Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Reexamination Certificate
1998-06-22
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
06236212
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application discloses subject matter in common with U.S. application Ser. No. 09/099,149, which is entitled INDUCED POLARIZATION METHOD FOR IDENTIFYING MINERALS ON THE OCEAN FLOOR, and is filed concurrently herewith, and which is hereby incorporated by reference.
FIELD OF THE INVENTION
The present invention relates to an induced polarization system for detection or identification of metals, metallic minerals and the like in sediment deposits on and beneath the sea floor.
BACKGROUND OF THE INVENTION
As was mentioned above and is discussed in more detail below, the invention concerns an induced polarization (IP) detector. 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 surface. 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 induced polarization detectors for 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.), and U.S. Pat. No. 3,984,759 (St. Amant, et al.). These patents are not concerned with detectors adapted for use in a subsea environment.
It will be appreciated that the use of IP detection in a subsea environment presents tremendous engineering obstacles, including current channeling in the seawater and noise problems because of the low signal-to-noise ratio of the signals involved. Because of these and other obstacles, to the knowledge of applicant, IP detection systems have not used previously in the detection of minerals and the like on the sea floor.
SUMMARY OF THE INVENTION
In accordance with the invention, an induced polarization detection system is provided which enables the detection of minerals and metal-containing materials on or below the sea floor and which does this without the need for implanting electrodes in the sea floor sediments being examined. Because IP detection is sensitive to surface area, rather than volume, the system of the invention enables efficient detection of very small amounts of material well below the sea floor, including, inter alia, titanium minerals and gold and platinum group elements where associated together, metallic particles of manmade origin, and certain other materials and items not detectable by existing magnetic or seismic detectors. As will be apparent, the system of the invention incorporates a number of important features which overcome the serious problems and obstacles encountered in any attempt to use IP in a subsea environment.
According to the invention, a system is provided for use in making induced polarization measurements under the sea, the system comprising: a streamer cable having a proximal end adapted to be towed from a ship in the sea, and a distal end positionable, in use, during towing of the cable, near or on the sea floor, said streamer cable including an active portion located at the distal end and comprising at least one transmitter for injecting a periodic electrical current into the sea floor, at least one receiver for receiving secondary signals produced by induced polarization of sources on or in the sea floor in response to the periodic electrical current, and at least one pre-amplifier connected to said at least one receiver for conditioning the secondary signals so as to combat noise and cross-talk effects in the secondary signals to thereby produce conditioned secondary signals; and on-board means, adapted to be disposed on-board the ship that tows the streamer cable, for supplying electrical current to said at least one transmitter and for receiving and processing said conditioned secondary signals from said at least one preamplifier.
Advantageously, the pre-amplifier comprises a common-mode-rejection three-electrode pre-amplifier. The on-board means preferably includes a signal processor and the pre-amplifier is advantageously connected by a grounded, shielded, co-axial line to the signal processor. Preferably, the system includes a floating ground independent of the signal processor and also of the ship that tows the streamer cable, and the co-axial line is connected to the floating ground. The at least one pre-amplifier is advantageously disposed in close proximity to the at least one receiver.
The at least one transmitter preferably comprises a titanium wire electrode connected by a transmitter cable contained within said streamer cable to the on-board means. As discussed below, the use of a titanium electrode provides important advantages, particularly with respect to combating the corrosive effects of sea water. In an advantageous implementation, first and second titanium wire electrodes are employed.
Preferably, the at least one receiver comprises a silver-silver chloride electrode. The at least one receiver preferably comprises a plurality of receiver electrodes disposed in equally spaced relation along the streamer cable so as to provide, in use, sampling at different depths. The plurality of receiver electrodes advantageously comprises first and second dipole pairs of receiver electrodes. In a beneficial implementation, four receiver electrodes are connected together in groups of three wherein each group includes a shared electrode and a central electrode in forming the first and second dipole pairs.
Preferably, the on-board means comprises a current transmitter circuit connected to said at least one transmitter, and a data processing unit or signal processor connected to said at least one pre-amplifier for processing the conditioned signals, the processing unit being connected to the current transmitter circuit through an optical isolation amplifier to control the output of the current transmitter circuit. The current transmitter circuit preferably transmits a square-wave signal to said at least one transmitter.
In an application wherein the system is particularly adapted for use in detecting ilmenite in a towed mode (i.e., a mode wherein the streamer cable is towed from the ship), the square-wave signal is transmitted at a frequency of 4 Hz since it has been discovered that ilmenite produces its strongest IP response at this frequency.
In an application wherein the system is adapted for use in a stationary “spectral” mode wherein the ship is maintained at selected position, the current transmitter generates square-wave signals at different frequencies to enable
Patidar Jay
The United States of America as represented by the Secretary of
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