APPLICATIONS FOR CAPACITIVELY COUPLED RESISTIVITY SUVREYS IN FLORIDA

 

K. Michael Garman and Scott F. Purcell, Subsurface Evaluations, Inc., Tampa, Florida

 

Abstract

 

The use of capacitively coupled resistivity (CCR) as a geophysical method has historically been of limited use in Florida due to the shallow water table and the time necessary to make multiple passes to collect resistivity data at depth. The induced current used by CCR instruments is stronger and can penetrate to greater depth if the surface materials are resistive, because the voltage measured at the receiver equals the current in the transmitter multiplied by the resistivity of the earth materials. The presence of shallow groundwater increases conductivity thereby reducing the CCR signal strength. The availability of a multi-channel CCR instrument, the Ohm-Mapper by Geometrics, Inc. has eliminated the need for multiple passes for a study. Subsurface Evaluations, Inc. has investigated a promising application for multi-channel CCR surveys in Florida: the identification of shallow deleterious soil conditions, such as clay and peat lenses, that might be missed by a standard drilling program and that are not readily detectable by ground-penetrating radar (GPR).

Introduction

 

In Florida, geophysical surveys for geotechnical engineering applications are usually performed by ground penetrating radar (GPR). Such surveys are typically limited to a maximum depth of about 4.5 to 9 meters (15 to 30 feet) due to attenuation of the signal by the shallow water table common in most of Florida. Despite the limited depth of penetration, GPR is widely used because:

  • The profiles generally show enough subsurface structure to identify buried depressions and other possible karst erosion features;
  • GPR surveys can be performed quickly (1.6 to 4.8 kilometers per hour or 1 to 3 miles per hour) and economically; and
  • The GPR profiles can be read and analyzed in real time by an experienced operator without processing.

 

In addition to the limited depth of penetration, GPR surveys do not provide information on the densities or compositions of the subsurface materials. Conventional multi-electrode resistivity (MER) surveys generally have depths of penetration of 30 to 61 meters (100 to 200 feet) and provide information on subsurface material composition based upon the calculated true resistivity values; but the data must be processed to produce a geologic cross section for interpretation. The data collection using MER is much slower at 30 to 152 meters per hour (100 to 500 feet per hour) and more costly compared to GPR.

 

Historically capacitively coupled resistivity (CCR) has been of limited use in Florida due to the shallow water table and the time necessary to make multiple passes to collect resistivity data at depth. However, the availability of a multi-channel CCR instrument, the Ohm-Mapper manufactured by Geometrics, Inc., represents an effective compromise between the relative high speed and low cost of GPR with information on material composition provided by resistivity surveys. The multi-channel CCR survey can be performed at a speed of 1.6 to 4.8 kilometers per hour (1 to 3 miles per hour) comparable to GPR. Although the data must be processed to create a geologic cross section, the cross section provides information on the materials present based upon resistivity. Like GPR, the presence of shallow ground water and conductive materials limits the depth of signal penetration.

 

Subsurface Evaluations, Inc. (SEI) recently tested multi-channel CCR for the identification of shallow deleterious soil conditions such as clay and peat lenses along proposed road projects.

 

 

Data Acquisition

 

In the field, an AC current is coupled into the earth by a transmitter and measured by receivers that are positioned one behind the other in a towed array (Figure 1). Up to five receivers may be used in the array to collect data from five different dipole spacings simultaneously. A four receiver array is shown on Figure 2. Geometrics, Inc., of San Jose, California, manufactures the Ohm-Mapper unit, which was used in this study.

 

The induced current used by the Ohm-Mapper is stronger and can penetrate to greater depth if the surface materials are resistive as the voltage (V), which is measured at the receiver, equals the current in the transmitter (I) multiplied by the resistivity of the earth materials (R) between the transmitter and receiver, V = IR. Therefore, for a given transmitter current (I), if the resistivity (R) is low, the voltage (V) will be low. If the voltage is too low, it cannot be distinguished from background noise and no data will be collected.

 

The Ohm-Mapper collects data using a dipole-dipole array, which provides high lateral resolution. The distance between the dipoles and the lengths of the diploes are adjusted to collect apparent resistivity readings from different depths. The longer the dipole and spacing configuration, the greater the depth of the survey as the depth from which data is collected is equal to about 15 to 20 percent of the total dipole and spacing length. After the apparent electrical resistivity data are collected, it must be processed to obtain a virtual cross-section of estimated true resistivity values.

 

 

 

Figure 1.: Schematic of Ohm-Mapper System (supplied by Geometrics, Inc.)

 

 

Figure 2.: Survey along a roadway using a four-channel Ohm-Mapper.

 

 

The CCR method was evaluated as a method to identify and map shallow peat and clay deposits and small zones of raveled soils that might adversely affect road projects. This project was ideally suited to the capabilities of the CCR method as it involved material identification based on resistivity and the depth of interest was between the ground surface and a depth of about 6 meters (20 feet). In this application, the CCR was able to identify conductive materials consistent with clay deposits and abrupt changes in resistivities consistent with possible peat deposits and karst features.

 

As part of the evaluation of the Ohm-Mapper, the length of the dipoles and the non-conductive tow link between the transmitter and receivers was varied to optimize the data that could be collected in a single pass. The configurations tested were: 5 meter dipoles with 5 meter link; 5 meter dipoles with 10 meter link; 10 meter dipoles with 5 meter link; and 10 meter dipoles with 10 meter link. In general, the 10 meter dipoles with 5 meter link was considered the best as the strength of signal was greater with the 10 meter dipoles and shallow information was still available with the 5 meter link.

 

Results

 

The results of the CCR survey along roads with settlement problems demonstrated that the method worked very well. One example from eastern Hillsborough County, Florida, shows discontinuities in the sand layers across a wetland. The discontinuities within the sand layer are locations at which test borings are recommended.

Figure 3.: CCR profile along road with settlement problems in a wetland area.

 

 

At another location in eastern Hillsborough County, Florida, the CCR profile showed the presence of a small low resistivity area within the surficial sand Figure 4. This low resistivity area was investigated by a piezocone penetration test boring. The boring identified raveled soil that was missed by the initial test boring program.

Figure 4.: CCR profile showing raveled, low resistivity zone in surficial sand.

 

At the University of South Florida northeast of downtown Tampa, sinkholes occur at relatively high frequency of 3.3 per square mile per year (from new sinkhole database maintained by Subsurface Evaluations, Inc.). Although the typical sinkhole is relatively small with a diameter of about 5 feet, they can damage structures and roads if they form beneath a structure. Therefore, GPR surveys are commonly used to evaluate sites for buried depression or evidence of sinkhole precursors prior to new construction. The Ohm-Mapper was tested at the University and proved as effective as GPR for identifying possible karst erosion features (Figure 5).

 

Figure 5: Comparison of GPR and CCR profiles over same location.

 

 

Summary and Conclusions

 

In low lying and wetland areas, the CCR method has proven to be very effective for identifying shallow clays and peat deposits. In this respect, CCR was far superior to GPR. The CCR is ideal for road projects where the depth of geotechnical investigation is limited to about 6 meters (20 feet). The CCR also proved effective for identifying potential raveled soil pipes and other possible karst features, which show up on GPR but may be missed by a drilling program that does not use geophysics. Although the CCR profiles must be processed before they can be interpreted, the CCR profiles are easier to interpret. The two surveys proceed at the same speed; but the long straight lines required for CCR, due to the array length, make it less useful on small sites.