Geophysical Environmental Surveys

EOL SURVEYS

EOL SURVEYS PROVIDE 1D RESISTIVITY LOGS,
2D MAPS & CROSS-SECTIONS, AND 3D IMAGES OF
FREE-PHASE HYDROCARBON PLUMES
FROM GRADE TO THE EOL RECEIVER WELL DEPTH

The Electromagnetic Offset Log (EOL) method is based on Newmont Mining's EMP (EM Pulse) induction method for surface-source to in-hole-receiver (STBH) measurements. EOL is a generic term coined by Gehm Environmental, Boonville, MO, in 1994. The EOL differs from most STBH techniques in that it operates in the near-field frequency domain (quasi-static domain) where the wave length used is several times the maximum depth of investigation. The EOL provides resolution that is an order of magnitude better than the resolution of surface-source to surface-receiver techniques.

The objective of EM geophysical measurements is to define resistivity (or conductivity) variations in the subsurface. Most of today's EM surveys for site-characterizations are designed and limited to mapping conductors at shallow depth. The EOL surveys, on the other hand, are designed to map and image subsurface resistivity anomalies caused by hydrocarbon plumes. to depths up to several hundred feet.

EOL surveys are based on sampled EM data. Signals received by a borehole receiver at different depths in a well provide a measure of time-varying EM current sources in the model space. The model space is from the surface to the depth of the receiver well. EM sources are found at boundaries of resistivity change. The primary EM source is a loop source laid flat on the surface. Image loop sources, known as eddy currents, are generated at boundaries of resistivity change in the subsurface. The eddy currents are known as secondary sources.

The EOL survey source is a flat-lying, large coil, surface source which uses a low frequency signal. The low frequency signal induces a long wave-length magnetic field below the source coil. The source coil is optimally tuned, has an area of four square meters, has 25 continuous turns of low resistance wire, and operates at a current up to 10 amperes. The effective EM moment of source is more than 1,200 ampere x meter x meter.

The magnetic-field signals are measured by an in-hole receiver which is finely tuned to the single-frequency source used. The frequency used in environmental EOL surveys is a low frequency chosen at one of the minimum-amplitude points of the noise spectrum commonly found at culturally and industrially noisy sites. The signals from the receiver are passed through a high-Q inverted-notch filter specified for the particular source-coil frequency. This step 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 of the source signal.

After a complete set of voltage measurements at 0.1-foot intervals along the receiver borehole is recorded, the source coil is moved to another surface location and new measurements are taken. This logging process is repeated at linear grid points until a matrix of voltage data is acquired to adequately model the subsurface objectives.

The final field processing steps are to digitally sample the log voltage and to plot field records for data quality review and field evaluation by an on-site technician. Once the digital data for and EOL is accepted, the data are stored for further processing.

The data is passed to a processing center to perform proprietary processing steps to:

Automatically edit and remove extreme noise and all unusable EOLs.
Automatically perform amplitude-statics corrections to eliminate variations in the logs caused by changes in source strength; in and around metal features, such as buried tanks, pipelines, etc.
Adjust data sets from multiple receiver wells to create a single-well data set . The adjustments are predicted by overlapping EOL data recorded from two or more receiver wells.
Automatically provide signal-to-noise enhancement (smoothing) using the 0.1 foot sampled EOLs to generate 1.0 foot sampled EOLs.
Generate apparent resistivity logs and separate them into 1st Order resistivity logs (gross character logs) and Residual resistivity logs (refined character resistivity logs).
Design two-dimensional and three-dimensional model weights.
Perform 3D surface-integral modeling.
Provide maps, images, and cross-sections of the free-phase TPH plumes. The processing provides volumetric estimates of the plumes.

The EOL survey and its predecessors, the 3DR Survey and the MagnaScan survey, were developed by and have been applied by Dr. James Pritchard on environmental surveys since 1988. Pritchard has performed, modeled, and reported on the EOL-type surveys for more than 75 projects, mostly in Texas, California, Missouri, and Hawaii.

EOL SURVEY "TPH TO RESISTIVITY" CONCEPT

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