SuperGrad – FAQ Using Gradiometric monitoring

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.

What is a magnetic gradiometer?
What is short-baseline magnetic gradiometric monitoring?
What are the advantages of short-baseline gradiometric monitoring?
What phenomena can gradiometric monitoring detect?
Where is magnetic gradiometric monitoring applied?

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What is a Magnetic Gradiometer?

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:

Removing diurnal effects. Diurnal and other temporal changes in the magnetic field are eliminated by synchronously taken readings.
Resolving complex anomalies related to overlapping sources.

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.

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What is Short-Baseline Magnetic Gradiometric Monitoring?

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.

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What are the Advantages of Short-Baseline Gradiometric Monitoring?

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:

Short-baseline gradiometers are about 100 times more sensitive than long-baseline magnetometers in measuring a dipolar source of piezomagnetic changes 10 km away or 10 times more sensitive for 100 km source distance. This is derived from Equation 1 where we see a direct relationship between sensor spacing (dH/dr) and depth (r).
For example, a 1 nT anomaly from a depth of 10,000 m would produce a gradient of 3 nT/10000 m = 0.3 pT/m. This can be detected by the SuperGrad with a signal-to-noise ratio of 6:1 at 1m spacings of sensors or 300:1 at 50 m spacings.
Short-baseline monitoring enables us to eliminate diurnal noise related to atmospheric effects.
It also eliminates the effects of cultural noise from nearby cities, etc. Although very sensitive, the system will not see a 10-ton truck at 1 km distance. Interferences at shorter distances can be identified and compensated for by special software developed by the ISORAD magnetics group. As a result, the system can work completely unobstructed with only few hundred meters of magnetically protected space in each direction i.e. a fraction of a square kilometer of surface area.

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What Phenomena can Gradiometric Monitoring Detect?

Gradiometric monitoring can be sensitive to a variety of phenomena in the interior of the earth, including:

Piezomagnetic effects related to pressure-induced magnetization (i.e. such as those assumed to preceed earthquake events)
Thermomagnetic effects related to the melting of rocks below the Curie point (point at which magnetic properties are retained)
Thermokinetic effects related to material flow (for example, within fractures) that generate corresponding currents and hence, magnetic fields

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Where is Magnetic Gradiometric Monitoring Applied?

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.

For more Information

For more information on Potassium technologies, please refer to the technical papers on this site. Specifications on the SuperGrad are provided in the GEM GSMP-20S3 brochure.