CALIBRATION OF SEISMOMETERS USING GROUND NOISE

We have developed and tested a new technique for calibration of seismo meters using continuous recordings of ground noise. The method is foun ded on analytic techniques recently developed for estimation of transf er functions in magnetotellurics. We find that the technique can produ ce precise, absolute calibration measurements on sensors that do not h ave calibration coils. The data used are obtained by placing two sets of sensors close enough together that we can assume they record the sa me ground motion. It is further assumed that one of the sensors has a known, absolute calibration. One then records ground noise of sufficie ntly high amplitude to guarantee that one is recording above the ampli fier noise floor across the entire frequency band of interest. Data ar e recorded for a time period that depends upon the lowest frequency th at is to be resolved. The data is then divided into a series of N part ially overlapping time windows, transfer-function estimates are calcul ated from each of these N time windows, and finally a robust mean esti mation procedure is used to produce transfer-function estimates at a s et of discrete frequencies. We applied this technique to produce calib ration estimates for four different types of sensors (GS-13, triaxial 4.5-Hz L-28, STS-2, and triaxial L4) in various recording arrangements . We found that the technique worked extremely well in every case at f requencies above the point where the sensor output dropped into the in strument noise floor. Problems were consistently encountered above som e high-frequency limit that depended upon the site and sensor being te sted, and, as a result, we conclude that obtaining reliable results at higher frequencies requires more care in the experimental procedure. We show results from 4.5- and 1-Hz passive sensors plastered onto the same pier, which show nearly perfect coherence out to 100 Hz, and exce llent agreement with theoretical predictions between 0.03 and 20 Hz. H owever, above 20 Hz, a systematic phase error plagues our results. Oth er cases were comparable when care was taken in the experimental proce dure, but differed in detail. We argue that there are fundamental prob lems recording ground noise at these higher frequencies as a result of the following three experimental problems that can be difficult to co ntrol: (1) coupling of sensors to a common, stable platform, (2) conta mination by acoustic and pier resonances in typical recording vaults, and (3) resonances of the sensor-pier-ground system.