With the advent of crosshole seismic technology in the 1980s, a new generation of high resolution geophysical tools has become available for reservoir characterization. The chief improvement is simply that the tools are deployed in boreholes so measurements take place much closer to the region of interest.
The apparent magnetic noise, obtained from the coherency function for two parallel magnetic sensors, generally overestimates sensor noise because the sensors do not measure the same signal. The different signals result from the nonparallel alignment of the sensors and from the additional magnetic signal induced in each sensor by its motion in the Earth's magnetic field. A magnetometer array experiment was completed in Grass Valley, Nevada, to determine the minimum magnetic signal that could be detected in the presence of background natural field variations and motion of the sensor. Superconducting quantum interference device (SQUID) magnetometers with internal biaxial tiltmeters were used to record the magnetic fields and the motion of the sensors. A least squares fitting program enabled the field at one site in the array to be predicted from a remote site and to produce a residual field with a standard deviation of 8 pT over a 1‐hour period with a low‐pass filter at 0.3 Hz. Consistent field‐to‐residual ratios of 40–60 dB were achieved, with some ratios exceeding 70 dB. The least squares fit uses only a linear combination of the magnetic and tilt fields at a remote site to predict the observed magnetic field. This procedure allows for correction of calibration and orientation errors as well as the removal of the apparent fields originating from sensor movement. Misalignment and motion of the sensor are shown to be the major sources of magnetic field noise. The orientation error is typically of the same magnitude as the noise induced by sensor motion. In order to achieve ratios better than 20–40 dB one must include both the orthogonal fields and the tiltmeter outputs. Inclusion of a frequency‐dependent transfer function should increase the prediction ability of the least squares model, as evidenced by the improvement to 70 dB obtained with simple band limiting of the original data. These techniques should be applicable to any type of artificial source survey where natural field fluctuations are the noise‐limiting factor. The ability to describe the observed signals should allow a dramatic increase in one's ability to detect an artifically generated signal, allowing signal‐to‐noise improvements of 40–60 dB without increasing transmitter power or the averaging time.
A field experiment was conducted at the University of California Richmond Field Station to demonstrate the sensitivity of borehole‐to‐surface resistivity measurements in groundwater investigations. A quantity of saline water was injected into a fresh water aquifer while the resistivity was monitored using a multichannel borehole‐to‐surface system. Two experiments were conducted using pole‐pole and pole‐dipole receiver electrode arrays. The data from the pole‐pole experiment were superimposed to simulate a dipole‐pole array and the data from the pole‐dipole array were superimposed to simulate a dipole‐dipole array. This superposition of the data was done to enhance the anomaly and facilitate interpretation. A numerical modeling study was performed in conjunction with the field experiment in order to interprete the results. A three‐dimensional modeling program was used to simulate the geological setting of the field experiment and the salt water injection. This modeling revealed that an asymmetric displacement of the salt water slug results in asymmetric current channeling which is observable as a 25 to 40 percent difference between preinjection and postinjection borehole‐to‐surface resistivity. In addition to demonstrating the sensitivity of subsurface arrays, this experiment demonstrated that the measurement of bulk resistivity can identify a groundwater flow pattern not detected by hydrological measurements.
A dipolar self‐potential anomaly of about 90 mV peak‐to‐trough amplitude and 5km peak‐to‐trough length has been measured over the East Mesa geothermal field in the Imperial Valley of Southern California. The anomaly does not appear to be related to surface features. A surface field similar in form to the measured self‐potential anomaly is generated by a source configuration consisting of dipolar current distributions along a series of three steeply dipping planes that are roughly coincident in location and depth extent with known faults. The source currents could be generated by the interaction of heat or fluid flow with changes in thermoelectric or electrokinetic coupling coefficients across faults. However, source potentials associated with these currents are considerably greater than those generated by estimated in situ coupling coefficients and heat and fluid flows. This implies that in situ coupling coefficients are much larger than those derived from room temperature measurements; that heat or fluid flows are much greater than those assumed for the East Mesa field; or that some other mechanism is responsible for the generation of the anomaly.
DEMONSTRATION REPORT: DISCRIMINATION A MULTISENSOR SYSTEM FOR THE DETECTION AND CHARACTERIZATION OF UXO MM-0437 SITE LOCATION: U.S. ARMY YUMA PROVING GROUND – OPEN FIELD AREA DEMONSTRATOR: LAWRENCE BERKELEY NATIONAL LABORATORY ONE CYCLOTRON ROAD, MS: 90R1116 BERKELEY, CA 94720 p.o.c. Erika Gasperikova, egasperikova@lbl.gov, 510-486-4930 TECHNOLOGY TYPE/PLATFORM: BUD/CART DECEMBER 2007
The FY06 Defense Appropriation contains funding for the 'Development of Advanced, Sophisticated, Discrimination Technologies for UXO Cleanup' in the Environmental Security Technology Certification Program. In 2003, the Defense Science Board observed: 'The problem is that instruments that can detect the buried UXOs also detect numerous scrap metal objects and other artifacts, which leads to an enormous amount of expensive digging. Typically 100 holes may be dug before a real UXO is unearthed! The Task Force assessment is that much of this wasteful digging can be eliminated by the use of more advanced technology instruments that exploit modern digital processing and advanced multi-mode sensors to achieve an improved level of discrimination of scrap from UXOs.' Significant progress has been made in discrimination technology. To date, testing of these approaches has been primarily limited to test sites with only limited application at live sites. Acceptance of discrimination technologies requires demonstration of system capabilities at real UXO sites under real world conditions. Any attempt to declare detected anomalies to be harmless and requiring no further investigation require demonstration to regulators of not only individual technologies, but of an entire decision making process. This discrimination study was be the first phase in what is expected to be a continuing effort that will span several years.
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