© 2004 - 2009 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.
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.