Abstract

New supplies of groundwater, both fresh and brackish, will be critical for meeting the future water needs of Texas. Conventional seismic reflection data (2D and 3D surveys with low-frequency sources and larger group and source intervals, as are typically acquired for oil and gas exploration and development) may provide important refinements to hydrogeologic understanding of deep fresh and brackish aquifers, including structural and stratigraphic interpretation, estimating aquifer extents, and imaging water-bearing geobodies. Seismic data cannot determine salinities, and conventional seismic quality is typically limited by acquisition geometry and source frequency parameters that were designed for deeper targets; however, reprocessing can help to mitigate these limitations.

To better define the ways that conventionally acquired seismic reflection data can help our understanding of the stratigraphic and structural features of brackish aquifers within Texas in the depth range of 1000–5000 ft, we undertook five tasks:

1. review of existing literature relevant to groundwater exploration using conventional data, along with a brief description of seismic techniques and a look at high-resolution data;

2. evaluation of seismic data availability, leading to statewide maps showing coverage from major vendors;

3. evaluation of seismic data quality and limitations, including research on sand-shale reflectivity versus depth in the Gulf Coast region;

4. development of an integrated workflow for using seismic data in brackish aquifer studies (including summary analysis of suitability of conventional seismic for various aquifers); and

5. testing of this workflow by reprocessing and interpretation of the Stratton 3D dataset in Nueces County, within the brackish part of the Gulf Coast aquifer system.

A dozen or so aquifers across Texas show great potential for use of conventional seismic data, and several more show limited potential. Of these aquifers, the Cenozoic aquifer systems of the Gulf Coast region are most prospective, due to abundant 2D and 3D data, slow seismic velocities that allow enhanced resolution, relatively deep brackish zones, and complex internal geometry. As seismic acquisition in these areas has focused on deeper targets, reprocessing is expected to improve imaging and understanding of the shallow water-bearing section. Reprocessing of the Stratton 3D dataset confirms this expectation, and shows that use of conventional seismic will improve our understanding of aquifer resources.