b'FeatureA beginners guide to seismic sensorsFigure 14. A selection of different nodes currently in use in order of their year of introduction (left to right).Figure 15. Distortion vs. tilt for two types of geophones and a GAC. 0.01%distortion is typically about -80 dB, 0.03% is typically about -70 dB.then you will need to either convert the acceleration curves (e.g.Figure 8) to velocity or vice-versa to enable a comparison with velocity sensors. It is not sufficient to simply compare natural frequency values as differences in sensitivity can result in a high-sensitivity 10Hz geophone being superior to a standard-sensitivity 5Hz geophone at low frequencies (e.g.Figure 6). Figure 16. Top: A cumulative histogram of sensor tilt measurements (n600000). 4.Use the noise level of the recording system (typically given inBottom: The resulting histogram of distortion for a conventional geophone.the system specifications) to generate a response curve that shows the true possible recorded bandwidth (e.g. Figure 5). If undertaking active seismic work you will need to compare the response to the ambient noise level (which would require example noise data) as the ambient noise level is typically considerably higher than the system noise level.5.Establish the likely deployment effects of the sensor. In particular, how does the form factor of the sensor (Figure17) affect our ability to plant it vertically and with adequate coupling? This is very important for 3-component recording where the horizontal components are particularly sensitive to coupling, the lower frequency geophones which have a lower tilt tolerance, and the sensor must be aligned with north. The combination of these factors makes placing them difficult as you can easily degrade the coupling whilst adjusting the node to make it level. It may, therefore, be more effective to create an appropriately shaped vertical hole to insert the node into. Low frequency seismometers (Figure18) often have adjustable feet on their base to allow for them to be very accurately (3) levelled. For nodes that employ external sensors, it must be considered that wind caninduce noiseFigure 17. Examples of 3C nodes, from left: SmartSolo IGU-16HR 3C, through the sensor wires (Dean and Sweeney 2019).NuSeis3C, and AllSeis-3C.Note that we have not addressed any operational or budgetary issues, well save those for a future paper.JUNE 2024PREVIEW 43'