Groundwater Flow Modeling Using Seismic Refraction

During the summer of 2005 Subsurface Evaluations Inc. (SEI) was contracted to perform Seismic Refraction Tomography in the area of the Cimarron River located in Oklahoma.  The local geology in the area of the survey consists of approximately 45 feet of unconsolidated materials overlying shale. Generally speaking, these unconsolidated materials consisted of sands varying in clay and silt content. The contact between the unconsolidated materials and the shale was known to be clean and rather abrupt, this information being available from direct soil recovery.

After considering the site’s geology and the level of confidence required by the client, it became Subsurface Evaluations’ position that a seismic refraction tomography survey would best suit the needs of the client. The direct intent of the refraction tomography survey was to map the interface between the unconsolidated materials and the deeper shale. The application of the data was to provide a greater understanding of the nature of the shale’s surface as it pertained to groundwater flow. SEI’s client’s main concern was the possible existence of channels and depressions in the surface of the shale strata. The idea being that these channels and/or depressions could represent possible preferential lines of flow or subsurface topographic lows where groundwater contaminants might accumulate. There was also an interest in knowing dip direction relative to the nearby Cimarron River.

Subsurface Evaluations Inc. used the following acquisition instrumentation and processing software. The source chosen was a Digipulse AWD-100 seismic source.  The source energy is provided by a 100 pound mass accelerated by use of elastomer bands.  Two 24 channel RAS seismographs were combined for a complete 48 channel seismic recording system.  GS-20DM 8 Hz Vertical Geophones were used for sensing refracted seismic energy for each of the 48 channels. First break picks were made for every line for use in refraction modeling/inversion. Software used for first break picks was SeisOptPicker from Optim Software Inc.  Refraction modeling/inversion was performed using software program SeisOpt@2D also by Optim Software Inc.  The final modeled velocity data was then contoured using Surfer 8 by Golden Software.

What makes this survey worthy of note is the fact that quality seismic data was acquired, processed, and interpreted within a timeline and cost framework previously unthinkable. Using innovative field techniques, the latest in software technology, and only a two person field crew with an additional logistics/GIS person, Subsurface Evaluations is able is provide high resolution seismic refraction and reflection to clients restricted by budgetary concerns.