Advancing Electrical Methods Of Geophysics


 

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During the summer of 2007 I was asked to provide an article, which follows, relating my memories of working with Dr. Ken Zonge for a book being compiled on the occasion of his retirement.

Background

In the summer of 1977 I accepted a position as manager of geophysical research with the Climax Molybdenum division of American Metals Climax (AMAX). The timing was fortuitous as AMAX had just discovered the Red Lady molybdenum deposit on Mount Emmons near Crested Butte, Colorado, so there was a natural laboratory available to test new geophysical methods.

Of course AMAX had been helping Dr. Ken Zonge with his dissertation research and, to our mutual advantage, we quickly partnered up. Also, Zonge Engineering and Research Organization (ZERO) had just run their first CSAMT survey (for Exxon if I remember correctly) with promising results and I quickly jumped at the chance to test this new technique on AMAX's porphyry sulfide deposits, especially Red Lady before drilling and development had proceeded very far and cultural contamination interfered with our research. As a result we started running CSAMT surveys in the summer of 1978 after the snow melted sufficiently (Mount Emmons is over 12,000 feet). As is usually the case, some redesign and modification of the equipment was required after the initial survey and it wasn't until the summer of 1979 that we really got going with CSAMT surveys at AMAX.

In the meantime I'd found that, with a few modifications, self potential (spontaneous polarization, or simply SP) surveys were a very cost-effective reconnaissance method for the porphyry sulfide bodies we were looking for. Before finding the Red Lady deposit, AMAX had been drilling in Redwell Basin on the north side of Mount Emmons for years (fifteen 4,000 foot cored diamond drill holes). The SP survey on Mount Emmons also indicated an anomaly in Wolverine Basin just east of Redwell. So we made a point of doing a CSAMT survey over the Wolverine SP anomaly. As there was also some interest by the geologists it wasn't much of a problem to get a drill hole in the center of the SP/CSAMT anomaly in Wolverine Basin during the summer of 1980. From what we'd seen to date from CSAMT results, and based on a 2-D interpretation, I rashly predicted ore-grade molybdenite between 2,500 (770 m) and 2,800 feet (860 m). The geologists had predicted that, in the unlikely event there was an ore body there, it had to be shallower than 2,000 feet (600 m). So I took some flak. However, when the drill hole intersected ore grade mineralization at 2,450 feet (755 m) we did a little celebrating. But I was off about the bottom of the deposit by about 100 feet (30 m) as the drill hole went out of ore grade around 2,700 feet (830 m).

Wolverine Basin didn't prove to be a big enough deposit to be of economic interest but with the boundary well defined by the CSAMT data we were able to drill it out with just five (5) drill holes compared with the fifteen (15) needed to drill out the similar Redwell Basin deposit. Thus, we saved the cost of drilling ten 4,000-foot holes and, as I recall, our drilling costs were running around $40/foot.

After that there was no question that CSAMT was the geophysical method of choice at AMAX.


 

But why did CSAMT work so well?

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It had been recognized for some time with induced polarization (IP) surveys that the polarization effects over sulfide deposits were directional. In fact, with some sulfide deposits there was no IP response in one direction, and obvious polarization in another direction. Conventional models for the IP mechanism did not account for directional effects but that inconsistency was brushed under the carpet.

We were seeing much the same thing with the CSAMT surveys, especially over the Red Lady deposit on Mount Emmons. In one direction the CSAMT apparent resistivity looked like the impedance of a series-resonant circuit over the center of the ore body. On Red Lady line 1 apparent resistivity over the center of the ore body was <10 ohm-m. On Red Lady line 2, run nearly perpendicular to line 1, the apparent resistivity over the center of the ore body was two orders of magnitude greater.

We found this same response at several other known deposits as well so we knew that it wasn't just a fluke at Red Lady. There was also the additional complication of the directional nature of the response. But we had no explanation for why the CSAMT apparent resistivity looked like a series-resonant circuit over the center of the deposits, or the directional effects.

ZERO by now was doing CSAMT surveys about as fast as they could put equipment together and field crews, and moving into the new building on Fort Lowell. But the mechanism continued to bother me. Late in 1980 it dawned on me where I'd seen this kind of behavior before. Plasmas in a magnetic field directionally polarize and the impedance through a plasma is effectively zero. So could it be that the free electrons in the semi- and metallic-conductor ore minerals acted as a solid-state plasma in response to the CSAMT applied electric field?

If memory serves, in February 1981 I went to Tucson and got most everyone from ZERO together in the conference room. I then fearlessly presented my wild hypothesis about sulfide ore bodies reacting as a solid-state plasma to applied electrical fields. I'd expected to have to spend at least a couple of hours defending the idea and at least a 50/50 chance it would be shot down completely. Much to my surprise, within half an hour I'd sold the idea to most everyone, although there remained obvious unanswered questions about why and what was the underlying mechanism, which needed research.

Obviously to do further research we were going to need an acronym for the project. Certainly the acronym would have to incorporate "plasma" and "solid state." The next two or three hours were thus spent debating a suitable acronym. Finally we settled on a project name of P lasma U tilizing S olid S tate E lectron Y ield and so Project PUSSEY was born. Now if blame must be assigned as to Y we chose that acronym I think the name Van Reed should be mentioned.


 

Epilogue

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We did some additional research on Project PUSSEY at Mount Emmons during the summer of 1981, as well as continuing CSAMT surveys over other AMAX prospects. But in 1981 metal prices collapsed and my research group at AMAX was first out the door. By the end of 1982 AMAX had gone from fifteen geophysicists to zero.

AMAX did drill one more hole based on the CSAMT results at the Pine Nut deposit in Nevada before I left. There I'd predicted the base of any sulfide mineralization to be at 650 feet (200 m) but no ore grade deposit. Core from the drill hole showed no sulfides below 200 m and nothing approaching ore grade.

The partnership with ZERO had included swapping people. So some of my group went to work for ZERO. In 1983 I went to Texas A&M as a visiting and adjunct professor and one, Mark Bieniulis, who had come to work for me at AMAX from ZERO, became my graduate student at Texas A&M. Also, AMAX very kindly allowed me use of all the data we'd collected during my tenure there.

By then I'd stumbled on to the idea that ore minerals might be ferroelectrics. That would account for the polarization we were seeing and provide a mechanism for the space charge separation required to maintain a solid plasma. The question then was whether ore minerals were commonly ferroelectric. The conventional wisdom was no they were not. However, I dragooned Mark into doing his masters thesis on the question of whether or not chalcocite, a common copper ore mineral, might be ferroelectric. The evidence from Mark's work is that chalcocite is indeed ferroelectric.

In the meantime I continued to collect data on the crystal properties of any other ore minerals I could find data for. That search was facilitated by my accepting a position as an associate professor of geophysics at the University of Missouri - Rolla (Missouri School of Mines) in the fall of 1984. Ore mineral properties were a major interest of the department and I eventually put together the tables on the optical, structural, electrical, and magnetic properties of ore minerals now found on the Zonge web site. That foray into solid state physics determined that at least 20 common ore minerals are ferroelectrics and the space groups of many more suggests some 60+ ore minerals may be ferroelectrics as well.

With the help of Dudley Emer from ZERO, and the use of their equipment, field research on Project PUSSEY was continued in the Patagonia Mountains of southern Arizona in December 1984.

Eventually, after returning to a previous incarnation as an oceanographer at Woods Hole, in 1994 I published the results of the ferroelectric studies accumulated during Project PUSSEY. A bibliography of papers done in conjunction with ZERO on CSAMT and ferroelectrics is given below. Surprisingly, the ferroelectric work is consistently one of the most accessed articles on my web site, averaging about 5 visitors per day. Neither Ken or I can account for the interest in such an esoteric subject.

However, the effects of Project PUSSEY have clearly spread far and wide. In reviewing an article recently for the Society of Exploration Geophysicists (SEG) I learned the Canadians are even considering ferroelectric effects as a mechanism for SP.

I think it safe to say that Project PUSSEY fundamentally changed electrical methods of geophysical exploration.


 

Relevant publications

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Corry, C. E., 1981, The role of the self potential method in the exploration for molybdenite: AMAX, Climax Molybdenum Co., Golden, CO, 53 p., 7 maps.

Corry, C. E., DeMoulley, G., and Gerety, M., 1983, Field procedure manual for self potential surveys: Zonge Engineering and Research Organization, Tucson, Arizona, 75 p., 1 map.

Corry, C. E., 1984, Ferroelectricity in sulfides and related ore minerals (abstract): EOS, Transactions, American Geophysical Union, v. 65, no. 45, p. 1080-1081.

Bieniulis, M. Z., Corry, C. E., Pandey, R. K., and Hoskins, E. R., 1984, Anomalous dielectric behavior in the transition metal chalcogenide, Cu2S, chalcocite (abstract): EOS, Transactions, American Geophysical Union, v. 65, no. 45, p. 1106.

Bieniulis, M. Z., Corry, C. E., and Hoskins, E. R., 1987, Ferroelectricity in natural samples of chalcocite, Cu 2 S: Geophysical Research Letters, v. 14, no. 2, p. 135-138.

Corry, C.E., 1985, Spontaneous polarization associated with porphyry sulfide mineralization: Geophysics, 50 , 1020-1034; also see discussion in v. 51, no. 5, p. 1153-1155.

Corry, C. E., Emer, D., and Zonge, K. L., 1987, Controlled source audio-frequency magnetotelluric surveys of porphyry sulfide deposits and prospects in the Cordillera of the United States: Proceedings, North American Conference on Tectonic Control of Ore Deposits and the Vertical and Horizontal Extent of Ore Systems, University of Missouri - Rolla, p. 204-213.

Corry, C.E., Carlson, N.R., and Zonge, K.L., 1988, Case histories of controlled source audio-frequency magnetotelluric surveys: 58th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 415-418.

Corry, C. E., 1989, Preliminary investigation of ferroelectric effects in sulfide deposits (abstract): EOS, Transactions, American Geophysical Union, v. 70, no. 15, p. 492.

Corry, C. E., 1994, Investigation of ferroelectric effects in two sulfide deposits, Journal of Applied Geophysics, v. 32, p. 55-72 (WHOI contribution 8403).

Corry, C. E., 1996, Optical, Structural, Electrical, and Magnetic Properties of Ore Minerals, Zonge Engineering and Research Organization.

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Added December 8, 2007

Last modified 2/15/14