GY: Geyokcha Array, Turkmenistan

• University of California, San Diego (UCSD), United States of America

As part of the Joint Seismic Program facilities in Central Asia an experimental high­ frequency array was installed in Turkmenistan in August 1993 adjacent to the GSN station ABKT. The Geyocha array was designed to try to push array signal enhancement capabilities to higher frequencies than existing array designs such as NORESS. This requirement was driven by a long-term goal of the JSP array group to try to extend the performance of seismic anays to wider frequency bands than those exploited in existing monitoring facilities. This is important because most seismic discrimination methods that use any form of signal processing are fundamentally spectral. That is, they look for some type of difference in the output of a source at different time scales (e.g. the Ms-mbdiscriminant compares long-period surface wave to P wave signal levels.). The high-frequency end of the spectrum is especially troublesome due to some fundamental processes related to interaction of seismic waves with the uppermost part of the crust. The Geyocha anay has proven valuable in the more conventional role of a seismic array. The existence of the Geyocha array significantly lowers the detection thresholds in thi s area of the world. In addition, however, this array has collected a wealth of fundamental data on high-frequency wave propagation that we can exploit for improving our basic understand ing of how high-frequency seismic waves propagate in the real world.

A unique design feature of the Geyocha array (Figure 2) is that it is a hybrid of two very different technologies. Twelve stations in the array are equipped with state-of-the-art STS-2 broadband sensors, which allow the array to record signals at frequencies as low as 0.0 I Hz. The remaining 36 stations in the array are equipped with 12 element strings of triaxial L-28 geophones. This equals a total of 1296 total sensors located at
432 different spots on the ground. This i s a large number of
sensors in a small piece of real estate, but the idea of this design is founded on well developed technology for data acquisition at freq uenc ies over 10Hz in reflection seismology .

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