b'FeatureA beginners guide to seismic sensorsmore an issue with geophones that have natural frequencies of 2Hz and below, the difference between 5 and 10 Hz geophones being minimal. They also tend to have higher distortion (e.g.the Sunfull PS-10R 10 Hz geophone has a distortion of 0.1% whereas its 5Hz equivalent, the PS-5GR, has a distortion of 0.2%) and lower tilt tolerance, i.e. they must be placed carefully to ensure they are vertical. Although experiments have been successfully conducted using co-located 2Hz and 10Hz geophones (Chiu etal. 2012) this is an inefficient way of collecting data.The poor low frequency response of geophones can be overcome by appropriate filtering to correct for the geophone response, in terms of both amplitude and phase (Bertram and Margrave 2011), but this relies on the signal strength of the lowest frequency of interest being sufficiently above the system noise. This is illustratedFigure 3. The response curves for damped and open 10 Hz geophones.in Figure 5; the graph at the top shows the amplitude of the signal in red (the effect of the sensitivity of the 10 Hz geophone clearly visible) versus the system noise in blue. The signal drops below the system noise at about 3Hz. After the correction (the lower graph) the signal spectrum is now flat, but the system noise has also increased. What this example shows is that although we can boost the low frequency content of data recorded using geophones by correcting for the geophone response, we cannot overcome the inherent signal to system noise limitations.As well as the natural frequency of a geophone we must also consider its sensitivity. The higher the sensitivity of a geophone the greater the voltage that will be induced by ground movement and the more sensitive the geophone will be. ForFigure 4. Relationship between the spurious and natural frequencies of example, Figure 6 shows the response of three geophones, thedifferent Sunfull geophone elements.JUNE 2024PREVIEW 39'