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 Seismics

Seismic techniques are commonly used to determine site geology, stratigraphy, and rock quality. These techniques provide detailed information about subsurface layering and rock geomechanical properties using seismic waves. Different lithologies transmit seismic waves at different velocities, bending and reflecting waves at interfaces. By configuring seismic sources and receivers into specific arrays and measuring the time spent for compressional (P) or shear (S) waves to travel from the source to the receivers, interpretation techniques resolve the layer velocities and interface depths. Seismic surveys provide detailed diagrams illustrating geologic structure, layering, and layer velocities beneath the area of exploration.

Refraction and Reflection

Refraction and reflection are the most commonly known seismic techniques. These methods determine geologic structure and rock velocities by either refracting or reflecting waves off boundaries between rock units with different seismic velocities. Seismic refraction and reflection determine:

  • General geologic structure
  • Rock rippability and quality
  • Water table depth
  • Faults and other hazards
  • Overburden thickness bedrock depth

Borehole Seismics

Borehole seismic surveys measure geologic boundaries and rock velocities in the vicinity of boreholes. They are most commonly used to measure various types of rock moduli in situ. P and S-waves are transmitted from the source on the surface to receivers in an adjacent borehole (downhole) or from a source located in one borehole to receivers located in another (crosshole). Borehole seismic surveys provide information about:

  • Dynamic rock moduli
  • Vibratory loading response
  • Rock velocities and quality
  • Geologic layering                                                     Borehole Vs. MASW Comparison

Multichannel Analysis of Surface Waves, MASW

Multichannel analysis of surface waves is a method for measuring shear wave velocities without the need for a borehole or well.  The method measures the velocity and dispersion of surface waves (which have compression and shear components) and backs out the shear wave velocities.  The result of a MASW study are two-dimensional profiles showing changes in shear wave velocity both laterally and with depth.  MASW is applied to:

  • Measuring Shear Wave Velocity
  • Detecting Low Velocity Zones
  • Liquefaction Studies                         
  • Earthquake Response studies

Vibration Monitoring

Measurements of ground vibrations and noise are often necessary during construction activities, highway expansion, and quarrying operations. Ground vibrations are monitored to determine if activities such as pile driving, foundation excavation, quarry blasts, or dynamic compaction operations pose potential hazards to existing buildings. Vibration monitoring includes:

  • Instant vibration and noise information
  • Acceleration, velocity, and displacement measurements
  • Frequency and waveform analysis
  • Permanent records

Tomography

Seismic tomography is applied to situations where detailed pictures of the subsurface are needed. Tomography utilizes both borehole to surface and borehole to borehole shooting. Sophisticated computer programs invert the field data into a tomographic image showing a very accurate two dimensional diagram of the velocity structure in the surveyed areas. Tomography is employed to detect or map:

  • Discontinuous low velocity zones
  • Fracture zone orientation
  • Fault orientation and thickness
  • Dynamic rock moduli and quality
  • Grout curtains

Combining Techniques

J R Associates often combines geophysical techniques to provide a better understanding of subsurface geology. Seismic refraction and resistivity are combined in groundwater explorations. Resistivity and magnetics are combined to locate faults. Magnetics and ground penetrating radar are often used together to locate buried tanks and drums. Because we offer a wide variety of geophysical techniques, combined geophysical surveys are often performed for the same cost as individual geophysical surveys.

At J R Associates we stand ready to apply our geophysical expertise to your particular requirements. Please contact us to discuss how our combination of techniques, knowledge, and experience can provide you with a window on what's underground.

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J R Associates, 1886 Emory Street, San Jose, CA  95126

Ph: (408) 293-7390 Email: jra@Greatgeophysics.com