Seismic Refraction

The Method

NAEVA performs seismic refraction surveys for measuring depth to bedrock, commonly for mapping bedrock topography. The travel time is measured for a wave to pass through a layer to another, refract along the interface, and return to the geophones at the surface.

Equipment Used

Bison Instruments 9024 Seismograph

Field Procedures

Prior to the collection of the seismic survey data, a test is performed to optimize analog filter settings, source spacing, number of source impacts, and geophone configuration. Once the survey design parameters have been confirmed, the data collection commences. Cables are laid out and connected to each geophone and the seismograph. The source is impacted at each station a sufficient number of times to record good signal. The seismograph digitally records and stores the data for each field record.

Equipment

NAEVA Geophysics operates a 24-channel Bison 9024 seismograph, a briefcase-sized signal stacking instrument featuring instantaneous floating point amplifiers, a high-speed digitizer, high and low pass filters, a high resolution thermal printer, and 500 MB of data storage. A 16 pound sledgehammer and metal plate are typically used as a seismic source for shallow investigations. Where information is sought for greater depths, or in "noisy" areas, a 10 gauge “Betsy Seisgun” or an elastic wave generator (EWG) is employed.

Data Processing and Presentation

NAEVA Geophysics uses either the Generalized Reciprocal Method (GRM), or a Delay Time Method for seismic refraction data processing. The GRM software used by NAEVA is Interpex's GREMIX, and the delay time method software is Rimrock's SIPT2. The interpreted data are usually presented as a profile illustrating depth to refractors below the seismic spreads. Alternatively, when a number of parallel lines have been surveyed, 3-D maps may be prepared. Seismic reflection data are processed using Interpex's Seistrix 3. Interpreted data are most often presented as cross-sectional images, with important events highlighted.

Applications

  • Determine bedrock depth and topography
  • Resolve strata and aquifer thickness
  • Measure overburden thickness
  • Calculate rippability
  • Engineering properties: bulk or shear moduli
Seismic Profile

The depth to bedrock is measured by calculating the travel time for waves to refract along the interface between bedrock and overburden