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Electromagnetic short-period seismographs

Electromagnetic seismometers and geophones are passive sensors whose self-noise is of purely thermal origin, and does not increase at low frequencies as it does in active (power-dissipating) devices. The signal level at their output is however comparatively low, so a low-noise preamplifier must be inserted between the sensor and the recorder (Fig. 20). Unfortunately the noise of the preamplifier does increase at low frequencies and limits the overall sensitivity. We will call this combination an electromagnetic short-period seismograph or EMS because in view of the superior performance of feedback instruments, it is now rarely used for long-period or broadband recording.

The sensitivity of an EMS is normally limited by amplifier noise. However, this noise does not depend on the amplifier alone but also on the impedance of the electromagnetic transducer (which can be chosen within wide limits). Up to a certain impedance the amplifier noise is nearly constant, but then it increases linearly with the impedance, due to a noise current flowing out of the amplifier input. On the other hand, the signal voltage increases with the square root the impedance. The best signal-to-noise ratio is therefore obtained with an optimum source impedance defined by the corner between voltage and current noise, which is different for each type of amplifier and also depends on frequency. Vice versa, when the transducer is given, the amplifier must be selected for low noise at the relevant impedance and frequency.

Figure 20:Two alternative circuits for a preamplifier with a low-noise op-amp. The non-inverting circuit is generally preferable when the damping resistor Rl is much larger than the coil resistance and the inverting circuit when it is comparable or smaller. However, the relative performance also depends on the noise specifications of the op-amp. The gain is adjusted with Rg.

The electronic noise of an EMS can be predicted when the technical data of the sensor and the amplifier are known. Semiconductor noise increases at low frequencies; amplifier specifications must apply to seismic rather than audio frequencies. In combination with a given sensor, the noise can then be expressed as an equivalent seismic noise level and compared to real seismic signals or to the NLNM (5.1). As an example, Fig. 21 shows the self-noise of one of the better seismometer-amplifier combinations. It resolves minimum ground noise between 0.1 and 10 s period. Other examples and general discussions are found in [Riedesel et al. 1990] and [Rodgers 1992a,Rodgers 1993,Rodgers 1994]. The result of all these considerations is easily summarized: most well-designed seismometer-amplifier combinations resolve minimum ground noise up to 6 or 8 s period, this is, to the microseismic peak. A few of them may make it to 10 or 20 s (this is, resolve the secondary microseismic peak near 15 s). To resolve minimum ground noise up to 30 s is hopeless, as is obvious from our Fig. 21: ground noise falls and electronic noise rises so rapidly beyond a period of 20 s that the crossover point cannot be substantially moved towards longer periods. Of course, at a reduced level of sensitivity, restoring long-period signals from short-period sensors may make sense.

Figure 21:Electronic self-noise of the input stage of a short-period seismograph. The sensor is a Sensonics Mk3 with two 8 kOhm coils in series and tuned to a free period of 1.5 s. The amplifier is the LT1012 op-amp. The curves a and b refer to the circuits of Fig. 20. NLNM is the USGS New Low Noise Model. The units are as in Fig. 18.

Amplifier noise can be observed by locking the sensor or tilting it until the mass is firmly at a stop, or by substituting it with an ohmic resistor that has the same resistance as the coil. If these manipulations do not significantly reduce the noise, then the seismograph does obviously not resolve seismic noise. This is however only a test, not a way to precisely measure the electronic self-noise. A locked sensor or a resistor do not exactly represent the electric impedance of the unlocked sensor.

next up previous contents
Next: Force-balance seismometers Up: Instrumental self-noise Previous: Instrumental self-noise
Erhard Wielandt