An Evaluation of Ground Penetrating Radar for Imaging the Near Surface Examples from Contaminant Map

Ground penetrating radar (GPR) is becoming established as a new geophysical technique for imaging the near-surface. In this presentation, we assess GPR in terms of its application in the petroleum industry, and evaluate its capabilities for mapping a contaminated site near downtown Calgary and for delineating the internal geometry of a modern coastal barrier sandspit. GPR is based on the measurement of the traveltime of a transient electromagnetic pulse which is reflected from boundaries in the subsurface across which there is a contrast in dielectric permittivity. Antennae frequencies from 25 to 200 MHz are available and propagation velocities in the earth range from about 0.05 to 0.2 m/ns. Vertical resolution is in the decimetre to metre range, and depth of penetration can be up to 30 m in electrically resistive environments. Most surveys are undertaken in a single-channel, fixed-offset mode, somewhat analogous to single-channel seismic recording. Applications of GPR have been primarily in geotechnical and environmental studies, including pipeline surveys and contaminant mapping projects. Recently, there has been increasing interest in GPR for shallow sequence stratigraphy and for addressing statics problems in reflection seismic data.

On the south bank of the Bow River in Calgary, at a location known as the Canada Creosote Site, dense non-aqueous phase liquid (DNAPL) accumulated in the ground during creosote treatment of railroad ties and power poles between 1924 and 1962. In 1989 these liquids were observed to be leaking into the river just west of the downtown core. Extensive refraction seismic and drilling programs were undertaken by Alberta Environment to define the morphology of the bedrock surface, as it was considered that DNAPL was accumulating at the base of the weathering layer. In a research study, we collected about 1.5 km of GPR data at the site. Good energy penetration was obtained through gravels beside the river, with reflections recorded from depths of up to 10 m. However, up on the flood plains, energy penetration was significantly less (1 to 3 m at best), probably due to conductive near-surface clays and fill materials. Near the river, good agreement was obtained between drillhole data, refraction seismic profiles and interpreted GPR sections. Of particular interest is an amplitude anomaly in the GPR data which coincides with an area of creosote contamination in bedrock fractures.

GPR is also being used for a detailed 3-D study of a barrier sandspit on the Washington coast. Knowledge of the internal geometry of the spit is useful as it is a modern analogue for reservoir sands in Alberta which were deposited in similar environments. Data were collected at 1 m intervals over a 50 x 50 m grid using 100 MHz antennae and show a shingle-like accretionary pattern of reflections which dips toward the ocean at about 1 degree. Some events can be mapped throughout the data volume, but others are discontinuous and are interpreted as individual lenticular-shaped sand packages.

Horizon maps and interpretations of sandbody geometries will be illustrated.