| A geophone is a seismometer that has been commercially available for decades. It is robust and affordable, but only measures local, high frequency signals (5–500 Hz). As a result, geophones can't detect large distant earthquakes, such as the Kobe or Northridge events of the mid 1990's. Broadband seismometers can measure the seismic signals from such large events after they have crossed the globe, but these instruments typically cost on the order of {dollar}10,000. This makes detecting large earthquakes across the Earth impractical for groups that have an interest in global seismology but are lacking in resources.; To address this need, a geophone, which costs {dollar}50, was used as the building block of an affordable, broadband seismometer. A geophone was first converted into a feedback seismometer, with the conventional geophone output, its coil velocity, as the feedback signal. Then, a geophone was modified to measure coil position capacitively. This results in improved low frequency sensitivity and resolution. After reducing environmental noise sources, a capacitive geophone with a resolution of 10−6 (m/s) √Hz at 30 mHz (33 sec period), the frequency of a typical signal from a distant earthquake, has been demonstrated. This improved performance allows a capacitive geophone to detect distant earthquakes. |
