Evgeniy V. Torgashov

Ground Penetrating Radar: Utility/Rebar/Cable Detection and Concrete/Subgrade Debonding

Ground penetrating radar (GPR) profiles acquired during seven separate field investigations are presented. The intent is simply to illustrate that GPR can be used to detect rebar, pre-tensioned tendons, deteriorated and/or detached concrete/fill, buried utilities, underground storage tanks, and buried foundations. The objective of each investigation is summarized, accompanied by brief descriptions of the nature of the target of interest. The application of GPR to the investigations described in this paper is not new. However, the presentation and illustration of multiple applications of this imaging technology in a single manuscript is somewhat unique.

A Description Of An Effective Sinkhole Investigation Approach: A Case Study Of A Site In Greene County, Missouri

A sinkhole had developed and led to the subsidence of a portion of the parking lot of a residential building in Nixa, Missouri. The sinkhole was characterized using a combination of the geophysical data (multichannel analysis of surface waves and the electrical resistivity tomography) and supplemental data (aerial photographs, topographic and geological maps and borings). The interpretation of the data indicated that a fault trending northwest – southeast runs across the site and is underlain by shallow, fractured limestone bedrock of the Burlington Keokuk formation. The sinkhole is understood to have been triggered by the excavation of a surface drainage channel and pit adjacent to the building in order to check flooding. However, the action led to the increased concentration of flow of surface water into the fault zone, hence causing an increased dissolution of bedrock and raveling of sediments into the formed cavities and subsequently sinkhole development.

FAULT DETECTION USING ELECTRICAL RESISTIVITY TOMOGRAPHY

Electrical Resistivity Tomography (ERT) data were acquired on the ground surface across an underground limestone mine access tunnel in an effort to characterize the roof rock. This investigation was conducted because simultaneous localized failure occurred on the ceiling of the access tunnel and elsewhere in the mine along a previously unknown northwest trending lineament. The interpretation of the ERT data indicates the limestone roof rock above the tunnel is dissected by several prominent clay-bearing near-vertical solution-widened fractures (joints or faults) that are not visually exposed on the ceiling of the tunnel. Roof rock failure in the mine access tunnel occurred at the intersection of projected lineament and one of the more prominent interpreted solution-widened fractures.

Pseudo-3D Electrical Resistivity Investigation of an Area in Proximity to the Tumbling Creek Cave, Taney County, Missouri

Three pseudo-3D ERT data sets were acquired at the southeast area of the Tumbling Creek Cave, Tane y County, Missouri. The objective of the study was to better delineate subsurface geologic structures and the hydrologic processes in the discharge area of the creek. The top of bedrock was mapped and the orientations of prominent dissolution-widened joints were delineated in the study area. The ERT results showed the bedrock was dissected by prominent joints trending in the north, northeast and northwest directions in the area. The jointing patterns observe d are consistent with the flow patterns exhibited by the Tumbling creek and other creeks around the study area. It is a standard practice in geophysical investigations to constrain geophysical interpretations with ground truth. But due to the unavailability of boring control in the study area, the interpretation of the ERT data was based on results of previous ERT studies in southern Missouri.

Geophysics and Geologic Hazards

Sinkholes in Florida pose significant geotechnical, engineering, and hydrogeological challenges for using the land in constructive ways. In some instances, the sinkholes may prove unstable, thus limiting the overburden stress that can be applied. Additionally, the sinkholes may provide a conduit for accelerated contaminant transport from surface activities. In this case study, we use electrical resistivity tomography (ERT) to understand the scope of sinkhole activity under a planned landfill. As part of their application, the landfill permit applicant submitted a dense network of parallel, twodimensional electrical resistivity profiles as described in the following. We provided an alternative, three dimensional analysis of this data set to enhance detection of subsurface sinkhole targets. Eighty five parallel resistivity lines spaced 6m (20ft) apart were coalesced into a large three-dimensional resistivity model to map the 14 hectare (35 acre) site. The results revealed that resistive sand-filled sinkholes could extend at least 30m (100ft) below ground surface with a diameter that ranged from 30 to 100m (100-300ft). The host conductive limestone was shown to have a complex undulating topography with eroded pinnacles. Using cone penetrometer technology (CPT), the edge of the limestone pinnacles were also shown to have significant raveling, which coincided with a narrow range of resistivity values. The implications of the correlation between direct characterization using CPT and indirect characterization with ERT suggest that raveling could cover as much as 17% of the site. Based on these findings, the site was determined to be ill suited for landfill construction.

Resistivity/Induced Polarization/Self-Potential Methods and Applications

Modern multielectrode and multichannel resistivity systems have made it relatively easy and rapid to collect time domain induced polarization (IP) data in near surface surveys. This paper will examine a wide variety of applications through case studies in a variety of geological settings in Western Canada. Case studies will show various applications and complementary features of IP surveys including distinguishing salt water from conductive clays, identifying faults, locating deeply buried structures underneath active facilities, and distinguishing landfilled debris from leachate. IP data sets will be correlated with other data sets including resistivity, seismic reflection, and borehole geophysical parameters.