RELIABLE AND ECONOMICAL HIGH-TEMPERATURE DEEP-BOREHOLE SEISMIC RECORDING

Recording earthquakes with borehole seismometers has become increasing ly popular in recent years as the advantages in noise reduction and al so the distorting effects of near-surface rocks, especially sediments, have become well known. Borehole recording can be extremely complex, involving active sensors, special cables, and downhole electronics. Su ch installations, however, are often not very reliable at the high tem peratures reached in active tectonic areas at depths of 1 km and great er. Here we describe a simple and reliable system for 3-component reco rding of local earthquakes at single and multiple depths greater than 1.5 km and temperatures up to 120 degrees C. Our system was designed a nd developed for experiments in the Cajon Pass Scientific Drillhole of Southern California. The borehole packages are made of titanium with spring-loaded clamping, allowing easy retrieval. Standard seven-conduc tor oil-well logging cables are used together with a specially designe d cablehead. The data recorded have been used for investigating earthq uake source scaling, attenuation in the mid-crust, and also near-surfa ce site effects.