Ground penetrating radar, a tool for determining complex geological structures from cavern boreholes

GPR surveys provide important spatial data on the geological structures within salt deposits for the development of salt mines and the construction of caverns. GPR measurements can be carried out in shafts, drifts, and boreholes during exploration. Because these GPR measurements are carried out underground in full space, it is important to record and interpret the data as a function of direction. The authors have further developed the GPR instruments and the methods for interpreting directional data. The transmitter emits pulsed electromagnetic energy into the rock. The receiver consists of a direction finding antenna, which allows the radial direction angle of reflections to be calculated for any point in the signal. The orientation of the logging tool with respect to north in a vertical borehole is determined with a compass. The principle of the spatial interpretation will be shown at an example of GPR borehole measurements. Then the results of a GPR survey in a salt deposit with a complex geological structure will be discussed. The survey was carried out in an oil filled single borehole of a cavern under production. The lower part of the survey was inside the cavern. The spatial modeling of the reflecting structures gives important information of the complex geological structures in salt. Three-dimensional interpretation of the results allows geologists and miners to better target their activities more safely. An other GPR survey in a different cavern gives an impression of possible ray traces from transmitter to receiver inside the oil filled cavern. In some cases it is possible to detect the shape of the cavern, in others not. The submitted energy into the rock and the amount of the reflected information depends on the conductivity, permittivity, frequency and geometry.