Saturday, 17 June 2017


Experiments with IP (induced polarization) as a New Marine Tool

The earlier land-based effort invited experiments with the IP method in the offshore environment. The USGS constructed a prototype marine IP streamer (that is, a 13-conductor cable with IP transmitter and receiver electrodes along with pre-amplifiers installed on the end; see figure 4). This was towed behind a ship off the Georgia coast and gave very promising results (Wynn, 1988; see figure 5). 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 use of seawater instead of planted electrodes has the serendipitous effect of suddenly making IP work highly mobile--we can now theoretically make more IP measurements in a week than have been made by geophysicists on land--worldwide--for the past 50 years!

Diagram of marine IP streamer being towed
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.

Marine IP data
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.

There is little doubt that finding the remains of Waratah is going to be a difficult and challenging task. What remains is probably largely silted over. In addition to using magnetometry and side-scan ultrasound, there might be scope for a modality such as this to detect large concentrations of copper and lead. This method has been used offshore, South Africa 2007, to detect titanium-sand deposits.

Fascinating developments in the search for wrecks:

Sub-bottom sonar profiling:

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