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Survey Over a UST Using Combined Magnetometer/EM61

by Ralph Soule

This memorandum compares the geophysical responses of three geophysical instruments from a 12,000 gallon Underground Storage Tank, 8 feet in diameter and 33 feet long, buried at a depth of 6.5 feet.

The three Instruments are:

  • GEOMETRICS 858G MAGMAPPER (MAG SURVEY)
  • GEONICS EM61 METAL DETECTOR (EM61 SURVEY)
  • AQUA-TRONICS A6 TRACER (EMA6 SURVEY)

As the following map and profiles show the UST had a Manhole access and Fill Pipe coming to the surface, a vent pipe and an underground power cable crossing the UST. Data were acquired along traverses having a 5 foot traverse interval and gridded and contoured. For the EMA6 SURVEY two orthogonal traverse orientations were used to study the "directional effects" of this instrument. Two orthogonal composite PROFILES were constructed along 15E and 30N to study geophysical responses.

As expected the UST produced large geophysical responses. The following observations and conclusions are made from the study:

  • The MAG SURVEY produces a strong anomaly but is shifted to the south of the UST as expected and is strongly influenced by the Manhole and Fill Pipe.
  • The EMA6 SURVEY detects the Manhole and Fill Pipe but does not detect the UST as a depth of 6.5 feet is about the detection limit for the A6 TRACER for USTS..
  • The EM61 TOTAL component is mainly detecting the Manhole and Fill Pipe. However, the EM61 DIFFERENTIAL produces an anomaly which best defines the location and shape of the UST. The EM61 DIFFERENTIAL data suppresses the effects from shallower objects thereby enhancing the anomalies from deeper objects.
  • These observations indicate that an EM61 DIFFERENTIAL SURVEY is the best method for locating deeper USTS.

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