High-resolution shallow reflection seismic integrated with other geophysical methods for hydrogeological prospecting in the Nylsvley Nature Reserve, South Africa

Abstract

Two perpendicular high-resolution shallow seismic and resistivity profiles were acquired to assist in imaging the near-subsurface sedimentary architecture for hydrogeological prospecting in the Nylsvley Nature Reserve along the Nyl River floodplain in South Africa. We deployed 48 channels of 14 Hz geophones with 1-2 m dense source-receiver spacing providing fold coverage of 24 (line 2) and 48 (line 1). The resistivity profiles were acquired with a Schlumberger electrode configuration with 2-3 m electrode spacing and a total of 40 steel electrodes. The raw seismic shot records were characterized by low-frequency, high-amplitude source-generated noise (surface and guided waves). Reflections from bedrock were not obvious. To enhance the reflected seismic signal, we employed an extensive seismic processing workflow which enhanced the seismic reflectivity on the stacked sections. The seismic reflection interpretation was constrained and integrated with seismic refraction and resistivity tomography, and the one-dimensional multichannel analysis of surface waves to generate the model that best represents the real subsurface geological model. The integrated results show the bedrock-overburden contact at 8-12 m depth, which correlates well with boreholes drilled in the area. In addition, the reflection seismic and refraction tomography show the bedrock undulation and velocity changes associated with erosional surfaces or weathered/fracture systems. We further interpret these characteristics to be associated with groundwater movement and storage related to the fractured/weathered zone within the bedrock. The integrated data also delineate the interface between the unsaturated sand and saturated sand-gravel that represents the groundwater water table. This study demonstrates the potential of combining several surface geophysical techniques for near-surface investigations, especially for hydrogeological prospecting.