Seismometer selection

In this section factors to be considered when selecting a seismometer are discussed.

The noise floors for the three Trillium seismoemeters are shown in the following figure.
 

The Trillium 120 and Trillium 240 are well-suited to teleseismic studies, with the latter able to resolve the MLNM from 100s to 10 Hz.

Clip Levels

The clip levels of the Trillium family of seismometers are shown in the following graph. All of the sensors are capable of detecting an M6 event at 100 km epicentral distance without clipping.


The clip level of a seismometer always rises up at the lower corner period of the instrument. Unfortunately this is of no practical use, because there is no energy in a typical local or regional event below 10 s period and because teleseismic events above M8 are so rarely observed. The clip level of the Trillium 120 and Trillium 240 begins to fall off above 1.5 Hz, taking advantage of the fact that the spectral content of a typical local or regional event does too.

Seismometer selection

The graphs above do not tell the whole story. Because performance is fundamentally limited by existing 24-bit technology, this must be taken into consideration at the same time as you select your seismometer.
The following pages discuss digitiser configurations for each of the three Trillium seismometers.

Digitizer Configuration for Trillium 120P seismometer

Resolution

Currently the industry standard is the 24-bit digitizer. The discussion which follows is applicable to all well-made digitizers in this class, including the Trident and Taurus families offered at Nanometrics. As long as care has been taken to ensure good linearity and does not use gain-ranging, the performance will be essentially the same.

 

Noise Floor

The noise floor of a digitizer can be estimated once from the resolution and the clip level. The noise spectrum will be white at high frequencies. This means the power spectral density is constant with respect to frequency. At lower frequencies (i.e. generally 1-5 Hz) the digitizer noise will be dominated by flicker or 1/f noise. This means the power spectral density is inversely proportional to the frequency.
 
The noise floor of a well-designed digitizer will move up and down as the clip level is adjusted, with minimal deterioration of dynamic range.
 

Clip Level

Digitizers used for seismic recording generally have maximum clip levels of ±20 V on a differential input. Most digitizers can have their clip levels adjusted to lower levels. Some require changes to the hardware to do this, others such as the Trident and Taurus families from Nanometrics can be remotely configured through software.
 
The choice of digitizer clip level is done by taking into consideration the expected site noise and event magnitudes.
 

Input Referral

In order to compare the clip level and noise floor of the digitizer to the expected site noise and event magnitudes, it must be referred to the input of the sensor by dividing by the sensor transfer function. Since broadband sensors have a bandpass response with 40 dB/decade rolloff at high and low frequencies, the noise floor and clip level of the digitizer when referred to the input increases by the same amount at high and low frequencies.

For the Trillium 120, the clip level and noise floors at minimum and maximum digitizer gain are as follows:

Relative to the Trillium 240 the Trillium 120 trades some performance at long periods for reduced cost and extended temperatrure range without recentering. With a 24-bit digitizer at high gain, the Trillium 120P is 4 dB above the NLNM at 100 seconds and below the NLNM to 10 Hz, but there is a possibility that extremely large teleseismic events will clip. If on the other hand the 24-bit digitizer is configured for low gain, there may be difficulty resolving the NLNM at 10 Hz but it should be possible to record regional M7 and local M5 events without clipping.

Digitizer Configuration for the Trillium 240 seismometer

Resolution

Currently the industry standard is the 24-bit digitizer. The discussion which follows is applicable to all well-made digitizers in this class, including the Trident and Taurus families offered at Nanometrics. As long as care has been taken to ensure good linearity and does not use gain-ranging, the performance will be essentially the same.

 

Noise Floor

The noise floor of a digitizer can be estimated once from the resolution and the clip level. The noise spectrum will be white at high frequencies. This means the power spectral density is constant with respect to frequency. At lower frequencies (i.e. generally 1-5 Hz) the digitizer noise will be dominated by flicker or 1/f noise. This means the power spectral density is inversely proportional to the frequency.
 
The noise floor of a well-designed digitizer will move up and down as the clip level is adjusted, with minimal deterioration of dynamic range.
 

Clip Level

Digitizers used for seismic recording generally have maximum clip levels of ±20 V on a differential input. Most digitizers can have their clip levels adjusted to lower levels. Some require changes to the hardware to do this, others such as the Trident and Taurus families from Nanometrics can be remotely configured through software.
 
The choice of digitizer clip level is done by taking into consideration the expected site noise and event magnitudes.
 

Input Referral

In order to compare the clip level and noise floor of the digitizer to the expected site noise and event magnitudes, it must be referred to the input of the sensor by dividing by the sensor transfer function. Since broadband sensors have a bandpass response with 40 dB/decade rolloff at high and low frequencies, the noise floor and clip level of the digitizer when referred to the input increases by the same amount at high and low frequencies.
 

For the Trillium 240, the clip level and noise floors at minimum and maximum digitizer gain are as follows:

The Trillium 240 is ideally suited to teleseismic studies. With a 24-bit digitizer at high gain, the NLNM can be resolved from 10 Hz to 100 s, but there is a possibility that extremely large teleseismic events will clip. If on the other hand the 24-bit digitizer is configured for low gain, there may be difficulty resolving the NLNM at 10 Hz but it should be possible to record regional M7 and local M5 events without clipping.