Using Ground Penetrating Radar to Investigate Karst at the Anna Limestone Quarry
Harvey Henson, Jr.

Since the mid-1970's ground penetrating radar (GPR) has been utilized to solve an assortment of shallow subsurface problems in a variety of disciplines, such as engineering, geotechnical, archaeology, forensics, and many others. Recently many papers and case studies have been published

Fig. 1 Air-photo of Anna Quarry

which detail GPR data acquisition and processing techniques and methods for successful high-resolution mapping of subsurface features buried within soil and bedrock stratigraphy.

This paper describes a simple high-resolution ground penetrating radar survey conducted within a southern Illinois limestone quarry to map karst features which interrupted quarry operations. The limestone quarried at the study area belongs to the Ste. Genevieve Limestone and St. Louis Limestone formations, which are Upper Mississippian (Chesterian) in age. Locally these units consist of interbedded chert and fine-grained limestone (wackestone and packstone) layers. The cherty layers are typically clustered in 4-5m thick bands. Normal quarry operations are capable of removing a 10m bench of limestone. The limestone bench actively quarried before initiation of this study was located 31m below the surface, as shown (Fig. 1) in the air-photo of the Hartline Pit at the Anna Stone Quarry located along Highway 51 So. near Anna, Illinois.

Clay-filled karst was encountered during normal operations of this limestone quarry. Attempts to determine karst extent with a drill rig became too dangerous.

Fig. 2 GPR field set-up.

Consequently, a ground penetrating radar (GPR) system (Fig. 2) was used to map karst extent and to determine any possible relationship to fractures observable along quarry walls. Georadar profiles were located adjacent to major fractures to intersect exposed portions of the karst feature.

The diagram shown in figure three illustrates the georadar reflection data acquisition method used in this study. Reflector velocity information was gathered using the common midpoint (CMP) method at selected locations along the radar profiles. Velocity information is useful for data processing and interpretation. Radar profiles were collected using 50 and 100 MHz antennae (white objects labeled T1,R1...). The 100 MHz setup resulted in the highest resolution data,

Fig. 3 GPR reflection mode.

and as expected the 50 MHz antenna provided slightly deeper penetration into the limestone.

Karst features filled with clay and air were imaged remarkably well, as shown on the interpreted georadar data profile below (Fig. 4). Stratigraphy associated with the karst includes 4m of massive limestone overlain by 5m of interbedded chert and limestone. This vertical transition in stratigraphy is observable on the processed georadar data, as are several fractures which intersect the main cavity (shown below). Fractures were conduits for local groundwater, which resulted in carbonate dissolution (the formation of caverns in the rock), and for sediment which later filled the void. Because this study was highly successful, karst extent was determined quickly and efficiently using simple ground penetrating radar methods and quarry operators were provided with information necessary to continue production of limestone materials.

Fig. 4 GPR reflection profile A1.