Under a contract with the National Disaster and Emergency Management Authority (AFAD) in Ankara Gaiacode will be installing three seismic borehole stations in Turkey. These stations will augment an existing seismic monitoring network in the country, which has one of the highest earthquake hazards in the world. Each station consists of a downhole seismic instrument and a multichannel Tau digitizer at the wellhead. The downhole instruments contain a set of three Alpha broadband sensors, three Sigma accelerometer sensors and a newly designed single jaw hole-lock to anchor the instrument in the borehole. These sensor elements and the hole-lock are stacked in a 316 stainless steel tube with a diameter of 89 millimeters. The whole system is pressure rated to a water depth of 900 meters.
Three locations in Turkey
The boreholes are located in Istanbul, in the far eastern Anatolian city of Erzurum and in Kütahya in the western part of the country. The instruments will be installed at borehole depths of approximately 110 meters each. Data will be transmitted in analogue form to a Gaiacode Tau digitizer at the respective wellheads.
Wide bandwidth and high dynamic range
The combination of Alpha and Sigma sensors in one borehole unit allows for the recording of seismic signals over a wide frequency range, from 120 seconds to 100 Hertz for the broadband sensor and DC to 350 Hertz for the Sigma accelerometers. The Sigma accelerometers are set to record strong ground motion of up to 4 g.
New Single-Jaw Holelock
The sensor stack is combined with a powerful single arm hole-lock, which locks the sensor system against the borehole casing with a force exceeding 30 kilogram. Together with the hole-lock two sets of “ears” (skids) at the top and bottom of the sensor stack force the sensor stack against the borehole casing. This geometry shifts possible parasitic resonances of the locked instrument package in the borehole far out of the seismic band. One of the main features of this unique hole-lock design is that the instrument package can be removed from the borehole without retracting the hole-lock arm.
