous selfpotential experiments will be conducted in addition to the literature search. The Townsend Brown Foundation will be asked to supply any other information available for this review. The purpose of this review is, in part, to insure that the experimental tests are repeated with the equipment arranged as nearly as possible in the same manner as that used in the previous experiment. In addition, a complete review of the previous work may identify other test factors or conditions which should be included in the new experiments.

B. Task B - Final Design of the Experiments

Following the literature search and review of the previous experiments, final designs for the new experiments will be formulated. Based upon knowledge gained during Task A, the tentative experiments outlined under Task C below will be changed or other experiments added. The revised design of the experiments will be submitted to the Townsend Brown Foundation for review and approval prior to the start of Task C.

C. Task C - Experiments

The experiments, which will be conducted during this project, will be designed with the objective of obtaining a full explanation of all observed electric potentials. The starting point, however, will be to repeat the past experiment conducted by T.T. Brown, with the experimental apparatus being as identical to the original test arrangement as possible. For this experiment, the Townsend Brown Foundation will be asked to supply the original apparatus, if available, or otherwise a complete description and specifications so that a duplicate test set-up may be fabricated. The original rock specimen or a similar rock specimen will also be required. The ability to repeat the earlier observations is necessary before an explanation of the results can be achieved.

Additional experiments will be conducted using an automated digital data recording system. The controller and data logger to be used in this system will consist of a Hewlett Packard 9825 desktop computer available in the Department of Geosciences. This computer will be made available for long-term dedicated use. The other data acquisition equipment required for the tests will be purchased as part of the project. The equipment recommended for this purpose is:

HP Model 3497A Data Acquisition/Control Unit with Option 001 - 5 1/2 Digit Digital Voltmeter and Option 010 - 20 Channel Relay Multiplexer.

This data acquisition system can be controlled by the RP9825 desktop computer to record data from up to 20 measurement channels at preprogrammed time intervals. The Digital Voltmeter supplied as option 001 of this system has resolution down to one microvolt and an input impedance of 109 ohms. The accuracy of this system should be more than adequate for the proposed experiments. The calibration of this system will be periodically checked using a standard voltage cell. In addition to this data acquisition system, three shielded temperature controlled chambers will be constructed or purchased.

The experiments tentatively planned to be conducted include:

Experiment #1: A second rock similar to that used in the repeat of the original experiment will be prepared in an identical manner and the potential monitored by the digital data acquisition system. This experiment will be conducted on a long-term monitoring basis and the data obtained will be compared with that of the repeat of the original experiment. The Townsend Brown Foundation may be asked to assist in obtaining suitable rock samples.

Experiment #2: The temperature, pressure, and humidity both inside of the environmental chamber and outside of the chamber will be recorded.

Experiment #3: An arrangement identical to Experiment #1 will be established except that the rock will be replaced by carbon resistors connected between the electrodes. The resistance value will be set to the measured resistance of the rock used in Experiment #1.

Experiment #4: If non-random electric potentials are observed in the other experiments then another experimental arrangement will be established which can be changed and tested without disturbing the long-term monitoring experiments. This last experiment will primarily be used to determine the origin of potentials recorded by the other experiments. When an outside source of potential is found which can be eliminated, then the other experiments will be modified to eliminate this source. If all observable electric potentials are eliminated by modification of the long-term experiments, or definitely identified as to their origin, then the project will be considered to have been completed.

D. Task D - Final Report

A final report describing all work performed on the project and all findings resulting from the experiments will be prepared and submitted to the Townsend Brown Foundation.

IV. Project Staffing and Task/Time Schedule:

The overall technical direction of the project will be the responsibility of Dr. B.M. Duff who is expected to devote at least 360 hours of time to the project. Mr. James C. Biard and Mr. David A. Anderson will assist in carrying out the experiments and together are expected to devote about 700 hours of time to the project. Professional Data sheets for these key individuals are included. In addition. a senior technician will assist in various ways. Dr. T.E. Owen, Director of the Department of Geosciences, will be available for consultation. The Task/Time Schedule on the next page presents a 15-month calendar schedule recommended for conducting the proposed project tasks. This schedule includes one visit by the Project Leader to the Townsend Brown Foundation in California during Task A.

V. Southwest Research Institute Qualifications and Related Experience:

A. General

Southwest Research Institute is a not-for-profit research corporation operating under the laws of the State of Texas to serve government, industry, and individuals in the fields of applied research, development, and engineering. Present Institute employment is 2024 of whom 863 are professional level scientists and engineers involved directly in technical work. The professional staff has over 697 degrees from more than 160 colleges and universities, including 102 doctorates in special fields of science and engineering.

During its 1984 Fiscal Year, Southwest Research Institute received gross revenues of $120,516,364 derived from more than 950 active projects. Sixty-six percent of these revenues were derived from research and development services for private industry and 34 percent from services for various agencies of government. The Institute does not engage in production manufacturing; however, it is particularly well qualified to perform applied research and development and to provide one-of-a-kind prototype quantities of experimental devices and equipment. Because it does not depend upon production contracts, the Institute is able to provide thorough and unbiased services regarding the survey, design, analysis, evaluation, selection, and recommendation of various technical approaches and equipment. Many past programs have been performed at the Institute because of this unbiased technical position.

Southwest Research Institute performs research and development work in a wide variety of technical fields as illustrated by the titles of its various research divisions and departments. These include:

Applied Physics Automotive Research Chemistry and Chemical Engineering Electromagnetics Electronic Systems Engineering Sciences.

Engines, Fuels, and Lubricants Instrumentation Research Quality Assurance Systems and Engineering Structures Research and Ocean Engineering U.S. Army Fuels and Lubricants Laboratory.

In addition to the operating divisions and departments representing the the various scientific and engineering disciplines, the Institute maintains several well-equipped in-house support facilities to provide a complete research and development capability. These include a Library facility presently containing more than 85,000 bound volumes and over 175,000 technical reports, a complete Machine Shop facility, and an up-to-date Editorial, Printing, and Reproduction facility.

Southwest Research Institute has its principal facilities and headquarters in San Antonio, Texas; however, active operating centers are also maintained in other locations. The SwRI-Houston Office is a primary center for air- and water-pollution studies. The Institute also maintains offices in Washington, D.C.

B. Electronic Systems Division

The Electronic Systems Division is comprised of four departments: Geosciences, Data Systems, Communications, and Bioengineering. The Department of Geosciences conducts applied research and development programs in geophysical instrumentation, specialized geophysical exploration systems, and electronic instrumentation. The Department of Data Systems performs digital system engineering and software development for applications in automatic testing, data management, and digital control. The Department of Communications provides in-depth experience in advanced communications hardware and electromagnetic compatibility. The Department of Bioengineering is engaged in the study of behavioral effects of electric power system fields and rehabilitation engineering for the handicapped.

C. Department of Geosciences

The Department of Geosciences is organized to explore and develop advanced concepts and methodology in geophysical and geosciences applications and to demonstrate those techniques and instrumentation in practical field studies. The principal technical applications of this work involve borehole geophysical probes, high-resolution seismic and electrical exploration methods and equipment, ground-penetrating electromagnetic and radar techniques, and environmental monitoring-instrumentation. In addition to these developments, geophysical applications and demonstration tests are being performed in probing both wet and dry boreholes for formation fracture conditions, detecting geologic anomalies in coal seams and mine roof structures, surface surveys to map man-made tunnels and abandoned mines, and collection of baseline environmental climatic data at nuclear waste repository field sites. Recently completed projects in Geosciences have included seismic methods for emergency mine communications, detection and delineation of soil sinkholes and ground subsidence anomalies, experimental analysis of electromagnetic parameters of oil shale, development of harmonic radar systems, military intrusion detectors, and remote battlefield sensors.

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