Offshore Industrial Mineral Studies Using a Marine Induced-Polarization Streamer System

Introduction

Figure 1. Exclusive Economic Zone (EEZ) of the United States and its affiliated islands is shown in yellow. The EEZ is larger than the land area of the United States. From McGregor and Lockwood (1985).

Studies suggest that the EEZ contains substantial mineral resources, including industrial minerals like ilmenite (a source of titanium, Ti) and monazite (a source of thorium and rare earth elements), as well as gold and platinum, which are sometimes associated with them. Ilmenite distribution off the East Coast of the United States is shown in figure 2.

Figure 2. Ilmenite (FeTiO3) detected in very limited sampling from ships off the East Coast of the United States; the colors indicate Ti contents in Vibracore samples. From Andrew Grosz (unpub. data, 1997).

Vast amounts of urban waste have been dumped offshore (for instance, offshore Miami, in the New York Bight, Long Island Sound, and Boston Harbor). These sea-floor waste deposits are significant health and even navigation hazards, and they are being mobilized by ocean currents. The hypodermic needles that washed ashore on the New Jersey coast several years ago came from the New York municipal waste barges that have operated in the New York Bight over the past several decades.

In addition to urban waste dumps, there are numerous places where unexploded ordnance (UXO) has been left over from World War II and peacetime military exercises. This UXO is generally buried beneath a shallow layer of sediments and is invisible. It poses a threat to divers, marine life, and fishermen.

Several approaches have been taken to characterize and map the mineral resources and hazards of the EEZ. Sidescan-sonar and high-frequency seismic-reflection ("chirp") methods can be used to identify shapes and features on the sea floor. Scientists have learned to correlate data from these methods with ancient beach deposits and, in places, with modern dump sites.

Figure 3. The R.V. John Wesley Powell off the Virginia coast; note the Vibracore unit on the fantail. Photograph by Andrew Grosz.

Sidescan-sonar images, however, can only identify shapes, and then only on a very gross scale; they cannot, for instance, directly identify mineral deposits or UXO. Grab-sampling and Vibracoring (where a plastic tube is vibrated into the sea floor to recover a vertical section of sediments; see figure 3) can sample the sea floor, but both techniques provide only point data, are labor intensive, and are very expensive. It became apparent that we needed something that could map large tracts of the bottom sediments down to at least 20 feet (6 meters) below the sea floor (the typical depth limit of a Vibracore) and directly detect different minerals and metals in very small concentrations. Experiments indicate that induced polarization surveys of the nearshore sea floor are a promising approach.

Induced Polarization (IP)

The IP method can detect pyrite (iron sulfide often called "fool's gold") in minute quantities, sometimes as low as 0.1-0.2 percent. 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 (for example, 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. With this wide spectral range, we can plot the behavior of the IP effect (the complex resistivity) on a graph of magnitude vs. 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 and engineering difficulties in controlling noise in the signal.

Experiments with IP: Ilmenite and Monazite

Since about 9,000-10,000 years ago, seawater levels worldwide have risen as the glaciers retreated northward in the northern hemisphere. Relatively recent, titanium-rich beach deposits are known to exist as far as 30 miles (50 km) off the coast of Georgia. Similar deposits off the coast of Sierra Leone are known to also host rich platinum resources. This is because platinum-group elements and ilmenite (and also gold) are frequently found in similar rocks, are eroded at the same time, and tend to be transported in a similar way.

Experiments with IP as a New Marine Tool

Figure 4. A schematic diagram of the marine induced-polarization (IP) streamer being towed. Note that there are two different receiver electrode pairs (1-3 and 2-4). Pair 1-3 is designed for shallow (up to 6 feet or 2 meters) detection, and pair 2-4 is for deeper (up to 20 feet or 6 meters) depths into the underlying sediments.

Figure 5. An example of marine induced-polarization (IP) data acquired with a prototype streamer off the Georgia coast. Note the IP peak over paleochannels in the sea floor observed in the bathymetry (in meters). A Vibracore collected nearby contained as much as 10 percent heavy minerals, including 4 percent ilmenite (Wynn, 1988).

These prototype experiments demonstrated that marine IP is a practical approach for mapping heavy minerals (including ilmenite) in the shallow offshore environment. Experiments with the spectral IP technique (where the IP effect is measured as both magnitude and phase shifts over a frequency range of 0.1-100 hertz) gave ambiguous results. This was because spectral IP measurements can be made only by using a stationary (not moving) streamer. Because of limitations with the navigation equipment available at the time (LORAN), we were unable to precisely reoccupy locations of some 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 65 feet (20 meters) wide by 1,600 feet (500 meters) long.

Offshore Industrial Minerals Project

Project goals include development of a geophysical and geologic toolkit for identifying and quantifying industrial mineral concentrations, UXO waste dumping materials, construction aggregate and beach-reclamation resources, and beach-reclamation sediments on the shallow offshore continental shelves of the United States. As this handout was assembled, no actual marine IP experiments had yet been conducted offshore in the search for UXO or urban waste, though IP had been proven effective for these targets on land.

The primary tool for this project, as outlined above, is a USGS-developed marine induced-polarization technology. We initially developed this new electrical streamer system to directly detect placer heavy minerals, certain clays, and disseminated metals directly on the sea floor from a moving vessel. We realize that it also can provide porosity information about layered sediments. During our surveys, we also acquire precise GPS (Global Positioning System) location information and integrate the two. Subsequent evolutions of the IP streamer system will also incorporate bathymetry. Additional tools that may be used include the following:

• Petrographic microscopes and an electron microprobe. These will be used in conjunction with laboratory electrical measurements to precisely characterize the cause of the unusually strong IP effect in some placer minerals.
• Magnetic separators and heavy-media separation chemistry.
• Specialized geophysical contouring and plotting software.
• Neural network technology to do real-time analysis of field data on shipboard.

Other Participants

We also anticipate future cooperation with other interested agencies and organizations. We propose to expand applications of the marine streamer system to resource data gathering in near-offshore shelf areas of both the east and west coasts of the continental United States and to Alaska and Puerto Rico, targeting not only heavy-mineral resources, but also waste-dumping sites and potentially even UXO.

Principal Activities During Fiscal Years 1997-1998

• Bring the former analog system to operational status as a digital system (this task is now completed).
• Begin resistivity, IP, spectral IP, seismic velocity, and magnetic susceptibility studies on grab samples and Vibracore splits in a newly established USGS unconsolidated-sediments laboratory.
• Conduct sea trials off the coast of Florida (completed in June 1997) to verify towed-cable behavior. Computer modeling has shown that electrical current penetration is ineffective if the streamer rides more than 3 feet (1 meter) above the sediments. The sea trials showed that the streamer tracked in the sediments and collected useful data at towing speeds up to 3 knots.
• Begin a series of sea trials to expand the applications of the system to other types of mineral resources, as well as manmade hazards (including dumped waste and UXO) on the shallow coastal sea floor.

References Cited

McGregor, B.A., and Lockwood, Millington [1985], Mapping and research in the Exclusive Economic Zone: Reston, Va., U.S. Geological Survey, 40 p.

Wynn, J.C., 1988, Titanium geophysics -- The application of induced polarization to sea-floor mineral exploration: Geophysics, v. 53, p. 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: Society of Exploration Geophysicists Special Volume on Induced Polarization, p. 280-303.