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.
About the Author(s)
Don Lawton graduated from the University of Auckland, New Zealand in 1979 with a Ph.D. in geophysics. He had previously worked as an exploration geologist with Amoco Minerals in New Zealand. Don has been employed by The University of Calgary since 1979 where he is currently an Associate Professor of Geophysics and is an Associate Director of the Consortium for Research in Elastic Wave Exploration Seismology (CREWES project).
Research interests include multicomponent and 3-D seismic techniques, physical and numerical seismic modelling, environment applications of geophysical methods and integrated geophysical and geological research studies in the Rocky Mountain Foothills. He is a past editor of the Canadian Journal of Exploration Geophysicists and is a member of CSEG, SEG, EAEG, ASEG and APEGGA.
Harry Jol earned his B.Sc. (1987) and M.Sc. (1989) in Physical Geography at Simon Fraser University. During his program at Simon Fraser, he worked on the Fraser River Delta conducting a high resolution shallow seismic program in cooperation with the Geological Survey of Canada. He then moved to the Department of Geography, University of Calgary and recently defended his Ph.D.
Harry's doctoral work brought ground penetrating radar into many fields of the earth sciences, particularly geomorphology and sedimentology. He has been to more than 300 sites to test ground penetrating radar methodology and has extensive experience on deltaic and coastal features. Harry is presently a University of Calgary Post-doctoral Fellow collecting and interpreting data from a wide variety of sites. He is also working with industry and looks forward to continuing the ground penetrating radar research program.