C – 44 Reservoir Project

A large, approximate 4,500 acre reservoir was planned for construction in Martin County, Florida.  In order to delineate the continuity and composition of shallow soil layers at the proposed reservoir site, SEI was retained to provide geophysical services to provide these subsoil evaluations.  Due to the large size of the site, the geophysical survey was only performed around the perimeter of the site, at the location of the proposed earthen dam surrounding the reservoir.  After preliminarily evaluating the anticipated soils present, the Capacitively Coupled Resistivity (CCR) was selected as the most cost-efficient and most viable method of geophysical exploration. 

CCR is a geophysical method of obtaining a virtual cross-section of subsurface soil and rock layers.  This method consists of two separate steps: 1) measuring the apparent (weighted average) electrical resistivity along a continuous survey line using a dipole-dipole array, and, 2) computerized processing of apparent resistivity data to obtain a virtual cross-section of the estimated true resistivity values.  The data is collected by towing a dipole transmitter and a series of dipole receivers spaced at successively greater distances from the transmitter along the cable array.  The cable array is towed behind an all-terrain vehicle (ATV) to allow the rapid collection of resistivity data.  A Trimble® Pro-XR differential GPS unit was used to locate the survey transects in UTM coordinates.

After the completion of the field survey, the CCR field data is processed using MagMap 2000 computer program, supplied by Geometrics.  The data is then modeled using two-dimensional inverse resistivity methods.  This modeling method consists of estimating the true resistivity of the ground at points arranged in a grid on a vertical plane based on the apparent resistivity values measured in the field.  The calculated true resisitity values are used to calculate the apparent resistivity values, which are then compared to the actual measured apparent resistivity values.  Adjustments are made in the program and the modeling progresses toward better estimates of the true resistivity by iteration using the least-squares method.  The calculated true resistivity values are exported into ArcView GIS software and contoured to produce a two-dimensional virtual cross-section beneath the survey line referenced UTM coordinates.

The results of the CCR survey at the site generally revealed relatively low resistivity soils with resistivity values less than 100 Ohm-meters.  These values were consistent with clays to clay/sand mixtures.  The CCR profiles also identified isolated locations with resistivity values greater than 100 Ohm-meters.  The higher resistive soils may represent discontinuities in the clayey/sand layer.