b'FeatureA beginners guide to seismic sensors4.5 and 10Hz geophones differ only in their natural frequency,springs from a frame, the entire sensor being smaller than a coin. whereas the 2 Hz geophone also has increased sensitivity. Unlike a geophone the resonant frequency of a MEMS sensor is If we know the noise level of the system, then we can determineabove the seismic bandwidth. If the acceleration of the sensor the effective minimum frequency, being simply the point atcauses the mass to move, then the central position of the mass which the sensitivity of the geophone drops below the systemis maintained through the application of an electrostatic force noise level (Figure 7). (Tessman et al. 2001). Unlike geophones that have an output proportional to velocity, MEMS have an output proportional Although now not commonly used, an alternate sensor to theto acceleration. Due to their relatively small size, they are often geophone is the geophone accelerometer or GAC (Figure2b).included in 3-component configurations. The acceleration The GAC consists of an individually calibrated geophone elementresponse of MEMS sensor is flat between about 3 and 200Hz. For combined with an operational amplifier (op-amp) and a feedbackgeophones the dominant source of noise is electronic whereas circuit to control the output response. The op-amp included in thefor MEMS the dominant source of noise is thermo-mechanical. GAC design responds to the voltage generated by the movementThermo-mechanical noise is proportional to the inverse of the of the geophone element and passes an electrical current backfrequency making them ill-suited to particularly low frequency to the geophone which heavily damps the motion of the element3 to 4 Hz recording (Margrave etal. 2012). When converted to coil. As the force required to keep the geophone coil stationaryvelocity the response is linear and increases at a rate of 6 dB/is proportional to the acceleration of the coil, the output voltageoctave. The major problem with the initial MEMS accelerometers of the GAC is proportional to acceleration. The heavy dampingwas their relatively high system noise, around 100 ng/ (Tessman reduces the range of coil motion by an order of magnitude, alsoet al. 2001) although this has been reduced to 30 ng/ or 15 ng/ reducing signal distortion by more than an order of magnitude.for the latest Accuseis and WiNG systems respectively (Figure9). Figure 8 shows the acceleration amplitude response curves forMEMS also suffer from relatively high power-consumption a 10Hz geophone and a GAC, the response of the GAC being(the geophone version of the Quantum node has a battery flat across almost the complete bandwidth of interest.life of 50days whereas the MEMS version is limited to 23 days, Another type of sensor now becoming more common inalthough weight has been reduced by 26%).geophysics is the MEMS (micro-machined-electromechanical- The most recently introduced sensor type is the piezo-electric systems). A MEMS sensor consists of a tiny mass suspended byaccelerometer incorporated into the lightweight (150g) Stryde Figure 6. Sensitivity curves for 10, 5, and 2 Hz geophones.Figure 5. Signal (red) and system noise (blue) strengths before (top) and afterFigure 7. Diagram showing the response of 5 Hz (SG-5) and 10 Hz (SG-10) (bottom) geophone response correction.geophones relative to the system noise level.40 PREVIEWJUNE 2024'