Geophysical Services

EGS provides Geophysical Testing Services using state of the art testing equipment and highly experienced staff.  EGS’s staff has extensive experience implementing field surveys and interpreting data collected using Electrical Resistivity Imaging (ERI), Ground Penetrating Radar (GPR), and Multi-Channel Analysis of Surface Waves (MASW).

Typical Uses and Applications of Geophysical Testing

Geophysical testing methods are based on geophysical principles including dynamic theories and electrical theories as they relate to the earth and soils.  Typical applications of geophysical testing include void detection, sink-hole investigation, rock surface mapping, roadway pavement thickness surveys, location of buried debris, utility location, mapping of steel reinforcement in concrete slabs, location of unmarked graves, and geotechnical site classification.  Geophysical test methods can be used to create 2-dimensional and 3-dimensional models of the subsurface conditions. 
  

Ground penetrating Radar (GPR)

GPR is performed by inducing high frequency electromagnetic waves from the ground surface and used to generate 2-dimensional and 3-dimensional models of subsurface conditions such as pavement thickness, buried debris, voids, potential karst conditions, rock surface mapping, and utility location.  GPR can effectively be implemented in a variety of site conditions ranging from heavily wooded areas to open roadways.  One key advantage that GPR has over any other geophysical test method is the speed at which it can be implemented in the field.  Data collection occurs continuously and is displayed on a “real-time” display.  This allows for preliminary field interpretation of results which is beneficial for time sensitive projects.

Typical GPR Off-Road Configuration

Typical GPR Roadway Configuration

Typical 2-Dimensional GPR Asphalt Thickness Contour Map

 

Electrical Resistivity Imaging (ERI)

ERI generally consists of installing a straight line of equally spaced electrodes along the ground surface over the limits of the exploration area.  The number of electrodes installed and spacing for each ERI test depends largely on the existing subsurface conditions and the desired depth and resolution of the survey.  Often, preliminary soil borings will be installed prior to performing the test to help optimize the configuration of the test.  Once the electrodes have been installed, a direct current (DC) is applied at alternating outer electrodes along the electrode line.  While the current is being applied to the outer electrodes, voltage measurements are being recorded from inner electrodes along the line and are converted to an apparent resistivity value.  The result of a typical ERI survey is a continuous 2-dimensional subsurface resistivity profile. 

Typical ERI Transect Line Configurations

The electrical resistivity measurements displayed in the profile are generally color coded based on the ranges of soil resistivity values such that variations in groundwater, soil layers, and rock layers can be delineated with use of soil boring data.  Correlation between soil boring data and electrical resistivity measurements can be accomplished by directly overlaying the soil boring logs onto the ERI profile.  Analysis of the results and the ERI profiles can easily be understood by non-scientists, which is one of the greatest advantages ERI has when compared to other geophysical test methods.  Another benefit of this test method is that it can be conducted on the ground surface, which gives the user the ability to investigate large areas to depict generalizations of the soil profiles between soil boring locations.

 

Typical 2-Dimensional ERI Subsurface Profile

In situations that call for the highest level of precision, use of 3-dimensional ERI modeling can provide an even higher degree of accuracy than the traditional 2-dimensional model. One of the biggest advantages of 3-dimensional modeling is that it allows the professional to isolate different areas of specific resistivity values to identify the area of concern in 3-dimensional space.  Based on the 3-dimensional model, volumetric quantities of the measured anomalies can even be made.  Development of a 3-dimensional ERI model is accomplished by installing a rectangular grid of equally spaced electrodes on the ground surface over the exploration area.  Similar to the 2-dimensional ERI survey, the number of electrodes installed and spacing for each ERI test will depend largely on the existing subsurface conditions and desired depth and resolution of the survey.  Once the electrodes have been installed, a direct current (DC) is applied at alternating outer electrodes throughout the electrode grid.  While the current is being applied to the outer electrodes, voltage measurements are being recorded from inner electrodes along the line and are converted to an apparent resistivity value.  The result of a typical ERI survey is a continuous 3-dimensional subsurface resistivity profile.  Another method used to create a 3-dimensional ERI model is to merge together two or more closely spaced parallel 2-dimensional profiles. 


Typical 3-Dimensional ERI Subsurface Profile

 

ERI surveys can even be performed below water.  For underwater applications, specialized waterproof electrodes are weighted to the bottom and placed by divers.  In some instances, electrodes may be towed on the surface of the water to cover large areas in a shorter period of time.  Both 2-dimensional and 3-dimensional ERI modeling have a number of typical applications relating to subsurface investigations including cavity and sinkhole detection, geotechnical site characterization, groundwater exploration, well drawdown studies, archaeological site investigation, detection of contaminant plumes, delineation of organic material, and detection of buried debris or utilities.  Detection of high profile buried utilities can be accomplished with ERI when traditional location techniques do not work. 

The use of ERI can greatly enhance many of the traditional procedures for performing subsurface investigations.  The traditional approach to subsurface investigations typically starts by first defining the nature of the subsurface investigation (i.e. is the purpose of the investigation for the analysis of a structure foundation, stormwater pond, etc.)  Once the nature of the project has been clearly defined, a search will usually be performed for any pre-existing geological data of the site.  Based on the review of the pre-existing data and layout of the proposed improvements, the number and placement of the preliminary soil borings are determined.  The field crews then mobilize to the site and install the preliminary soil borings.  After a review of the subsurface data obtained from the preliminary soil borings, the ERI study is designed to focus on the area(s) in question.  The ERI study is then conducted using the preliminary subsurface data to calibrate the ERI model.  Using both the preliminary subsurface information and ERI results, additional soil borings may be needed to verify the presence of any anomalies identified in the ERI model.  The ERI study when coupled with traditional soil borings can produce better more focused evaluation of the subsurface conditions resulting in lower costs to the client and better confidence that the problem was adequately identified and evaluated.    

 

 

 

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Cascade Park
Selena Drive
Yellow River