SEISMICS

THE METHOD

Two types of seismic surveys are commonly performed:  seismic refraction and seismic reflection.  In seismic refraction surveys, 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.  For shallow investigations (less than 100 feet), refraction is commonly utilized for mapping bedrock topography.  Seismic reflection surveys make use of travel time and amplitude of all the reflected acoustic energy returning to each geophone.  Reflection surveying can produce detailed images of subsurface geologic structures.  This method is often applied to map faults, and fracture zones which may represent migration pathways for contaminants.
 

Three-dimensional image of bedrock topography below land surface. Volume of removable overburden material was calculated from this suvey for landfill use.

FIELD PROCEDURES

The collection of the seismic data commences with a test to determine the optimum analog filter settings, source spacing, number of source impacts, and geophone array configuration.  Once the test has been completed and the survey design

 parameters have been confirmed, the data collection production phase will ensue.  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 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 sledgehammer and metal plate are often used as a seismic source in shallow investigations.  Where information is sought for greater depths, or in "noisy" areas, a 10 gauge 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
APPLICATIONS
  • Determine bedrock depth and topography
  • Determine groundwater depth
  • Resolve strata and aquifer thickness
  • Map fault and fracture zones
  • Measure overburden thickness
  • Calculate rippability
  • Engineering properties: bulk or shear moduli

is 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.
 

Seismic records are reviewed after each shot to ensure quality data. Digital survey data are transferred from the seismograph to a personal computer for processing.