This Technical Bulletin highlights how field collection techniques, multiple survey methods, integrated interpretation, and data processing can clarify subsurface features under difficult conditions.  This example is a typical mark-out at a former commercial facility to identify underground utilities, storage tanks, and other subsurface features.  The Site encompassed approximately 1.5-acres and historic data suggested the possible presence of abandoned UST’s on the property. 

            The initial stage of the project consisted of equipment selection considering the geographic area and site-specific conditions.  Every geographic area has unique geologic and soil characteristics that will affect geophysical instruments differently.  For instance clayey, highly saline or saturated soils, areas covered by steel reinforced concrete, foundry slag, or other highly conductive materials greatly reduce GPR signal and effectiveness (see Reliance on Ground Penetrating Radar Technical Bulletin).  Since these conditions were anticipated, the recommendation was made to the client to incorporate an Electromagnetic (EM) survey in addition to the typical utility locating tools.  The EM method uses the principle of electromagnetic induction to measure the variability of conductivity in subsurface materials.  The large EM response to metal makes this technique particularly well suited to identifying buried metal objects such as underground storage tanks, buried drums, pipelines, reinforced building foundations, or other metal components of buried structures.  An additional advantage of EM methods is that it can be processed to enhance the data quality in noisy or unfavorable conditions. (see Locating Objects Below Reinforced Concrete Technical Bulletin).

               The survey approach for this project consisted of running the EM survey, processing this data to identify potential anomalies, and following up with GPR to obtain depth information and verify EM anomalies.  GPR signal penetration was a maximum of approximately 4 feet below ground surface over the bulk of the Site, and reduced to approximately 2 feet or less below the concrete pads because of signal attenuation.  As is often the case, an important EM anomaly was located beneath a concrete slab, and the GPR data provided extremely poor information using the basic system operating settings. Delta geophysics uses the advanced GSSI SIR-3000 GPR unit, which is considered the industry leader in GPR technologies because of signal bandwidth and the flexibility of its data collection software.  As a result our field personnel had the ability to adjust the GPR collection parameters for these site specific conditions, and we were able to obtain a GPR signal from the buried object.  The recovered signal was a classic hyperbolic pattern typically associated with an underground tank. 

The differences in the raw data collected using default parameters is compared to the field adjusted and final processed displays below.