b'FeatureA beginners guide to seismic sensorsA beginners guide to seismic sensorsTim Dean and Matt Grant Anglo American Steelmaking Coal, Brisbane, AustraliaEtim.dean@angloamerican.comIntroduction response or sensitivity curve that gives the output voltage in terms of the velocity of the ground movement across a range The last ten years have seen a revolution in seismic acquisition,of frequencies (e.g. Figure 3). An ideal seismic sensor has a high primarily driven by the proliferation of cheap, light-weight, andsensitivity and a consistent response across the full bandwidth of reliable nodal recording systems (Dean et al. 2021; Dean, Grant,interest (although not shown here, the geophone also has a phase and Nguyen 2020; Dean and Sweeney 2019). Given the number ofresponse that varies across the bandwidth). As shown in Figure3, units being offered, we are occasionally asked for advice on howhowever, the sensitivity of a geophone peaks at its natural to choose between them. Often such a choice is heavily impactedfrequency, in this case 10 Hz (the most commonly used geophone by operational considerations, but before addressing those wetype). To flatten the response curve around the natural frequency first need to determine what would constitute the most ideala resistor is added across the geophone, such a geophone is system geophysically. In this paper we aim to give the reader ideasreferred to as being damped. The response of a geophone is on how they can help determine the most appropriate systemoften simply referred to by its natural frequency, i.e.a 10 Hz for their specific application. We begin by looking at the differentgeophone is simply a geophone with a natural frequency of types of sensors available and their relative performance.10 Hz, as this is colloquially thought of as the lower limit of the We then discuss the importance of the deployment of thesegeophone, although as we shall see, this is not quitetrue.sensors, in particular coupling and tilt. Finally, we detail a suggested process that can be used to help compare differentThe upper frequency limit of a geophone depends on its spurious acquisition systems. Note that we do not address operationalfrequency. Just as the natural frequency is the resonant frequency considerations, we save this subject for a future work of the geophone element in the desired direction (usually vertical), the spurious frequency is the resonant frequency perpendicular to the desired direction (Faber and Maxwell The sensor 1996). As the natural frequency of the geophone decreases the The original, and still the most common, sensor used for landspurious frequency also decreases (Figure 4) thus the choice of seismic surveys is the geophone. The geophone consists of a caseelement (assuming one is possible) is most often determined by that contains the geophone element (Figure 1) and a spike orthe range of frequencies that we may wish torecord.plate to ensure it is coupled to the earth (e.g. Figure 2a). A modernComparison of the spurious frequency plot with the typical geophone element contains a coil that is suspended within aseismic bandwidth explains why the 10 Hz geophone is magnetic field by a spring. When a vibration occurs the coil remainsmost popular, despite the better low-frequency response of stationary while the case moves, generating a small magnetic field.5Hz geophones. 4.5/5 Hz geophones have a lower spurious The amount of voltage generated by the movement is thefrequency (typically 90-150Hz), which often puts them within response of the geophone and is usually summarised by athe bandwidth of interest. Low-frequency geophones also have additional drawbacks. They tend to be heavier, although this is a bFigure 1. Photo of a geophone element, the damping resistor can be seenFigure 2. A standard geophone with a spike (left) and a GAC geophone connected between the two terminals. mounted on a plate (right). Both contain a geophone element, but the GAC contains additional controlling electronics.38 PREVIEWJUNE 2024'