An Integrated Real-Time MultidisciplinaryGeophysical Borehole Observatory

The world's first integrated geophysical borehole observatory has been installed by GaiaCode on the Kapidag Peninsula on the South coast of the Marmara Sea in Türkiye (40^O29'22” N, 27^O58'44” E). The downhole instrumentation consists of a low noise ultra-broadband three component feedback seismometer, a strain-meter, a dilatometer, a continuous pore-pressure sensor and a temperature probe. All these instruments and sensors were designed and built in house by GaiaCode. To complete the observatory, these sensors will be augmented by a MEMS accelerometer and a three axis 4.5 Hz geophone installed downhole as well as a weather station at the well head. The latter instruments were supplied by other vendors but integrated into the observatory by us.

 

The observatory has two boreholes which are about 4 m apart. Each hole is approximately 110 m deep with minimal vertical deviation. The first set of instruments (dilatometer, strain-meter and a set of borehole geophones) has been cemented into the bottom of the first borehole. The ultra broadband seismometer “ALPHA” with 360 seconds low frequency corner and 200 Hz upper frequency corner with 5 decades of frequency range  will be installed in a special casing in the same borehole.

 

This seismometer will also serve as a downhole tilt-meter, using its mass position outputs. Unlike triaxial (tilted Galperin) seismometer, classic orthogonal topology has the advantage over tilted Triaxial Galperin seismometers of providing precision tilt measurement without the requirement of a separate expensive tilt sensor. The broadband seismometer is equipped with a stable  single jaw hole-lock for easy retrieval. The real time pore-pressure sensor will be cemented into the second borehole.

 

The analogue measurements from these instruments will all be transmitted to their respective well head, where they will be processed by a fleet of TAU digitizers. TAU digitises can transmit 6 concurrent sample rates, ideally suited for Multidisciplinary seismic station data acquisition. Gaiacode's OMEGA software is used for recording and controlling-configuring the sensor system and the digitisers.

 

This new observatory is a major extension of the joint research initiative by the German Geoscience Research Center (GFZ) and the Turkish Disaster and Emergency Management Presidency (AFAD): the GONAF Project (Geophysical Observatory at the North Anatolian Fault). Its main objective is to measure seismic and aseismic tectonic deformation transients along the Marmara section of the North Anatolian Fault in northwestern Türkiye. This section is overdue for a major earthquake. The observatory will be jointly operated by GFZ and AFAD.