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Magnetic Gradiometer Sensor Separation Issues
A brief introduction to the sensor separation required when using a magnetic gradiometer in order to maximise resolution, whilst filtering out back ground noise and still giving usable detection range off the survey line. The document shows that whilst it is tempting to have a very short base line for the gradiometer array this can lead to very poor detection range and not yield any better result than that obtained by a single sensor magnetometer.
Magnetic Gradiometer Sensor Separation Issues (PDF 1.5MB)
Seismic Cables
Many of us think that as our seismic spread cables, look in good condition, and there are no dead channels, or maybe one or two, that they will be OK for acquiring good quality data.
This can be a very misleading assumption. At Geomatrix Earth science we regularly check all our seismic cables (and resistivity cables) for continuity and leakage using a high voltage insulation tester. Continuity checks for dead channels, which can be a broken wire in the end connector requiring re-termination of the connector, or a broken wire in the takeout which will require re-moulding of the takeout. Experience has shown that testing for cross channel leakage using a regular ohm meter can give very misleading results and a more thorough test is to use a high voltage insulation testing meter, which Geomatrix regularly perform on all of our seismic cables. The two examples below show what a dramatic effect cables with a poor leakage value can have on your data. Both data sets were acquired with a Geode seismograph set to exactly the same acquisition and display parameters, no acquisition or display filters are used, and no geophones are attached, so just background noise is being recorded.
The image labelled Good Cable shows what you would expect, low and equal noise across all channels and across the whole record length.
The image labelled Bad Cable shows noise spikes visible on several channels, and one channel in particular showing a far higher noise value than the others. Clearly this noise would be recorded along with your seismic data and although it is of low amplitude would detract from the fidelity of your recoded seismogram.
If you would like your geophone cables testing for leakage and continuity prior to any important surveys, please call our customer service department for a quotation.
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