© 2004 - 2006 WELLOG
Seismic exploration is
conducted by
WELLOG 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. Obviously, 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.
Look for Vertical Seismic
Profiling (VSP) online to learn more!
VSP: AN IN-DEPTH
UNDERSTANDING: http://www.cseg.ca/recorder/pdf/2001/09Sep/11_Sep2001.pdf
Shallow Surface Seismic:
Depth to a water saturated
zone and bedrock can be determined with this method.
Tops and bottom of coal
beds can also be determined without boreholes.
Shallow seismic operations
are conducted using a hammer energy source or weight drop to produce
compression waves or shear waves at the surface.
A computer is used to
acquire seismic reflection waveforms and perform interpretation.
For more information on
seismic methods contact sales@wellog.com.