Recently, we were asked to conduct a ground conductivity survey (EM31) to determine the lateral limits of unconsolidated fill and debris, and a high resolution resistivity survey (Sting) to determine depth to the bottom of the undocumented materials at a redevelopment project in San Francisco. The site had previously been utilized as a dump site for concrete debris by a nearby concrete batch plant; therefore, the fills were resistive in nature.

Using GPS, the EM data could be collected in a very quick and systematic manner. Data were collected in two directions in less time then it would have taken to set up a grid and collect data in one direction. The main purpose of the EM survey was to delineate the lateral limits of the undocumented materials.

A Sting profile (indicated in red) was conducted across a portion of the site which contained relatively resistive materials delineated during the EM31 survey. The target fill was expected to be resistive concrete debris overlying an electrically conductive bay mud.

The Sting results are very graphic, and illustrate areas of high resistivity materials overlying a conductive material. These areas were interpreted to be buried fill and concrete debris overlying bay mud. Subsequent drilling of the site provided subsurface data consistent with the geophysical evaluation

Some benefits to consider:

• The survey work was conducted despite ongoing operations at the facility. 
• These surveys can be conducted in endangered habitat or other sensitive areas.
• No permitting is required for conducting these surveys, and therefore, can be scheduled ahead of drilling.
• When conducted in conjunction with exploratory borings or test pits, it can increase the overall extent of subsurface information as well as aid in boring and test pit location.
• Reconnaissance geophysical studies can reduce the number of exploratory excavations, and help to focus the intrusive testing.

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We have used Resistivity Surveys to: 

• Map faults or landslides
• Map fracture zones for water well sighting
• Map large karst voids and sea caves
• Find depth to groundwater, in certain specific situations
• Generally characterize subsurface geology.
• Aid environmental contamination flow projects when used to evaluate locations of paleo stream channels or fault zones. 
• Find buried bedrock valleys, which can be an important source of groundwater in unconfined aquifers. 
• Delineate vertical joints or cracks, which can indicate subsidence features.