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Jeff Wynn, Project Chief
|| Summary || Introduction || A promising new approach || A new marine tool || The offshore industrial minerals project || Other participants || Principal activities through 2007 || References
Based on:Wynn. Jeff; and Laurent, Kevin, 1998, A high-resolution electrical geophysical approach to mapping marine sediments in the Atlantic coastal shelf and the Gulf of Mexico: Extended Abstract, Society of Exploration Geophysicists Annual International Meeting, New Orleans, Sept 12-18 1998.
Wynn, Jeff; and McGinnis, Tim, 2001, Two different electrical properties can improve trans-oceanic cable-route mapping: proceedings of the 11th International Offshore and Polar Engineering Conference (ISOPE-2001), Stavanger, Norway, June 2001, pp. 37-41, 4 figures.
Wynn, Jeff; McGinnis, Tim; and Williamson, Mike, 2001, A new marine geophysical method for mapping placer sands, buried metallic objects, and androgenic sediments beneath the seafloor: Proceedings of the Oceanology International Americas annual meeting, Miami, Florida, April 2-6, 2001, 17 pages, 5 figures.
Zonge, K.L.; Wynn, J.C.; and Urquhart, S.A., 2005, Resistivity, induced polarization, and complex resistivity: chapter 9 in the Society of Exploration Geophysicists Special Volume on Near Surface Geophysics, pp. 265 - 300.
This page documents a multidisciplinary research effort involving several fields of science and engineering, laboratory and sea-trial testing, with a range of partners from inside and outside of the USGS. The primary goal has been to elaborate a geophysical and geologic toolkit for identifying and quantifying industrial mineral concentrations, unexploded ordnance, waste dumping sites, cables, pipelines, and androgenic sediments from urban sewage outlets in rivers, estuaries, and the shallow offshore continental shelf of the United States and its territories. The primary tool is a streamer designed to make induced polarization (IP and Spectral IP) measurements over the seafloor from a moving vessel. Because the IP measurements are made continuously as fast as a ship can travel, one recent offshore survey has itself acquired more IP data than all previous IP measurements made on land in human history. In 2001 two patents were granted covering marine IP surveys on or proximal to the seafloor.
There are more than 3,000,000 square miles (~8,000,000 square kilometers) of "land" out there that belong to the United States that we know relatively little about. These are the vast offshore regions in the 200-mile-wide Exclusive Economic Zone. While much effort has gone into studying this huge region (for instance the "Gloria" side-scan sonar mapping program), we still know relatively little about it. What little we DO know from sparse sampling suggests that there are substantial mineral resources, including exotic industrial minerals like ilmenite (FeTiO3, a source of titanium) and monazite (a thorium and rare-earth-bearing mineral), as well as gold and platinum, which are sometimes intimately associated. Figure 1 shows the distribution of ilmenite inferred from very sparse USACE vibracore sampling in the Atlantic Continental Shelf.
We also know that there are vast deposits of urban waste that have been dumped offshore (for instance offshore Miami; in the New York Bight; in Long Island Sound; in the Willamette River and Puget Sound). This sea-floor waste is proving to be a significant health and even a navigation hazard, and we now know that they are being constantly remobilized by ocean currents and are "on the move". The hypodermic needles that grabbed headlines when they washed ashore on the New Jersey coast several years ago came from New York City municipal waste barges that have operated in the New York Bight over the past several decades. Clostridium-infected sludge from 300 years of sewage dumped into the Connecticut river poses a health hazard to communities on the north shore of Long Island. Storm-related surges have periodically overwhelmed the sewage systems of Seattle, WA, and Vancouver, BC, with resulting outflow into Puget Sound.
In addition to waste hazards, there are numerous places where UneXploded Ordnance (so-called "UXO") have been left over from World War II and peacetime military exercises. This UXO is generally buried beneath a shallow layer of sediments and is otherwise invisible. In the late 1980's, a lobster served at a dinner table in Boston exploded in spectacular fireworks: it was full of white phosphorus, probably from an unexploded shell that had corroded over time, exposing sea-floor creatures to its highly toxic contents.
Finally, induced polarization is a surface-sensitive physical property of a number of minerals and of virtually all metals, so finely-dispersed metallic debris (e.g., an 18th Century wreck) buried beneath meters of inert mud are visible ONLY by this method, as the targets otherwise provide insufficient contrast to be detected by magnetic or side-scan sonar methods.
Faced with these potential resources and known hazards, some of them dangerous to divers and casual beach-goers, several tacks have been taken to try to characterize and map them. Side-scan sonar and high-frequency reflection seismic ("chirp") methods are techniques that can often identify shapes and features on the sea-floor. Scientists have learned to correlate some of these with ancient beach deposits or in some cases dump sites.
Side-scan sonar, however, can only identify shapes, and then only on a gross scale; it cannot directly identify mineral deposits or most UXO. Grab-sampling and vibracoring (where a plastic tube is vibrated into the sea-floor to recover a vertical section of its sediments) can sample the sea-floor in great detail, but both techniques provide only extremely limited "point" data, are labor-intensive, and are very expensive.
It became apparent that we needed something that could continuously sample the sea-floor, map large tracts of the sub-bottom sediments down to at least 6 meter (20 foot) depths (the typical depth limit of a vibracore), and directly detect different minerals and metals in very small fractions.
The induced polarization method (for obvious reasons often called "IP") works by injecting current into the ground through an array of electrodes, then detecting a secondary voltage signal using additional special non-polarizing electrodes after the inducing voltage is temporarily switched off. The IP method has been used on land for nearly a century to search for disseminated (very low-grade) sulfide ore-deposits. It works by causing ions in the groundwater to migrate (adsorb) under a high inducing voltage onto individual mineral or metallic grain surfaces. When the inducing electrical field is removed or temporarily turned off, the finite "bleed-off" time for these charges to move back to the ground water (or in this case seawater) can be measured as a phase-shift (or time-lag between the inducing transmitted voltage and the measured receiver voltage). This finite bleed-off, or time-shift between the induced and the measured voltages, is the IP effect. If we see an IP effect in our field measurements, it tells us that there are sulfides or certain other minerals such as ilmenite (or sometimes clays - a similar effect from a different electro-chemical mechanism) beneath our array between the high-voltage transmitter and the receiver (detector) dipoles.
It has been demonstrated on land that the IP method can detect pyrite (iron sulfide, often called "fools gold") in minute quantities, sometimes as low as 0.1% - 0.2% by volume. In the early 1970's, a variant of this IP method called "Complex Resistivity" or "spectral IP" was developed. Instead of measuring just one or two physical parameters (e.g., resistivity and phase shift) at a single frequency, this approach measures BOTH resistivity and phase-shift (also sometimes called the "chargeability") over a wide range of frequencies (typically 0.1 to 100 Hz). With this wide spectral range, we can plot the behavior of the IP effect (the Complex Resistivity response) in the complex plane, or on a graph of magnitude versus phase, and see distinct "signatures" for different mineral assemblages. Until the mid 1980's, however, the IP method had never been tried at sea because of the high conductivity of seawater (leading to current-channeling or short-circuiting between transmitter electrodes) and the tremendous engineering difficulties in dealing with abrasion, saltwater infiltration, electrode corrosion, and supressing streaming noise and cross-talk/coupling in the received signal.
Laboratory and field experiments by the USGS in the early 1980's (Wynn and others, 1990) showed that the IP method was extremely sensitive to certain titanium- and thorium-bearing industrial minerals (e.g., ilmenite and monazite). In experiments with these minerals in various concentrations, measured IP responses were unusually strong, up to five times stronger than pyrite. Industrial minerals like these are commonly found in crystalline rocks, but can be economically mined only from ancient beach deposits where they have been deposited and concentrated by long-shore currents after weathering and river transport. They are concentrated in these beach deposits in many areas of the world, including Australia, India, Sri Lanka, South Africa, and the southern and eastern United States.
In the last 9,000 years, seawater levels worldwide have risen as the last glaciers retreated from the northern hemisphere. Relatively recent, titanium-rich ("black sand") beach deposits are now known to exist as far as 50 kilometers (30 miles) offshore Georgia in the southeastern US. Similar deposits offshore of Sierra Leone are known to also host rich platinum resources in anastamosing paleochannels buried beneath meters of more recent mud. This is because platinum group elements and ilmenite (also gold, zircon, and diamonds) are commonly eroded from their source rocks and frequently transported by similar riverine systems to the sea, where they tend to accumulate together in placer heavy-mineral deposits.
The earlier land-based heavy-placer IP studies invited experiments with the IP method in the offshore environment. This led to the construction of several evolving prototype marine IP streamers. Initially, this was a 13-conductor, multi-coax cable with IP transmitter and receiver electrodes attached along with pre-amplifiers installed on the bottom end (see Figure 3, a schematic diagram of the IP streamer). This streamer was towed behind a ship off the Georgia coast and gave very promising results (Wynn, 1988; see an example of these data in Figure 4, below). On land, electrodes must be dug or pounded into the ground to make IP measurements; at sea one simply uses stainless steel or titanium wire wrapped around the streamer, and seawater conductivity makes the return path for the transmitted current. This has the serendipitous effect of suddenly making IP surveying highly mobile-- we can now make more IP measurements in a month than have been made by geophysicists on land -- worldwide-- for the past 50 years! It's never quite as easy as it sounds, of course, and issues with streaming potential, electrode corrosion, coupling noise, transmitter cross-talk, saltwater penetration of down-chain preamps, noise suppression, abrasion, limited penetration due to current channeling, etc., had to be worked out.
Figure 4 shows one of eight strong anomalies detected while towing the prototype IP streamer for 22 kilometers over the seafloor on the Atlantic Continental Shelf of Georgia. Note the correlation of the IP "hit" with the paleostream shown in the bathymetry. A vibracore taken at this approximate location showed 10% heavy minerals and up to 3% - 4% ilmenite (Bob Woolsey, personal communication). About a third of the offshore paleochannels we could see in the bathymetry correlated with an IP "hit" in this survey. These prototype experiments demonstrated that marine IP is a practical approach for mapping placer heavy minerals in the shallow offshore environment. Experiments with the Complex Resistivity (spectral IP) technique (where the IP effect is measured as both magnitude and phase-shifts over a 3-decade frequency range) gave ambiguous results. This was because spectral IP measurements can only be made using a stationary (not moving) streamer. Due to limitations with the navigation equipment available at the time (LORAN) we were unable to precisely reoccupy locations of some "hot" (mineral-loaded) vibracores, or even to reoccupy with sufficient precision some of our towed-mode IP "hits". Examples of ilmenite-bearing deposits mapped nearby onshore are usually highly localized, typically being 20 meters wide (often stacked en echelon) by 500 meters long.
Following the initial successes with the prototype streamer, a USGS project was established to further extend and experiment with innovative applications using this promising new approach. The Offshore Industrial Minerals project is a multi-disciplinary, multi-regional effort intended to involve a wide range of partners both inside and outside of the USGS in mineral and hazards research in the shallow offshore environment. A key goal of this effort is to develop exclusive licenses to the patented USGS-developed technology.
Project goals include further development of a geophysical and geologic toolkit for identifying and quantifying economic heavy-mineral placer concentrations, UXO, waste dumping sites, androgenic modifications (including mine waste and sewage) in rivers, estuaries, sounds, and the shallow offshore continental shelf of the United States and its territories. Preliminary survey results suggest that the marine IP system can also map sub-bottom facies changes, and also can detect magnetic and non-magnetic metallic debris, cables, pipelines, and wrecks buried beneath thick layers of inert modern sediment on the seafloor (Wynn and Laurent, 1998; Wynn and others 2001).
The primary tool for this project, as outlined above, is a USGS-developed marine induced polarization technology. We developed this new electrical streamer system originally to directly detect placer heavy minerals, certain clays, and disseminated metals directly on the sea-floor from a moving vessel. Multiple different depths are mapped simultaneously by using an array with different transmitter-receiver dipole separations. As we carry out the marine IP survey, we acquire precise GPS (Global Positioning System) location information and integrate the two together, compensating for the lag of the active streamer behind the GPS sensor. Subsequent planned evolutions of the IP streamer system will also incorporate bathymetry and magnetometer channels. Additional tools that may be used during the project lifetime include:
A. Set up an unconsolidated-sediments laboratory to carry out resistivity, IP, Spectral IP, seismic velocity, and magnetic susceptibility studies on grab- samples and vibracore splits. This laboratory effort has given us a phase-shift-vs-ilmenite-content calibration curve, and it has also shown that androgenic sediments (Clostridium-laced effluent from three centuries of outflow of the Connecticut River into Long Island Sound) can be identified and uniquely characterized using Complex Resistivity (spectral IP).
B. Fabricated a second prototype streamer and conducted operational sea-trials offshore of Florida. These trials verified the towed cable behavior and depth of penetration. Earlier computer modeling had shown that electrical current penetration into the sediment is ineffective if the streamer rides more than 1 meter above the sediments, and previous sea-trials suggested that the cable "flies" too far above the sediments at 6 knots but rides on the sediments (and collects useful data) at 2 knots. Diving observations at the Egmont Shoals offshore western Florida showed that a streamer with a torpedo-weight tracks in the sediment up to about 3.5 knots; at higher speeds it is no longer in contact with the sediments.
C. Conducted a series of sea-trials offshore of Mississippi, the Carolinas, and Georgia to expand the applications of the system to other types of resources, as well as man-made hazards (including dumped industrial waste and UXO) and wrecks and wreck debris on the shallow coastal sea-floor. A preliminary survey offshore of Mississippi shows that the system can rapidly and efficiently map ilmenite concentrations, and can also detect buried metallic debris (these new data and more can be seen in Wynn and Laurent, 1998). Work off of Cape Fear (Wynn and Laurent, 1999; Wynn and others 2001) reinforced the UXO-and-wreck-finding claim.
D. Two patents for the technology were awarded in 2001, and a third patent is pending.
E. Began the commercialization of this new, USGS-developed technology in the Bismarck Sea (2005) and off of the east coast of South Africa (2007).
Wynn, Jeffrey C., 1988, Titanium geophysics - the application of induced polarization to sea- floor mineral exploration: Geophysics, 53, pp. 386-401.
Wynn, J. C., Grosz, A. E., and Foscz, V. M., 1990, Induced polarization and magnetic response of titanium-bearing placer deposits in the southeastern United States: in: Society of Exploration Geophysicists Special Volume on Induced Polarization, pp. 280-303. (also: U.S. Geological Survey Open-File Report 85-756, 41 p., 16 figures).
Wynn, J.C., and Laurent, Kevin, 1998, A high-resolution electrical geophysical approach to mapping marine sediments in the Atlantic coastal shelf and the Gulf of Mexico: Extended Abstract (7 Figs.), Society of Exploration Geophysicists Annual International Meeting, New Orleans, Sept 12-18 1998.
Wynn, J.C., and Laurent, Kevin, 1999, Mapping buried metallic wrecks and titaniferous placers in the Mississippi Sound, Gulf of Mexico : Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems, the Annual International Meeting of the Environmental and Engineering Geophysical Society, Oakland, CA, 14-18 March 1999, p. 803-810.
Wynn, Jeff, and McGinnis, Tim, 2001, Two different electrical properties can improve trans-oceanic cable-route mapping: proceedings of the 11th International Offshore and Polar Engineering Conference (ISOPE-2001), Stavanger, Norway, June 2001, pp. 37-41, 4 figures.
Wynn, Jeff, McGinnis, Tim, and Williamson, Mike, 2001, A new marine geophysical method for mapping placer sands, buried metallic objects, and androgenic sediments beneath the seafloor: Proceedings of the Oceanology International Americas annual meeting, Miami, Florida, April 2-6, 2001, 17 pages, 5 figures.
Zonge, K.L.; Wynn, J.C.; and Urquhart, S.A., 2005, Resistivity, induced polarization, and complex resistivity: chapter 9 in the Society of Exploration Geophysicists Special Volume on Near Surface Geophysics, pp. 265 - 300.
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