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| USGS scientists and students prepare
source sites for combined reflection and refraction survey
outside Flagstaff, AZ. Cabled geophones (ground sensors)
are visible at the bottom right. |
Scientists use seismic imaging techniques to map the geological
structure below the earth's surface. These maps reveal the
depth and area of
basins ,
fault
networks, and the physical
properties of rocks. The basic technique involves generating
seismic waves from a "source"
and recording them on seismographs in the area
being studied. The paths of the seismic waves are reconstructed
from their traveltimes (time it takes a wave to travel from
its source to the seismograph). Seismic imaging techniques
have analogs in medical science. Waves transmitted (refracted)
directly through the earth produce a "catscan"-type image,
while waves
reflected back to the surface from layer boundaries or
faults produce a "sonogram"-type image. Traveltime observations
and variations in amplitude, frequency, and waveform are combined
to produce a model of the geologic structure. The model must
be consistent with other geophysical and geological data,
such as measurements of the strength of gravity, measurements
of the strength of the magnetic and electric fields, and laboratory
measurements of the speed of seismic waves in rock samples.
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