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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.
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.
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.
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.