COMPLIANCE INFO - Laserfiche WebLink

Home Browse Search

4.0 INVESTIGATIVE METHODS 4.1 Visual Survey Geocon personnel conducted a visual survey of the Site prior to the geophysical survey and the advancement of soil borings. The visual survey was used to help identify areas of apparent petroleum hydrocarbon soil staining, indications of buried utilities, the possible location of the product lines associated with the AST, and possible obstructions to the advancement of borings. 4.2 Geophysical Survey Caltrans specified the geophysical methods used for this investigation. The methods chosen by Caltrans were the GPR and EM methods. GPR and EM are standard methods for the detection of buried objects and anomalies in the subsurface. Spectrum Geophysical performed the GPR and EM portions of the geophysical site investigation as a subcontractor to Geocon. 4.2.1 GPR Method The geophysical site investigation included a GPR survey as specified by Caltrans. GPR is a standard method for the detection of buried objects and anomalies in the subsurface. The GPR survey was undertaken using a Geophysical Survey Systems, Inc. (GSSI) Model l0A sled type 500-MHz antenna with an internal receiver with built-in hard drive and video display. This instrument has a color display that enabled convenient acquisition, storage, and preliminary analysis of data in the field. For each area of interest, GPR data were collected along parallel traverses spaced approximately 1-m(3.28 feet) apart. The instrument was calibrated to record over a 50-nanosecond (ns) vertical scale and dragged along each traverse. Records of each GPR profile were recorded for post survey processing, and reviewed in the field for indications of USTs or other suspect buried features. The GPR emits radio waves, which are focused and directed into the subsurface under the GPR antenna. Dense buried objects such as those made of metal reflect the radio waves. GPR can also be used to detect a former excavation or area of disturbed soil due to the density contrast between the backfill and the surrounding undisturbed soil. The higher the frequency of the waves emitted, the greater the imaging resolution. However, depth of investigation decreases with an increase in frequency. The 500-megahertz antenna and 50-ns vertical scale were used for this survey as they provided a good compromise between resolution and depth of investigation for this project. 4.2.2 Electromagnetic Method The EM survey was performed using an EM-61 transmitter. The EM-61 generates short pulses of EM energy which travel downward and outward and have a primary field associated with them. The energy associates with conductive materials and causes a secondary magnetic field. The receiver measures the voltage of the decay curve of the secondary magnetic field which is proportional to the conductivity of Project No 58475-06-70 -5- July 9,2003