Increasingly, researchers are looking in to the applications of magnetics alone or in combination with seismic or radon measurements for earthquake research. GEM’s new SuperGrad technology is the highest sensitivity gradiometer ever developed for these types of applications.
Below is a set of commonly asked basic questions about SuperGrad.
A magnetic gradiometer is an installation comprising two magnetic sensors oriented either vertically above each other or horizontally beside each other. Gradients are calculated by subtracting the value of measurements made at one sensor from those made at the next sensor, and dividing by the distance between sensors.
Many surveys measure the vertical gradient; however, surveys can be expanded to measure all three orthogonal gradients. The basic value of gradient measurements lies in:
In terms of 3-axis measurements, the value lies in mapping the three-dimensional structure of anomalies that are related to physical phenomena of interest.
Magnetic gradiometric monitoring is the process of using gradients to measure, determine, separate and eliminate the different patterns of the geomagnetic field perturbation.
With short-baseline monitoring, the distance between operating magnetometer and base station is small compared to the depth of investigation. This differs, for example, from total field magnetic measurements where a separate, isolated magnetometer is used for diurnal correction (i.e. the baseline is effectively at infinity).
Magnetic gradient measurements (for a dipole response) are described by the equation:
dH/dr = -3H/r (1)
where dH is the difference in the total field, dr is the distance between sensors, H is the total field and r is the depth to the anomaly.
Relatively recent developments in low field magnetics (GEM Systems Potassium Supergradiometer) have opened a new way of detection of piezomagnetic or piezokinetic changes in stressed rocks: short base gradiometers.
Magnetic gradient sensitivities of 1fT/m (10-15 T/m) are readily achievable with the Potassium SuperGrad and a sensor spacing of only 50m. In comparison, the long-baseline method may have many kilometers between measuring and reference magnetometers, and the sensitivities are on the order of 1nT on a long term basis.
The advantages of short-baseline monitoring are:
Gradiometric monitoring can be sensitive to a variety of phenomena in the interior of the earth, including:
Magnetic gradiometric monitoring is currently being applied in Israel by ISORAD, the Geological Survey of Israel and the Survey of Israel. Monitoring activities involve a gradiometric system set up (2001) in the southern sector of the Dead Sea Rift. Project objectives are to study the potential of using gradiometrics in earthquake hazard related research in the vicinity of the active rift.
In other disciplines, the Istituto Nazionale di Geofisica e Vulcanologia in Italy is investigating long-base and short-base measurements for characterization of volcanic eruptions as a possible means of predicting eruptive events.
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