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Seismic exploration is conducted using
surface and sub-surface methods.
Seismic data provides valuable information about the
density and thickness of the layers of alluvium and rock below the surface of
the earth. The Seismic method has proven to be the most important geophysical
technique in terms of dollars spent on geophysical services.
Technique:
The basic technique involves generating
seismic waves and measuring the time required to travel from a sound source to
a series of geophones usually set in a line along the surface or suspended on a
wire-line within a borehole or well. Geophones are specialized acoustic sensors
that convert vibration into electric waveforms.
Seismic waves may be generated from
explosive devices, specialized vehicles having a hydraulic or pneumatic ram,
weight drop or steel strike plate and sledge hammer. Drilling rigs can also be
a source of sound energy.
Seismic Data While Drilling:
Rotary Drilling frequently employs the
use of a drill hammer. Geophones placed
on the surface in the vicinity of the drilling rig may be used to collect
geophysical information while drilling.
Subtle changes in seismic reflection give immediate feedback on entry
into specific formations. Examples include entry into a saturated zone or
bottom of coal bed.
Cable tool rigs generate sound waves
that can be measured for hundreds even thousands of feet. The
time-speed-distance relationship of sound waves measured at the surface gives
important information about formations and potential water level.
Sound waves also known as seismic waves
travel at different speeds in materials having different density. Generally,
more dense materials conduct seismic waves at greater speed. Speed is measured
in feet per second and inversely, time is measured in microseconds per foot.
Metric units may be used as appropriate. Accurate time – speed – distance
calculations can be made.
Seismic waves are subject to change as
they travel thru formations in the earth. Waves may be refracted, reflected and
changed in amplitude and shape.
VERTICAL SEISMIC PROFILING:
Vertical Seismic profiling is obtained by lowering the
seismic receiver (geophone) to different depths and measuring the seismic wave
travel time at each depth. Differences in travel time at
different depths provides information on formation travel times. Given
additional information on formation matrix (sandstone, limestone, etc) , allows computation of formation Porosity. Up going waves
are waves reflected from formations below and can give a “forward look” for
drilling purposes.
View an illustration of Vertical Seismic Profiling.
Check shot calibration, zero
offset, fixed offset and other variations of VSP allow clients to obtain
information about the subsurface in different directions surrounding the well.
Offset is the distance geophones are placed from the well head.
Environmental impact:
Seismic exploration when done without
large trucks or explosives is friendly to the environment.
WELLOG is currently developing down hole geophones that help geologists and geophysicists
determine the precise travel time, shear wave velocity, and compression wave
velocity thru various lithologies. This gives information to better understand
formation density, thickness, and other physical properties that control
porosity, permeability and produce-ability in water, gas, CBM, and oil wells. The method has proven to have value in other
applications involving mineral exploration, bedrock determination, and other
subsurface geophysical definition.
The WELLOG “Geophone Jack” is a
motorized “screw jack” that is used to press a geophone against the borehole
wall. An accurate depth – position readout combined with high speed digital
data acquisition completes the system.
Learn more about Vertical Seismic
Profiling (VSP) at www.wellog.com/vsp.htm !
Shallow Surface Seismic:
Depth to a water saturated zone and
bedrock can be determined with this method.
Download an MS Excel [Reflection Seismograph]
Simulation.
Download an MS Excel [Refraction Seismograph]
Simulation.
Download an MS Excel [Refraction SeismographV2]
Simulation.
Download an MS Excel [Refraction SeismographV3]
Simulation.
LITHOLOGY:
Tops and bottoms of coal beds can be
determined without boreholes.
Shallow seismic operations are
conducted using a hammer or weight drop seismic source to produce compression
waves or shear waves at the surface. A computer is used to acquire seismic
waveforms and perform interpretation.
Download an MS Excel [Field seismograph]
viewer.