You are visiting the web site of the EPSCI Program at the U.S. Dept. of Energy's Ames Laboratory, Ames, IA
On-site demonstration by contractor
EOL uses a surface source coil with a very low frequency signal to optimally induce time varying magnetic flux subsurface of the survey area. The source coil is an optimally tuned coil with an area of four square meters, a current of ten amperes, and 12 turns of low resistivity wire all of which create an effective EM moment of 500 ampere-meters squared. Voltage data (which are a measure of the magnetic flux) are remotely sensed by a EM receiver (in-hole probe orsonde).
EOL, as are other geophysical surveys and subsurface geologic analyses, are based on sampled data. The signals received at different depths in the well are the result of the superposition of time-varying magnetic flux from the surface coil in features of differing resistivity, and from the magnetic flux of the source coil itself. The magnetic flux from the "eddy currents" is known as secondary flux and the magnetic flux from the source coil is known as the primary flux.
The magnetic flux signals are measured by a receiver, finely tuned to a specific source- signal frequency being used. This frequency is relatively low and is at one of the minimum-amplitude spectral points of noise spectrum commonly found at culturally and industrially noisy sites. The signals from the probe are passed through a High-Q, inverted-notch filter specific to the source-coil frequency. This enhances signal over noise. The filtered signal is then passed to an integrator which performs additional signal-to-noise enhancement by summing and averaging the signal over many tens of cycles.
After a complete set of vertical offset log data representing the formations below the coil is recorded, the source coil is moved to another surface location and the process is repeated. A 3-D matrix of resistivity data is generated from this process until the survey objectives are accomplished.
The last steps of the field processing involve digital sampling of the integrator voltage output, field plotting of records of the output for quality control, then field evaluation of existing anomalies. The digital data accepted as having proper quality is stored on floppy disc for further processing.
The EOL survey conducted at the Marshalltown site consisted of approximately 100 offset logs obtained from a 20 foot square grid pattern. Data acquisition was under the control and direction of Dr. James Pritchard under contract by Entech Engineering, Inc. Project management was performed by The GEHM Corporation. The survey was conducted April 28 to May 1, 1994. The survey identified a high resistive anomaly which appears to come almost to the surface near the center of the site.
Contact: GEHM Environmental Corp., Boonville MO, (660) 882-3485
Please e-mail comments to: epsciwebkeeper@ameslab.gov.
Last Modified: 1 January 2002 by dave eckels
Expedited Site Characterization: etd/technologies/projects/esc/technologies/emloggingdemo.html