Most of the signal processing examples are done with the SEISAN earthquake processing software (Havskov and Ottemöller, 2000) (www.geo.uib.no) and there are also examples using the SEISLOG data acquisition software (Utheim et al, 2001). Most of the book was written during a sabbatical of one of the authors (JH) at the University of Granada with support from the University of Bergen. Lennartz), Paul Passmore (RefTek), Christoph Kundig (GeoSig) and Robert Leugoud (Earth Data), Nathan Pierce (Güralp Systems), Ogie Kuraica (Kinemetrics), Dieter Stoll (www.geo.uib.no).
Introduction
- Sensor
- Recorder
- Stations and networks
- Arrays
- Instrument correction and calibration
Assuming that the number 1 from the digitizer corresponds to the actual input signal (and not digitizer noise), then the larger number corresponds to the dynamic range. The use of groups makes it possible, for example, to "direct" the sensitivity of the network to a source point, with an improved signal-to-noise ratio (SNR) on the verification of seismology and the ban on nuclear tests in local and regional seismology. , including seismo-volcanic sources, to be used as a powerful tool for seismologists.
Seismic sensors
The standard inertia seismometer
Intuitively, you would expect the mass to initially remain stationary while the floor moves up. The sign is negative because the maximum amplitude of the mass motion will be delayed relative to the ground motion, see Figure 2.2.
Seismometer frequency response
If the frequency is low, the z term will dominate and the equation can be approximated as From the shape of the curve and (2.10), we can see that the seismometer can be considered a second-order high-pass filter for ground displacement.
Seismometer frequency response, alternative solution
As can be seen, low damping (h
The velocity transducer
- POLARITY CONVENTION
Dash damping was replaced by coil damping moving in a magnetic field (redrawn and modified from Scherbaum, 2001, courtesy of Kluwer Academic Publishers). In an electromagnetic transducer sensor, as explained above, a voltage E is induced in the coil that is proportional to the speed E=Gz&, where G is the generator constant.
Instrument response curves, different representation
This means that the final form of the frequency response, to use in practice, (2.24) is multiplied by –1 and the displacement response becomes . = ) (ω dv. Note that the shape of the accelerometer response is the same as that for the mechanical seismometer, only ω0 has changed.
Sensor response to transient signals
Thus, it is the ground acceleration that is the sole cause of the forced motion of the mass, regardless of the response of the sensor, which may be flat to ground displacement, velocity or acceleration in a given frequency band. Nevertheless, it must be kept in mind that it is precisely the ground acceleration that generates the motion of the mass.
Damping constant
It can be shown from (2.38) that the ratio of the amplitudes of two consecutive extreme values z1/z2 can be written as Thus, the actual attenuation of a given seismometer can be estimated by measuring the amplitudes of two consecutive extreme values.
Construction of seismic sensors
- THE WOOD ANDERSON SHORT PERIOD TORSION SENSOR
- LONG PERIOD SENSORS
- GARDEN-GATE
- INVERTED PENDULUM
- LACOSTE
For a mass of small size m compared to the length L of the string, the natural frequency is L. The sensor uses a "zero-length spring" designed so that the force is F=k · L, where L is the total length of the string. spring.
Sensor calibration coil
An astatic leaf spring suspension is comparable to a LaCosta suspension but is much simpler to manufacture (Figure 2.17). Once the motor constant and calibration coil mass are known, all other constants can in principle be determined as explained in Chapter 10.
Active sensors
As the servo system will have delays, the system will have a limited upper frequency response and additional phase compensation is required in the loop to ensure stability e.g. The natural frequency of the whole system is electrically increased, since the servo system will act as a stiff spring (see 2.15).
Accelerometers
This is usually done either by including a phase lead circuit (advancing the phase, Usher et al. 1977)) in the feedback or an integrator that follows the transducer. Additional amplifiers can be added to the accelerometer to change the value of G, but essentially the generator constant is determined by only 3 passive components, of which the mass per definition is flawless, the resistor is a nearly ideal component, and the force transducer can be made very precise because the movement is so small.
Velocity broadband sensors
The speed response of the BB sensor is mainly determined by 4 passive components (R, C, mass and feedback coil) and can be considered to behave like a normal speed sensor. This is the upper frequency limit of the feedback system and the response is no longer equal to the velocity.
Extending the frequency response, inverse filtering and feed back
The strong negative feedback is equivalent to a very high damping since the feedback force is proportional to the mass velocity, and, as we have seen from (2.15), if the damping is high, then the output from the seismometer is proportional to the acceleration. velocity for a mechanical sensor) within a limited band centered on the resonance frequency. So we can say that a strong negative velocity feedback is equivalent to a negative external damping resistance.
Theoretical aspects of active sensors
- GENERAL RESPONSE OF A FEEDBACK SYSTEM
- THE BASIC FORCE-BALANCED ACCELEROMETER
- BROADBAND FEEDBACK SEISMOMETERS
- OTHER FEEDBACK TECHNIQUES
We assume that connecting the output of one system to the input of another system does not affect its individual response (Figure 2.24). T's new input is now X =U−β⋅Y, but since its output must be Y =T⋅X, or. This result is important because, for a high loop gain, a) the elements within the main (forward) path in the loop (mechanical system and displacement transducer) have little influence on the closed-loop response, e.g. the sensitivity of the transducer D does not appear, and b) the response is controlled by passive (in this case), stable elements in the feedback path (resistance and coil).
Sensor self noise
Only at 0.1 Hz will the noise be equivalent to the NHNM level, as can also be seen in Table 2.1. The figure shows the noise contributions of the sensors themselves and the total noise.
Noise in passive sensors coupled to amplifiers, theoretical aspects
This transformer also connects the equivalent circuit to the actual signal coil circuit. To obtain the total voltage PSD at the output of this amplifier connected to the seismometer, we include the noise sources of the noise model in the specified circuit.
Some new trends in seismic sensors
- SEISMOMETERS WITH ELECTROCHEMICAL TRANSDUCERS
- MICROMACHINED ACCELEROMETERS AND SEISMOMETERS
This is shown in Figure 2.37, along with the equivalent input noise acceleration due to the sensor alone. The idea is to build the mass-spring system within the multilayer structure of an integrated circuit, using the same photo-lithographic techniques (Figure 2.39).
Sensor parameters
- FREQUENCY RESPONSE
- SENSITIVITY
- SENSOR DYNAMIC RANGE
- SENSOR LINEARITY
- SENSOR CROSS AXIS SENSITIVITY
- SENSOR GAIN AND OUTPUT
Dynamic range is the ratio between the largest and smallest signal that the sensor can record. For mechanical constraints, the dynamic range is estimated from the ratio of the output for the largest mechanical movement to the input noise of the amplifier.
Examples of sensors
- EXPLORATION TYPE 4.5 HZ GEOPHONE
- SHORT PERIOD SENSOR, THE L4-C
- ACCELEROMETER, THE KINEMETRICS EPISENSOR
- BROADBAND SENSORS, THE STRECKEISEN STS-1 AND STS-2 AND GÜRALP CMG-3TGÜRALP CMG-3T
- NEGATIVE FEEDBACK SENSORS, LENNARTZ LE-3D
- BOREHOLE SENSORS
It is often used with circuits that extend the frequency range, see above and the Lennartz sensor example below. The negative feedback sensor is relatively new to the sensor market and one of the first models sold was made by Lennartz.
Summary of sensor specifications
Most sensors are weatherproof, have some sort of calibration input, and have an attenuation close to 0.7. If sensors are sold in single-component and three-component versions, only the three-component version is listed.
Which sensor to choose
They are expensive, require very elaborate and expensive earthquake protection and are generally difficult to install. Long-lasting passive sensors are not a suitable choice for new installations and are no longer sold.
Seismic noise
- Observation of noise
- Noise spectra
- Relating power spectra to amplitude measurements
- Origin of seismic noise
If you compare the amplitudes with figure 3.3, the noise level can be considered to be worse or better than the average depending on which filter band is used. The spectrum in Figure 3.5 shows a relatively high noise level at lower frequencies compared to high frequencies.
Analog to digital converter
Example of a simple analog to digital converter, the Flash ADC
The ADC should now have the output range +2 to -2 counts instead of the 0-3 counts (when the offset of 2 is subtracted). Flash ADCs are not widely used in seismology, but illustrate some of the features of the ADC.
Basic ADC properties
This way of giving ADC dynamic range is the most common and there is no confusion as to what is meant. The smaller amplitude signal (right) has an offset of V/2, and the input signal that can be picked up by the ADC is now limited to +V/2 to –3V/2.
A typical ADC, the Ramp ADC
A small offset is of no consequence, but any offset will limit the dynamic range as the ADC will reach its maximum value (positive or negative) for smaller input values than its nominal full scale. An improvement on the ramp ADC is the successive approach ADC, which is virtually identical to the ramp ADC except that it has a more sophisticated control circuit.
Multi channel ADC
The ADC has a so-called multiplexer on the front end that connects the ADC to the next analog channel as soon as the conversion is complete. If the ADC is fast, the skew can be very small, but in the worst case, the ADC has enough time to take all the samples, and the skew is the sampling interval divided by the number of channels.
Digitizers for a higher dynamic range
The input signals are therefore not sampled simultaneously and there is a time shift, called skew, between the channels. The skew must be known and corrected for some types of data analysis.
Oversampling for improvement of the dynamic range
No electronic circuit is ideal, and thus limitations in the accuracy of the actual components will introduce oversampling limits. This is because the difference in dynamic range in the AD7710 is not only a matter of sampling, but also of the degradation of the performance of the electronic circuits due to the high sample rate.
Sigma Delta ADC, SDADC
- HOW SIGMA-DELTA IMPROVES DIGITIZATION NOISE: THEORY
Digitizing this gives 5 which corresponds to 0.5 V and the average is 0.5 which is our first estimate of the input signal. This is now converted back to analog and fed to the inverting input of the input amplifier.
Aliasing
The only way to avoid this problem is to ensure that the input signal does not contain energy above the Nyquist frequency. Therefore, the ADC must have a low-pass filter (anti-aliasing filter) to remove unwanted high-frequency energy above the Nyquist frequency before sampling.
Anti alias filters
If we follow (4.4) consistently, the output of the filter is always determined by the N original samples and the filter is always stable, since it depends only on the weighted average of the N samples. Zero phase filters are difficult to implement accurately and as a result IIR filters always have phase distortion within the filter passband.
Examples of digitizers
Most specifications given are 'typical' and as dynamic range changes with sample rate, most dynamic range specifications are given at 100 Hz or in a few cases at 50 Hz. So either a higher gain or a higher dynamic range ADC (with the same max input voltage) must be used.
Seismic Recorders
Analog amplifier
- DIFFERENTIAL INPUT-OUTPUT
Ideally, the input stage of the amplifier should be optimized for the sensor to which it will be connected. 1 A more accurate procedure is to connect a resistor equivalent to the output impedance of the sensor (eg coil resistance in the case of a geophone) to the input of the amplifier, as the noise current can contribute significantly to the total noise.
Analog filters
- AMPLIFIER SPECIFICATIONS AND NOISE
The amplifier's rejection of this noise is characterized by the PSRR (Power Supply Rejection Ratio). The amplifier's output is balanced, so it can be connected to a long twisted-pair cable.
Analog recording
One of the best-known analog field recorders still sold is the Sprengnether MEQ800, which can record with either ink or smoked paper. The amplifier and filter control are on the back, and to the right is the clock display.
Introduction to digital recorders
Ring buffer means that data is overwritten after some time (hours to months depending on the size of the storage medium). This can be done through a communication port and/or by replacing the storage media such as tape, CD or hard disk.
Digitizing
If digitized data does not have a time stamp, the recording computer must output the time stamp of the data. The data acquisition software can now request a retransmission of the data if the digitizer has this capability, or delete the data if not.
Time stamping of data
So there are two main issues involved: keeping the internal real-time clock on time and timestamping the digitized data. The sample rate is constant and nearby recorders will acquire the samples at the same time within the accuracy of the real-time clock.
Storage media and recording in a ring buffer
The most common compression method is to use the difference between the samples (eg the Steim compression, Steim and Wielandt, 1985) instead of the samples themselves. Extracting part of the entire ring buffer is just a matter of a simple file copy.
Seismic triggers
The end of the event is declared when the STA/LTA ratio reaches the trigger level. All the above trigger methods have considered that they are non-lookahead algorithms, that is, the current value of the trigger parameters depends only on current and previous values of the signal.
Summary of trigger parameters and their settings
The advantage is that, during the event detection, STA/LTA will reflect the true STA/LTA of the earthquake signal and the trigger will end at the true trigger ratio. This parameter enables the recording to capture the end or the coda of the earthquake signal.
Communication and data retrieval
General purpose computer based data acquisition is much easier to use for downloading data, as general purpose software such as ftp can be used to copy recorded event files. As the trend is toward more general-purpose computers for recorders, even more general-purpose communications equipment will probably become available.
Public domain data acquisition systems
One of the most important issues is whether the drive is hot-swappable, which means that the drive can be removed while the system is running and a new drive can be put in its place so that no data is lost. The role of the manufacturer will then be to supply the hardware, which is specialist work, especially when it comes to watertight mounting and guaranteed operation under extreme climatic conditions.
Seismic recorders in use
Due to the high cost, special requirements and low production numbers, most OBS are produced by government institutions. On the left are the main elements of the OBS and on the right is an OBS starting its journey to the bottom of the ocean.
The next generation recorder
A flashing light is attached to the left side, a radio beacon to the right and a sound release device to the rear. From Table 5.3 we can see that some recorders on the market now meet many of these criteria.
Examples of recorders
Until recently, the unit was one of the smallest and with the lowest energy consumption on the market. MARSlite MO with 0.5 Gb magneto-optical disk is half the size and one of the smallest on the market.
Which recorder to choose
The cycle time depends on the accuracy required along with the accuracy of the idle clock. One broadband station therefore has the same price as eight of the most economical stations.
Correction for instrument response
Linear systems
If A(ω) is completely known, then the amplitude of the harmonic input signal X(ω) can be calculated from the measured signal as By varying the frequency, the input and output amplitudes can be measured at different frequencies to produce an amplitude response function.
Spectral analysis and the Fourier transform
Note that the spectral amplitudes are normalized so that changing the length of the time window will not change the amplitudes. The same computer algorithms can be used just by changing the sign of the exponent.
Noise power spectrum
To obtain a constant value, we must use the power density spectrum (same argument as for the amplitude density spectrum) and (6.44) must be multiplied by T. Considering again only positive frequencies, the seismic power amplitude density spectrum Pnd must be calculated as.
General instrument correction in frequency and time domain
In the range 0.1 to 10 Hz, the earthquake signal almost disappears into the microseismic background noise. So any small amount of instrumental noise present at low frequencies will blow up the instrument correction.
General representation of the frequency response function
There are many different types of analog filters, but one of the best and most widely used is the Butterworth filter. A nice feature of this filter is that the response to the corner frequency remains constant for each order of the filter.
Anti alias filters
Instrument correction and polarity
The middle trace shows the signal filtered with a zero-phase-shift Butterworth filter while the bottom trace shows the signal filtered with a phase-shift Butterworth filter.
Combining response curves
In addition to the 3 curves that shape the response function, there is also the constant gain of the seismometer, the amplifier, and the ADC. The curves are made with SEISAN (Havskov and Ottemöller, 1999) and the total gain at 1 Hz is 2.51·109 counts/m, which is a typical value for many sensitive SP seismic stations.
Common ways of giving response information
- SEED
But looking at the complete response file in Table 6.1, it is difficult to see which parts originate from which part of the recorder. If the data from Table 6.2 is to be used in a GSE response file, add a zero to the poles and zero representation to make it offset.
Seismic stations
- Geographical location of a seismic station
- Site selection and seismic noise survey
- Installation of the seismic station
- Sensor installation
- BROADBAND SENSOR INSTALLATION
However, in the case of BB sensors, the sensor must be double insulated (see below). The GPS may be on the roof of the house, but for safety reasons it is most likely in the mast.