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.
J
R Associates,
Ph: