Micro-resistivity and GPR surveys for locating thermal lines in limestone bedrock cuts

Site Background

An industrial client asked us to locate thermal lines in the backyard of his main building where he wanted to construct an additional facility on 12 piers. He wanted to make sure that the piers did not break the thermal lines during the construction. Thermal lines come out under the main building and were installed into bedrock cuts (weathered limestone), under the few feet of soil, and into the bedrock as deep as 200 feet. Locations of thermal lines under the building and in the backyard were not known, and thus we were contracted to locate them.

We really did not know in de- tail what the thermal lines were all about until we arrived at the site. We accepted the scope of the work as locating utility lines and brought with us 6 geophysical instruments (conductivity - EM31, EM61, magnetometer, ground penetrating radar, and resistivity) . After talking to the client, we learned the following: thermal lines, which are plastic pipes a quarter of an inch in diameter, are part of a geothermal heat pump which is a central heating or cooling system that pumps heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer). This design takes advantage of the moderate temperatures in the ground to boost efficiency and reduce the operational costs of heating and cooling systems.

 Geophysical Methods

After getting more information about the thermal lines, and inspecting the site, we chose to perform only GPR and resistivity surveys. Resistivity imaging is a survey technique that aims to map of the electrical properties of the subsurface by passing an electrical current between electrodes and measuring the associated voltages. This technique has been widely used in mapping contaminant plumes, karst features (voids), and subsurface structures, such as faults and fractures. In this study, the Advanced Geosciences, Inc. (AGI) Super R1 Sting/Swift resistivity meter with the dipole-dipole resistivity tech- nique is used. This technique is more sensitive to horizontal changes in the subsurface, and provides a 2-D electrical image of the near-surface geology. Electrode spacing was held to 2 feet along all profiles and a roll-along survey method was employed. The depth of the investigation was about 13 feet.

The 400 MHz antenna (shallow mode) was used with a cart system to collect GPR data. GPR is the general term applied to techniques that employ radio waves in the 1 to 1000 megaHertz (MHz) frequency range to map near-surface structures and man-made features. Depth penetration of the radio waves is limited by the antenna chosen and the conductivity of the soil. The ability of a GPR system to work successfully depends upon two electrical properties of the subsurface, electrical conductivity and relative dielectric permittivity (i .e., dielectric constant) . The value of dielectric constant ranges between 1 (for air) and 81 (for water). The dielectric constant for sandy clayey soils varies between 10 and 15. A dielectric constant of 12 was chosen for the study area, and the depth exploration with the GPR unit was about 6 feet. Thus, differences in the dielectric constant of subsurface soils along distinct boundaries, such as voids (bedrock cuts), can cause highly significant reflections in the radar signal, which are recorded and displayed by the system.  In summary, GPR radar reflections occur when GPR waves encounter a change in velocity due to dielectric contrast. The bigger the change the more signal is reflected (Geophysical Survey System, Inc. GPR SIR-2000 Manual, 2000).

 Geophysical Survey Design and Results

First we conducted pilot GPR surveys along the concrete walk ways and across the new construction area. The GPR data collected along the concrete walk way (line G1) showed four distinct and similar anomalies 2 feet long along the profile, as indicated by letters A, B, C and D (Figure 2) . These anomalies are about 20 feet apart from each other. We further investigated these anomalies with a roll-along resistivity survey. We took the electrode spacing 2 feet because of the length of the GPR anomalies were about 2 feet (Figure 2). The resistivity profile (line R1) located 50 feet away from the GPR profile also indicated very unique low resistivity anomalies (blue in color). These anomalies are also shown with letters A, B, C and D, and correlate very well with the GPR anomalies. The shape of the low resistivity anomalies suggests that these locations were disturbed and excavated. It should be noted that GPR surveys along the resistivity profile did not show any anomalies due to the absence of dielectric constant between the undisturbed and the disturbed soil.

We saw the results of our work while we were at the site (Figure 3): An excavator dug a trench along the resistivity profile and located the bedrock cuts of A and B. We later learned that the rest of the anomalies were also found, and the construction was completed without any problem. And we felt like another mission was completed to the satisfaction of our client and ourselves!

P.S. This work was published in FastTIMES  v. 15, no. 1, March 2010