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The Technology
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GF3000
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The GF3000 seismoelectric survey instrument is designed specifically for detecting electrical signals generated by the passage of seismic impulses
through layered rocks, sediments and soils. Its design is portable, rugged and simple. The remarkably high signal to noise ratios achieved with the
patented design means that productivities are high; up to twenty sites can be surveyed by one operator in a day’s work. The depth from which signals
originate can be estimated in the same way as in a seismic reflection survey by utilizing the travel time of the downgoing seismic impulse. The quality
of the aquifer is estimated by semi-empirical relationships established between signal bandwidth and amplitude and aquifer permeability. These are
supported by thousands of observations over the past decade. It is also possible to estimate water table depth by noting the downward transition
from a disordered signal geometry to signals having focused radial symmetry. Systematic surveys along traverses or in grids allow the lateral
variability of aquifers to be mapped.
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SIGNALS
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The GF3000 and a seismic source are used together. The seismic source is used to create a sharp sound pulse. When the sound pulse moves through
porous and permeable aquifers the pore water moves relative to the rock matrix. Ions in the water are dragged away from their partners bound to
the rock and an electrical dipole is set up that “flickers” at seismic frequencies. Where there are changes in the rock, as at bedding planes,
the electromagnetic disturbance is caused to propagate to the surface at the speed of light. Each signal produced this way is detected at the
surface by two dipole antennas. Each signal is separated in time from its neighbors by the propagation time of the down going seismic pulse.
This gives unambiguous depth and thickness data, as with reflection seismic. The form of each signal gives information about the depth,
thickness and quality of the aquifer and this is used to estimate the likely water yield from a borehole drilled at the survey site.
The following factors, roughly in order of importance, determine the quality (as determined by the signal/noise ratio) of seismoelectric data:
- Signal strength as a function of all three spatial coordinates and time
- Power line noise field strength
- Antenna impedance
- Common mode (i.e. power line noise) rejection ratio of the detector
- Input impedance of the detector
- Source coupling
- Receiver coupling
- Detector input noise
There are numerous interrelationships between these factors.
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WHY USE SEISMOELECTRICS?
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Seismoelectrics is nearly unique in looking primarily at the fluids in rocks and soils. Nuclear magnetic resonance technology does too but is
unwieldy and costly compared with seismoelectrics and has fundamental limitations, especially an inability to give unambiguous depth information.
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