Each year, earthquakes injure more than 17,000 people and cause more than $40 billion in property and environmental damage globally.
In looking for ways to mitigate these losses, researchers are investigating different methods, including seismic, strong motion, GPS, electromagnetic, magnetic, radon and others.
Gradiometry is emerging as a promising method based on reports of piezomagnetic and/or piezokinetic effects prior to large earthquakes. Studies show large amplitude magnetic responses weeks and hours before events. Smaller events appear to exhibit less coherent patterns; likely due to the lack of sensitivity of traditional magnetic instruments.
GEM’s new SuperGradiometer is designed to improve detection of subtle responses and potentially lower the threshold of detectable earthquakes. SuperGrad is the highest sensitivity total field measuring device ever developed with a 0.05 pT root-mean-square (rms) sensitivity at a sampling rate of 20 Hz (averaged over a 1 second interval). This sensitivity is well over an order-of-magnitude more sensitive than any other system.
In its first year and 3 months of operation (on the research project described below), SuperGrad acquired over 5.25 billion readings – the highest sampling of magnetic data acquired by any organization over this length of time. Initial results show a sensitivity of 0.1 pT which is modulated by weak diurnals due to the surrounding rocks – a reflection of initial installation in a tunnel setting. Diurnals can be suppressed – increasing background noise to 1 pT peak-to-peak or about 0.25 pT root mean square (rms).
The GSMP-20S3 can achieve gradient sensitivities of 1fT/m (10-15 T/m) with a sensor spacing of 50m – a major advantage over traditional long-baseline magnetic measurements which have sensitivities on the order of 1nT. The GSMP-20S3 also minimizes cultural noise (i.e. from nearby infrastructure), and 1/f noise that typically degrades results from other measurements (ex. Electromagnetic). f is defined as the frequency of the piezomagnetic signal from the event.
After evaluating results, researchers are deploying a second 3-sensor SuperGrad on a magnetically quiet new site near the same rift. In a separate project, another GSMP-20S3 has been deployed at the Geological Survey of Canada’s observatory in Ottawa. This installation will be followed by another one on Canada’s west coast in 2004.
GEM also offers the Integrated SuperGrad/Radon system (ISGR) as an option for researchers seeking to integrate two types of information-rich, independent data into joint interpretations.
In a joint project with ISORAD, Israel Geologic Survey, Survey of Israel and GEM, researchers are integrating SuperGrad results with Radon data from the Dead Sea Rift, Israel. Radon studies to date have examined the temporal relationship between hundreds of weak earthquakes (M < 4.6) and the start time of 110 Radon flux signals. Earthquakes located within the three pull-apart grabens of the Dead Sea rift valley were found to occur preferentially within
the first 3 days after Radon start time.
These results demonstrate a statistically significant correlation between week earthquakes and Radon flux. In contrast, but in agreement with geological reasoning, earthquakes outside the Rift do not show a connection to anomalies recorded within. Ultimately, integration of data from gradiometric sources will serve to complement the Radon studies both inside and outside of the Rift. To access a summary poster, click here. Links to answers to specific ISGR questions are below:
The GSMP-20GS3 was developed with the Russian research group of Dr. E. Alexandrov in response to the United State Geological Survey’s (USGS) need for an ultra-high sensitivity gradiometer. Selected general gradiometry concepts are covered below:
Implementing a SuperGrad Installation
Key technologies include optically pumped Potassium sensors for ultra-high sensitivities. In the Israeli project, the system is arrayed with a vertical sensor and another two horizontal sensors oriented in an “L-shaped” pattern. Horizontal sensors are located at specified distances (ex. 50m or 100m) for maximum sensitivity. A GPS receiver records Universal Time.
Raw data are acquired at 20 samples per second along with filtered (running average) 1 sample per second data. This very large volume data is transmitted to a computer and formatted in 1 hour files for transfer via telephone, cell phone, or Internet to the central station. A standard program, such as Laplink, is used for remote control operation.
Subsequent data analysis is based on different techniques including visualization of events for time prediction and calculation of other parameters (ex. ratios) to help evaluate event geometry or direction.
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