GROUND PENETRATING RADAR TECHNIQUE TO LOCATE UNDERGROUND COAL MINING VOID FEATURES AT BASTROP, TEXAS

The objective of this research is to comprehend the efficacy of the ground penetrating radar (GPR) geophysical technique to locate the possible underground source of subsidence features found near the town of Phelan, two miles north of Bastrop in central Texas. The area was the site of lignite coal mining that flourished briefly in the early twentieth century. The history of specific mining operations that took place at Bastrop in the early 1900’s has been difficult to ascertain. Hence, the use of a geophysical technique, like GPR, to identify hidden voids and potential subsidence features could reduce the safety hazard and property risk facing residents of this area. The field site is a residential area situated in the floodplain of Colorado River. Surficial erosion is distinctly visible in the top soil horizon, in association with small gullies and sand accumulations. There exists a large, subsidence feature on the property, possibly a sinkhole related to a collapsed underground mine room. GPR data, employing a bistatic 25 MHz system, were collected along several profiles and, in one region near the sinkhole opening, in a 3D grid pattern. The penetration depth of the radar signal extended to approximately 20 m. Likely underground mine void spaces are represented in the GPR sections as parallel, concave—upwards hyperbolae cresting at the top and bottom of the mine workings. The hyperbolae often are overlain by a hazy radar signature possibly indicative of a collapse feature. Post-processing of the GPR data provides a 3D cube representation of the likely voids. Resistivity profiling was also conducted at the site, mainly to support the findings of GPR. The possible void spaces are associated with high resistivity anomalies confirming the presence of underground mine voids at the site. The GPR data, with validation from the resistivity measurements, are helpful to understand the utility of 25 MHz GPR in locating underground void spaces to depths in excess of several meters. We will discuss how the GPR data can be utilized to better quantify the safety hazard and the property risk.