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High Sensitivity Magnetic Gradiometer for Earthquake Research Applications
ISRAEL 2005
Ivan HrvoicH. Ginzburg, H. Zafrir, G. Steinitz, B. Shirman, G. Hollyer
Overview
• Magnetic Earthquake Research Methods– Introduction to Past and Current Methods
– Detectability of Earthquakes by Gradiometers
– Short Base (Gradient) Measurements
• Potassium SuperGradiometer
– Installation and Data Records
• Summary
Introduction to Magnetics
• Several decades of investigation
• Based on theory of piezomagnetism
and / or electrokinetics
• Possibility of detection is related to gradual pressure build-up prior to earthquakes or “events”
Monitoring Systems (1)
• Traditional Methods– Magnetic sensors (0.1 nT sensitivity)– Long base measurements
• Some startling results• No duplication of results
• Recent Work– Induction coils
• Improved sensitivity to 25 pT• But, limited bandwidth (0.01 Hz)
Monitoring Systems (2)
• Induction Coils– Detect first derivative of magnetic
field– Detect all 3 components– Skin Effect Problems
• 50 km @ 0.01 Hz• 1.6 km @ 10 Hz
Detectability of Earthquakes by Gradiometers
• Assuming earthquakes create dipolar anomalies, can calculate detectability of earthquakes of given magnitude:
• Where o is magnetic permeability, M is magnetic moment, and α is the angle between radius vector, r, and dipole direction.
• r is distance from hypocenter to the observation point
Magnetic Moment (1)
• Assuming cos2 = 0 for simplicity, the previous equation can be solved for M, Magnetic Moment, as follows:
• Results are extended to look at datafrom the Loma Prieta earthquake of 1989 which clearly shows magnetic precursors.
Magnetic Moment (2)
• Loma Prieta (M7.1) B=2.8 nT anomaly at 17 km depth to hypocenter with 1.7 x 1011 Am2 magnetic moment
• San Juan Bautista (M5.1) 20 pT anomaly at 9.4 km depth to hypocenter with 1.7 x 108 Am2 magnetic moment
Radius Calculation (1)
• Note the 3rd potential drop off of the magnetic field and the 4th potential of the magnetic gradient with radius:
• Extreme sensitivity needed to measure (dB / dr)• Prospects for elimination of man-made
noise are better
Radius Calculation (2)
• Maximum distance at which an event can be detected by different sensors
• Skin effect not taken into account
Potassium SuperGradiometer
• 3 sensors arranged according to terrain (horizontal or vertical)
• Sensor spacing up to 140m• Long term integration is promising
140m
100m
S2
100mS3 S1
100m
Computer
SuperGrad Array
Data (1)
Data (2)
Data (3)
Installations
• Currently in use near Dead Sea, Israel
• Measures fields at 3 sensors, 20 times per second with 50 msec and 1 sec integration
• 6 channels of data to 1 fT resolution
• GPS receiver provides Universal Time
• Noise background is about 0.1 pT for 1 sec integration; giving 2 fT/m sensitivity at 50m spacing
• Since July 2002, has acquired more than 10 billion readings; likely the most ever recorded for this type of application
Initial Installation - Israel
Data - Israel
pT
8 hours
Additional Installations
• System deployed in Magnetic observatory of the Geological Survey of Canada near Ottawa
• Test of basic system configuration for 6 months
– Remote operation– Downloading of data via internet or telephone
• Installation in Mexico (Oaxaca State)
– Photos and records from Mexico
• Also seeking other jurisdictions for siting system; potentially in regions of more active tectonism
Site Location- Mexico
Site Selection - Mexico
Site Survey Data - Mexico
Gradient Survey Data - Mexico
745550 745600 745650 745700 745750
1751050
1751100
1751150
1751200
1751250
1751300
-30-25-20-15-10-5051015202530354045505560657075
nT/m
Sensor Installation - Mexico
Data Acquisition & Communication - Mexico
Data - Mexico
pT
sec
Integrated Grad / Radon
• Complementary radon emanation measurements.
• Correlation with weak earthquakes in Dead Sea rift region
• The combined Supergradiometer / Radon system is now available for application by various groups pursuing earthquake research studies.
Summary
• Based on earlier assumptions (and not considering geometrical effects), we can conclude:
– Magnetometers of 1 nT sensitivity can detect only the strongest earthquakes (M >7)
– Induction coils are good for M>6
– SuperGradiometer in Fast mode is effective for M>6
– SuperGradiometer in Slow mode is effective for M>5