b'FeatureA beginners guide to seismic sensorsof a well coupled vertical geophone whilst the red line is a poorly coupled horizontal geophone (horizontal geophones are particularly susceptible to the rocking effects of poor coupling). This effect is also clearly visible on field data, Figure 13 shows power spectral density traces recorded with five differently coupled three component (3C) geophones (only the horizontal components are shown), as the coupling improves (e.g. from blue to red to green) the coupling induced resonance frequency clearly increases. Given the large variety of shapes of nodes currently available it is not possible to compare their ability to record data under all different surface conditions. It is possible, however, to test their coupling ability by simply inserting them into the ground and measuring how much force is required to remove them using a simple digital force meter.When a geophone is deployed, we also need to ensure that Figure 11. Estimated signal to ambient noise improvement for low frequencyit is oriented correctly (e.g. vertically for vertical geophones). (10 Hz data, (top) the theoretical relationship ( and (bottom) a more realisticConventional geophones have relatively strict tilt (the angle the relationship based on field measurements using 20 cm spaced geophones. Thegeophone can deviate from the vertical) criteria, for example the dashed red line in the top plot indicates the maximum values on the scale of themaximum angle for a 10 Hz geophone is 10. If the tilt limit is lower plot. Estimates are based on a linear array covering 30 m. exceeded, then distortion increases (Figure 15) and eventually the geophone will not operate as there will not be enough gravitational force on the spring inside the geophone to stop the coil moving to the top of the case. As the natural frequency of the geophone decreases the tilt criteria increases. Although geophone tilt tests are available on most recording systems the tests are essentially a test of geophone string sensitivity and may be affected by other faults. The distortion resulting from tilted geophones also depends on the direction of tilt, with geophones tilting perpendicular to ground-roll propagation being particularly susceptible to distortion (Dean, Claasen, and McCarthy 2014), this is particularly important for sensors whose performance is not affected by tilt, i.e. a vertical geophone will not work on its side, on the other-hand an accelerometer will work but will obviously record spurious data that may be difficult to identify.The tilt angle at which a geophone is considered bad is usually ~20. As nodal surveys generally deploy far fewer sensors (although have a larger total channel count) than geophone-array surveys more time can be spent ensuring that the Figure 12. Modelled coupling results (a) shows the results for a well coupledgeophones tilt and coupling is optimal. Even with this ability, tilt vertical geophone with a coupling resonance frequency of 200 Hz and a poorlymeasurements from a typical survey (Figure 16) show that 5% coupled horizontal geophone with a resonance frequency of 30 Hz. This resultsof sensors have a tilt 19, 80 % have a tilt 12, the average tilt in an anomalous ratio peak (b) at 30 Hz (Dean, Shem, and Hasani 2018). being 7.6 (Figure 16 top). Using Figure 15 to convert this to an estimate of distortion for conventional geophones indicates that distortion due to sensor tilt cannot be overlooked.Discussion and conclusionsSometimes (but not always) we are faced with a choice about which sensor to use for seismic acquisition (Figure 14), be it active or passive. We have developed the following process to help guide such a decision:1.Determine if single-component or three-component recording is required, the latter is most commonly used for passive seismic data acquisition.2.Determine the bandwidth of interest, for active seismic this is most likely the bandwidth of the Vibroseis sweep, for Figure 13. Normalised average PSD spectra obtained using a sledgehammerpassive seismic we are likely to be primarily interested in source at an offset of 20 m for the horizontal components of 3C geophoneslowfrequencies.sitting on their spikes (blue), with the geophone pushed half-way into the ground (red), with the spikes fully inserted (green), buried beneath the surface3.Compare the response curves for the different sensors (magenta) and without legs (cyan) (Dean, Shem, and Hasani 2018). (e.g.Figure 3), if including accelerometers in your comparison 42 PREVIEWJUNE 2024'