ARCHIVED REPORTS_XR0004249 - Laserfiche WebLink

Home Browse Search

i • ,• AA NORCAL Pacific Environmental Group, Inc July 10, 1996 Page 5 (Plate 2) shows the variations of the vertical magnetic gradient tfiroughout the site These variations include several high magnitude variations around the perimeter of the site, as well as in the center of the survey area We interpret the magnetic gradients along the southeast border as representing effects caused by above ground storage drums, light poles, and metal bumper poles in the survey area The circular gradients located in the northwest and southwest corners represent effects from metal poles severed at the ground surface Four areas with anomalously steep gradients that could not be associated with effects from above ground cultural features are located in the center of the survey area and are considered anomalous, as described above These anomalies are labeled A through E, as shown on Plate 2 Anomaly A is characterized as a high amplitude bi-polar anomaly with a peak to trough value of 450 nanoteslas per meter (nT/m) This anomaly represents effects from the known UST mentioned above We obtained GPR data over this anomaly to identify the surface trace of the UST This data resolved reflection patterns within • the upper two to three feet that are characteristic of disturbed subsurface materials However, no hyperbolic patterns characteristic of a UST were evident on the records Therefore, the UST is buried deeper than the detection limas of the GPR The EMLL investigation over this anomaly resolved buried metal in an area of approximately 4 by 6 ft These dimensions approximately correspond to a 550 gallon UST The EMLL method also resolved the surface trace of an undifferentiated utility trending from the UST vault lid to the concrete pad associated with a former building This utility is shown on Plate 1 Anomalies B and E are characterized as high amplitude mono-polar anomalies with values of approximately 400 nT/m Anomalies C and D are characterized as bi-polar with values of 250 nT/m Since these values are close to those resolved over Anomaly A (known UST), it is possible that they may represent additional small UST's Furthermore, It has been our experience that anomalies of these magnitudes and areal extents may also be due to metal sources such as buried debris and/or utility vaults The GPR data obtained over these anomalies resolved localized hyperbolic reflection patterns that are typical of small isolated objects and utility lines The GPR data do not indicate large hyperbolic signatures within the upper two to three feet that could represent an UST Therefore, the source of these anomalies may be very localized or buried deeper than the detection limits of the GPR The results of the EMLL investigation resolved the surface trace of several Undifferentiated utility alignments in this area, as shown on Plate 1 The surface trace of these . utilities, as well as the possible extent of the known UST, were marked on the ground surface with spray paint