Application of ground penetrating radar imaging to deepwater (turbidite) outcrops

Abstract

Ground Penetrating Radar (GPR) provides a high resolution image of bed-scale features within the interior of an outcrop. This geophysical technique has been used successfully in fluvial-deltaic sequences, but its successful use in deepwater deposits is less documented. The present paper demonstrates the ability of GPR to define the nature of channel margins and to distinguish internal channel facies at turbidite outcrops in Wyoming (Cretaceous Lewis Shale), USA. Profiles obtained behind the Lewis Shale outcrops show channel-fills having complex, slumped margins between channel sandstones and adjacent levee thin-beds, as well as onlap of sandstone beds onto the channel margins. Within the channel-fill, facies distinguished by GPR vary from interbedded debris flows and turbidites on one side of a channel to cross-bedded sandstones on the other side. Lateral accretion surfaces and onlap features, which cannot be observed at the outcrop scale, are present on GPR profiles. They may represent cross-channel sediment transport and deposition within a sinuous channel. These examples suggest that, when used together with geologic mapping, photomosaics, and available behind-outcrop drilling and logging, GPR surveys spanning large outcrops may provide valuable sub-seismic scale architectural information for building more robust 3-D models of deepwater reservoirs.