The geophone and front-end fidelity

Seismic acquisition on land is expensive. Even small improvements in data quality can be costly in terms of increased field effort. Instrument performance should never be allowed to become a limitation, and for this reason digital systems have been designed to record to an accuracy which more than matches the potential of processing techniques. However, the system is no better than its weakest link and it is therefore desirabie that we identify this and avoid practices, however standard these may have become, which place unnecessary restrictions on recording fidelity. When specifying a seismic system, we need to give particular attention to the performance of the analogue elements between ground motion (or water pressure ) and the analogue-to-digital converter. Digital specifications are robust and easily checked, but specifications at the analogue 'front end' can be ambiguous or misleading and can be difficult to verify, especially in the field. This part of the system calls for careful consideration, especially of its performance at low frequencies. One of the characteristics of land seismie data is the high content of source-generated and other noise which travels in various surface modes. This noise is usually of lower frequency than most of the signal and may, at the points of detection, be stronger than the signal by orders of magnitude. This is particularly the case where surface sourees are employed. The industry-wide popularity of geophones with natural frequencies of 8 to 10 Hz suggests a lack of adequate consideration for the problems which surround this analogue interface. The following offers three separate arguments which favour natural frequencies approximately an octave higher than those popularly in use. These relate to: (1) Low-frequency performance of the geophone. (2) Low-frequency performance of the analogue 'front end' electronics. (3) High-frequency spurious modes within the geophone. Each of these arguments is, of itself, convincing. Taken together the case for higher frequency geophones appears to be unanswerable. The popularity of the lO-Hz geophone remains unchallenged through much of the seismic contracting industry because its disadvantages are rarely acknowledged.