FIELD DOCUMENTS - Laserfiche WebLink

Home Browse Search

NORCAL ' URS Greiner January 25, 1999 ' Page 3 ' VMG Contour Map Interpretation ' Generally, the vertical magnetic gradient is very small except in the vicinity of ferrous metal objects. Areas where the VMG is large (positive or negative) are represented by closely spaced contours. These areas are defined as anomalies if they are not associated with above ' ground objects. If the source of a particular anomaly is an isolated object or a group of closely spaced objects, the contours may form circular or elliptical closures. A large accumulation of buried objects may appear as a group of closely spaced anomalies or one large anomaly. Actual anomaly magnitude and shape are dependent on the relative position and size of the buried objects with respect to the location of the measurement points. In general, anomaly magnitude will decrease and anomaly width will increase as the distance (depth) to the source ' increases. Anomalies may or may not have paired high and low values creating what are known as magnetic dipoles. UST's typically give rise to VMG anomalies with amplitudes ranging from several hundred to several thousand nanoTeslas per meter (nT/m). Small UST's, such as 500 gallon waste oil tanks, typically are manifested by anomalies that range from 400 to 1,000 nT/m. The lateral extent of UST anomalies are usually somewhat larger than the UST itself, depending on its depth of burial. In addition, the contours often indicate positive values above the UST, and negative values just to the north of it. ' GPR and EMLL Analysis We examined the GPR records for hyperbolic reflection patterns characteristic of UST's and ' underground utilities. We also reviewed the records for changes in reflection character that could indicate the presence of fill material associated with an excavation. The EMLL instrumentation indicates the presence of buried metal by emitting an audible tone. There are no recorded data to analyze. The locations of buried objects detected with the EMLL method were marked on the ground surface with white marking paint. ' RESULTS ' The results of the geophysical investigation are presented on the Site Map and Geophysical Survey Map, Plates 1 and 2, respectively. The Site Map shows the limits of each survey area, the structures or above ground cultural features that may be in close proximity to the site, and ' the locations of the GPR traverses. The Geophysical Survey Map shows the VMG contour map and GPR anomalous zones. The contour map represents the variations in the vertical magnetic gradient throughout the site. Magnetic variations that could not be attributed to ' above ground cultural effects are considered anomalous, as described above. Also shown on