Seismic Imaging, Inc. offers a wide variety of geophysical services for the purpose of investigating various characteristics in the subsurface. Clients have found these techniques particularly attractive because they can be carried out cost effectively in remote, densely wooded settings. Geophysical exploration can provide laterally continuous data in the form of cross-sections rather than spot subsurface characteristics common with many geotechnical drilling methods. Geophysical methods are also nondestructive, so they can often be conducted with minimal clearing of vegetation and no digging or drilling (except in the case of seismic crosshole surveys). The following chart outlines Seismic Imaging's exploration capabilities and indicates which geophysical methods are preferred for various applications.
Seismic Refraction
This method is utilized to determine the depth and lateral extent of soil,
partially weathered rock and bedrock in a given area. Useful applications of
this information include planning grade and invert elevations, determining the
rippability of a material and anticipated excavation methods, and creating a
bedrock surface map for plotting possible pathways for contamination plumes.
The first step in acquiring seismic data is to generate a compressional wave at
the ground surface. As the compression wave passes downward through the
underlying materials, it is bent (refracted) at each increase in material
density. These correspond to underlying layers of soil, partially weathered
rock, and bedrock. We employ a multi-channel seismograph and an array of geophones
to measure the time required for the wave to travel through these layers
of contrasting hardness in the subsurface. Depths of investigation using our
instruments are approximately 40 feet to 80 feet in most cases, but can be
altered to meet specific project needs. Seismic data collected in the field is
analyzed and then used to produce a cross-sectional profile of the subsurface
below each traverse.
Electrical Resistivity
Contamination plumes, voids and caverns, and average rock depth can often be
located using electrical resistivity imaging. Our resistivity systems detect
contrasts in resistance to current flow by applying direct current to the ground
surface through two electrodes and measuring the potential difference between
two different electrodes.
Changes in subsurface resistance cause variations in
potential difference measurements. Tomographic resistivity surveys use
electrode arrays including Dipole-Dipole, Pole-Pole, Schlumberger and Wenner.
After analysis and tomographic processing of redundant electrical resistivity
measurements, final results are presented to our clients as color cross-sections
with contours of resistance to current flow.
Ground Penetrating Radar (GPR)
Subsurface geology and other objects or structures buried at shallow depths
can sometimes be delineated using ground penetrating radar. Electromagnetic
waves are emitted from an antenna that is dragged along a line or grid in the
area of interest. When electromagnetic waves hit a conductive anomaly in the
subsurface, they reflect, and the returning waves express this heterogeneity as
a parabola. The antenna receives the returning waves and instantaneously
records the incoming radar data as the system is pulled along the ground.
Crosshole and MASW Seismic Surveys
Crosshole surveys with downhole geophones and sources to obtain shear wave velocity models have largely been replaced by MASW (multi-channel analysis of surface waves) and similar surface-run techniques. These cost effective, non-intrusive methods measure the variation of wave velocity to frequency (dispersion) since lower frequency waves penetrate deeper into the earth. The resulting model of the ground's shear wave, obtained by inversion, can be used to determine a site's IBC site classification for seismic design purposes.
Electromagnetic (EM) Surveys
By measuring natural and inducted electromagnetic fields in the subsurface,
depths to layers or objects of contrasting conductivity can be identified. Grid
data collected in the field is used to produce contour maps resolving the
electromagnetic anomalies.
Magnetics
Variations in the earth's magnetic field can be measured in the field using a magnetometer, then plotted and analyzed. Mineral deposits and buried objects with significant iron content cause local variations in the magnetic field. This method is only sensitive to ferrous objects.
Gravity and microgravity
Locating voids and caverns is possible by mapping changes in the gravity field
over a relatively small area using a tight grid system. Variations in gravity
are measured in the field, then analyzed and contoured.
