Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Reexamination Certificate
1998-06-18
2001-05-22
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
Using electrode arrays, circuits, structure, or supports
C324S357000
Reexamination Certificate
active
06236211
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for detection or identification of metals, metallic minerals and the like on and beneath the ocean floor, using a towed streamer and employing induced polarization.
BACKGROUND OF THE INVENTION
A number of different methods have been used in the detection of metals and metallic minerals in sediments on the sea floor. The methods include seismic, magnetic and resistivity profiling techniques. Although these prior art methods are well developed, for a further discussion of some of these methods, reference is made, for example, to U.S. Pat. No. H1490 (Thompson, et al), U.S. Pat. No. 5,430,389 (Wynn, et al.), U.S. Pat. No. 4,617,518 (Srnka), U.S. Pat. No. 3,562,633 (Swain), U.S. Pat. No. 3,182,250 (Mayes), U.S. Pat. No. 3,052,836 (Postma), U.S. Pat. No. 2,872,638 (Jones), U.S. Pat. No. 2,839,721 (Dewitte) and U.S. Pat. No. 2,531,088 (Thompson). Briefly considering some of these patents, the Wynn, et al. patent discloses a sensor for detecting magnetic fields and current dipoles of materials and objects buried in sediment using a towed coil, while the Srnka patent discloses a towed system including an electric dipole current source which emits an alternating current and a plurality electric dipole detectors which measure potential differences between the detectors. The remaining patents generally relate to ocean floor logging methods using towed detectors which measure and log either changing potential differences or resistivity along the path of the towed detectors.
As discussed below, the method of the invention employs induced polarization (IP) in the identification process. The IP effect is a current-induced electrical response detected as a delayed voltage in certain minerals and, as described below, the method has been used for some time in the detection of these minerals in the ground. One manifestation of the response is that the voltage on an array of detectors or receivers lags the primary or inducing voltage (produced by a transmitter) by a finite amount of time. This is usually expressed as a phase-shift, i.e., a slight shift of the wave-cycle between the transmitter and receiver, and is usually reported in units of milliradians, where one duty cycle of the transmitter is 2&pgr; radians.
For many decades it has been known that pyrite, most other metallic-luster minerals, and certain clays give rise to an IP effect. Geophysicists have taken advantage of this fact to discover and map large disseminated sulfide bodies (primarily copper and molybdenum) since the 1950's. The phenomenon is based on a complex double-layer interaction of ions in the electrolyte (the ground water) and the individual mineral surfaces. Because of this, IP is more sensitive to surface area than to volume and finely disseminated minerals make the best targets. An IP survey typically gathers both resistivity information, which is generally a measure of the porosity of the substrate, and polarization information, which is a measure of the reactivity of certain minerals (i.e., those described above) disseminated throughout the subsurface. Computer modeling can then be used to arrive at models that best fit the observed data acquired on the surface, with the purpose of providing a true map of the three-dimensional nature of the subsurface. The use of two physical characteristics (resistivity and polarization information) instead of just one makes the interpretation much more reliable.
There are, of course, a number of patents relating to the use of induced polarization in the detection of minerals and other materials and among these are the following: U.S. Pat. No. 5,671,136 (Willhoit), U.S. Pat. No. 4,467,642 (Givens), U.S. Pat. No. 4,041,372 (Miller, et al.), U.S. Pat. No. 3,984,759 (St. Amant, et al.).
SUMMARY OF THE INVENTION
In accordance with the invention, a method is provided which can be used to detect, on the sea floor and below the sea floor to depths of the order of about five meters, very small quantities (fewer than 1%) of certain economic titanium minerals as well as other metallic debris or materials including those containing fine-grained pyrite (e.g., waste sludge).
In accordance with the invention, a method is provided for detecting minerals and metal-containing materials which exhibit an induced polarization response, in and on the sediment of the sea floor, the method comprising: trailing a streamer cable in the sea behind a ship, the streamer cable including at least one transmitter and at least one receiver at the free end thereof and the streamer cable being trailed such that the free end is at least in close proximity to the sea floor; using the at least one transmitter to transmit an electrical current into the sediment of the sea floor; terminating the transmitting of said electrical current and using said at least one receiver to detect secondary signals produced by an induced polarization response of a source located on or in the sea floor in response to the termination of the electrical current; and processing the secondary signals to determine measurement parameters characteristic of the source of the secondary signals.
In one preferred embodiment of the method of the invention (referred to as the “towed” mode), the streamer cable is preferably towed in an expanding pattern to provide coverage of an area the sea floor. Advantageously, the streamer cable is towed back and forth in this mode along spaced paths in a grid or “lawn mower” pattern to provide the desired coverage.
Preferably, the method of the invention further comprises simultaneously acquiring positional data to provide an indication of the horizontal location of the source of a particular secondary signal. Acquiring of this positional data advantageously comprises acquiring data from the Global Positioning System.
In the towed mode, transmitting and interrupting of said electrical current is preferably carried out on a continuous basis during towing of the cable streamer over an area of interest and continuous detecting of secondary signals is carried out based thereon.
In a second preferred embodiment of the method of the invention, the streamer cable is positioned over a location on the sea floor which has been determined to contain an induced polarization source of interest, further induced polarization measurements are made at this position, using the at least one transmitter and the at least one receiver, over a range of frequencies, and the induced polarization measurements are used to generate a spectral signature of the source. Advantageously, the location in question is determined on the basis of secondary signals detected during towing of the streamer cable and the global positioning data acquired simultaneously with the detection of the secondary signals. Preferably, the signals produced by the induced polarization measurements are stacked and averaged, and thereafter signal amplitude and phase-shift information derived from the stack and averaged measurements is used to generate the spectral signature.
The transmitting and interrupting of the electrical current is preferably effected using a square wave voltage signal.
The secondary signals are advantageously amplified, using a pre-amplifier located at the free end of the streamer cable and providing common mode rejection, to provide an amplified secondary signal. The processing of the secondary signals preferably comprises processing steps wherein the amplified signal is Fourier transformed and then deconvolved against a calibration signal.
Although as was indicated above and is set forth in more detail below, the method of the invention can be used to detect and identify a number of different minerals and metal-containing materials on the sea floor, one aspect of the invention concerns the discovery that certain titanium containing minerals, and, most importantly, ilmenite, have a strong IP response. In an embodiment of the invention particularly useful in detecting ilmenite, a square wave transmitting signal is used during the towing mode which has a frequency of about 4 Hz.
O
Koltos E. Philip
Patidar Jay
The United States of America as represented by the United States
LandOfFree
Induced polarization method using towed cable carrying... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Induced polarization method using towed cable carrying..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Induced polarization method using towed cable carrying... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2482803