b'FeatureA beginners guide to seismic sensorsFigure 8. Amplitude response curve for a 10 Hz geophone and a GAC. Note that this is acceleration not velocity. The frequency limits of interest (2 to 150 Hz) are shown by the red lines. Figure 9. From left: Sercel WiNG, INOVA Accuseis and Styde nodes.node (Figure 9 right). The node has a quoted noise level of 22ng/(Dean, Dupuis, and Hassan 2015). We can estimate the low-and benefits from the piezo being a passive sensor (i.e.itdoesfrequency ambient noise attenuation at low frequencies using not require power) and the use of the internal battery as themeasurements made with very close geophone spacings sensors reaction mass (Manning, Ablyazina, and Quigley 2019). (e.g.0.2 m) that are then filtered to a maximum frequency of 10Hz. These results (Figure 11) show that the improvement is Arrays far smaller than that predicted (indeed the lines cannot even beplotted on the same scale), with very little improvement Modern digital sensors, be they GACs, MEMS, or piezo-electricbeyond the use of three or four sensors over 30m.accelerometers, are recorded individually and not formed into the arrays commonly employed in the past for geophoneDeploymentrecording. Despite their increasingly rare usage, primarily due to the proliferation of high channel count systems, we will brieflyWhen a geophone is deployed there are two major describe their attributes. Within an array, geophones can beconsiderations, the coupling of the geophone with the ground, connected in series, parallel, or a combination of the two (Table1).and the tilt angle at which the geophone has been deployed. We The ideal geophone array is one that has high output and lowbegin by looking at coupling. The term coupling describes how impedance (high impedance increases electrical noise pickup onwell a geophone is attached to the ground and therefore how the cable, static noise pickup and harmonic distortion (Pap 1984)). faithfully measurements made using it reflect ground motion. Just as the response of a geophone depends on the natural frequency Table 1. The resulting output and impedance for a stringof the geophone, the coupling response also has a resonant of geophones connected in different ways. E and Z are thefrequency (Krohn 1984). If this resonant frequency appears inside output and impedance of a single geophone respectively,the bandwidth of interest, then an anomalous amplitude peak N is the number of series elements in one leg of thewill occur. This is demonstrated using a simple modelled result pattern and M is the number of parallel legs (Pap 1984). in Figure 12; the blue line in Figure 12a shows the response Connection Output ImpedanceSeriesNE NZParallel NE (1/M)ZSeries-parallel NE (N/M)ZFigure 10 shows the resulting output and impedance values for different configurations of a string of 24 geophones. When the geophones are connected in series the output is high but so is the impedance, when connected in parallel impedance is low but so is output. Typically a series-parallel configuration is used as this gives the best compromise between output andimpedance.In terms of coherent noise suppression point receivers are considered to be superior as they allow digital noise removal rather than the fixed response of a geophone array (Regone 1998). Ambient noise reduction is improved by using strings of sensors, although generally less than the theoretical valueFigure 10. Different configurations of a 24 geophone string. N is the number of due to correlation between the noise measured by the sensorsgeophones in one leg and M is the number of parallel legs.JUNE 2024PREVIEW 41'