Diffraction imaging of sub‐vertical fractures and karst with full‐resolution 3D Ground‐Penetrating Radar

Abstract

Vertical fractures with openings of less than one centimetre and irregular karst cause abundant diffractions in Ground‐Penetrating Radar (GPR) records. GPR data acquired with half‐wavelength trace spacing are uninterpretable as they are dominated by spatially undersampled scattered energy. To evaluate the potential of high‐density 3D GPR diffraction imaging a 200 MHz survey with less than a quarter wavelength grid spacing (0.05 m × 0.1 m) was acquired at a fractured and karstified limestone quarry near the village of Cassis in Southern France. After 3D migration processing, diffraction apices line up in sub‐vertical fracture planes and cluster in locations of karstic dissolution features. The majority of karst is developed at intersections of two or more fractures and is limited in depth by a stratigraphic boundary. Such high‐resolution 3D GPR imaging offers an unprecedented internal view of a complex fractured carbonate reservoir model analogue. As seismic and GPR wave kinematics are similar, improvements in the imaging of steep fractures and irregular voids at the resolution limit can also be expected from high‐density seismic diffraction imaging.

Diffraction signatures of fracture intersections

“…Figure 1 displays a small subvolume of the larger Cassis 3D GPR survey acquired with 5 × 10 cm trace spacing and 200 MHz antennae (Grasmueck et al, 2013). Laterally extensive fractures are defined by several such bright spots aligned in the same fracture plane (Grasmueck et al, 2013). When following the reflection bands of these subhorizontal fractures in time slices, it appears that they are lined by small bright spots.…”

“…By acquiring very dense 3D GPR data with a grid spacing of less than quarter-wavelength in all directions and properly sampling diffractions, we have been able to produce images of fractures and karst networks with unprecedented resolution and clarity (Grasmueck et al, 2005(Grasmueck et al, , 2013. By acquiring very dense 3D GPR data with a grid spacing of less than quarter-wavelength in all directions and properly sampling diffractions, we have been able to produce images of fractures and karst networks with unprecedented resolution and clarity (Grasmueck et al, 2005(Grasmueck et al, , 2013.…”

“…To see clear diffractions at the reservoir level, special care should be taken during the acquisition, processing and interpretation of seismic data to preserve these very weak signals (Grasmueck et al, 2013): (1) Acquire very dense, vertically stacked single sensor data with a sufficiently high signal-to-noise ratio. Although GPR equipment only has 16 bit dynamic range, the standard 24 bits of seismic analog-to-digital converters are sufficient to also record weak diffraction signals originating at reservoir depth.…”

Enhancing stratigraphic, structural and dissolution features in GPR images of carbonate karst through data processing

“…Given the importance and the notorious difficulty of imaging and interpreting karstified carbonate rocks (Grasmueck et al . ), we present here a flow for processing GPR data in carbonate karst. This flow was tested on outcropping karstified carbonate rocks of the Jandaíra Formation, Potiguar Basin, Brazil.…”

Introduction to special section: Geophysical imaging and interpretation of outcrops

“…Recent advances in geophysical technologies and development of complementary tools have the potential to contribute to such studies (e.g., Malehmir et al, 2012;Grasmueck et al, 2013;Fernandes et al, 2015). This special section of Interpretation presents a collection of papers that span a broad spectrum of methods and applications.…”