Low frequency seismic sensor based on GPS interferometry

The ground deformation before, during and after a seismic event contains valuable information to understand the mechanics of earthquakes. Traditional seismometry is based upon the integration of an acceleration signal delivered by an elastically suspended mass near the resonance frequency. This approach is optimal for relatively high frequency deformations. However, there is evidence that non-negligible deformations take place at much lower frequencies, and a suitable detection device is therefore needed. To better understand and experiment with a non-accelerometric seismic sensor sensible to low frequency perturbations (10 Hz and below), we have developed a prototype seismometer based on GPS interferometry. Our breadboard consists of six NovAtel/CMC single frequency SmartAntennas configured in a two-dimensional array with spacing ranging between 1 and 5 meters. We have developed a real time software to log data from the six receivers and to compute and interpret the phase differences between pairs of receivers. The knowledge of the nominal coordinates of the receivers is used to solve and to monitor the integer ambiguities. We demonstrated that the data processing at each epoch, from this net, leads to relative coordinates between the receivers with root-mean-square repeatability between 4 and 8 mm horizontally and between 13 to 19 mm vertically. The resulting horizontal strain rates range from 0.8x10E-3 to 8x10E-3 1/s at a frequency of 1 Hz. The sensor is therefore effective only for large earthquakes (magnitude ≥ 5.9 and 7.5 for angular separations of 1� and 10� respectively). The precision of the results is limited mainly by multipath. The effect of multipath can be mitigated using a calibration signal optimized for the site where the sensor is placed.