Portable Seismic Stations

GEObit provides compact solutions for quick deploy seismic stations based on the SRi32L digitizer/recorder and C100 sensor. The SRi32L recorder has 3 input channels and integrated sensor electronics. The digitizer is based on a powerful, wide dynamic range, 32 bit sigma-delta analog to digital converter (ADC). It has very low noise characteristics and excellent power supply rejection. The sampling rate can be set to 50, 100, 200, 250, 500 and 1000 samples per second. The sensor response is 10 sec – 98 Hz. The data is stored on a removable microSD card. Ultra-low power consumption allows over one month of operation if powered by an ordinary 12 V/65 Ah lead-acid battery.

The LCD, displays the state of health, time, date and other information related to the instrument’s operation. The digitizer is intended for installation in harsh environments. The instrument supports an embedded SeedLink server for real time data telemetry.

Applications

  • Aftershock studies
  • Local and regional seismicity monitoring
  • Seismic tomography
  • Reservoir monitoring
  • Induced seismicity monitoring
  • Geothermal monitoring
  • Structural monitoring (dams, bridges, ancient castles)

Here we present the instrumentation which has been used in several passive tomography projects, as well as in several micro-seismicity and induced seismicity projects, explaining why we propose using this kind of equipment.

Micro-Seismicity Monitoring Instrumentation SRi32L Compact Sensor/Digitizer/Recorder Unit

Figure 1: Seismic Event

So why choose a GEObit unit? 

1. Why a wide band sensor?

For microseismicity monitoring experiments and applications, the seismic events usually have a Richter Scale (R) magnitude from –2 R, up to 4 R, and their frequency spectrum is in the band of 0.8 Hz to 20 Hz. Therefore, a wide band seismometer with the range of at least 0.5 Hz (2 sec) up to 30 Hz is necessary for the recording of such seismic events. In microseismicity monitoring , we are using sensors with a recording bandwidth from 0.1 Hz to 100 Hz to record the seismic events with maximum quality. The low frequency response gives us the ability to calculate moment tensors as well.

Figure 2: Seismic Event Plot


Figure 3: Seismic Event Spectrum Plot


Somebody may ask, why not broadband sensors? The answer is that broadband sensors are much more expensive making the survey inefficient in terms of price. Using broadband sensors doesn’t give us any additional information in recording local microearthquakes. For the next experiment, we used both wide band and broadband sensors for the seismic network. Below is an example of a recorded earthquake.

Figure 4: Seismic Event Recorded from Wide Band and Broadband Sensors


Figure 5: Seismic Event Spectrum of the Broadband Sensor


Figure 6: Seismic Event Spectrum of the Wide Band Sensor


Figure 4 shows a typical seismic event recorded from a wide band sensor and eight broadband (Nanometrics – Trillium 40) sensors. Trace 19 is the broadband one, and it could not be recognized from others at first look at the signal plot. The BB sensor and the wide band sensors’ frequency spectrum are presented in figures 5 and 6, respectively. These spectrum plots represent frequencies of recorded signals from the same earthquake. Even with the use of the wide band sensor, the recording result is equal.

Why not use simple geophones? 

Commercial geophones with small dimensions usually have a natural frequency 4.5 Hz or 10 Hz. Using these kind of geophones, the lower part of the signal spectrum is lost (low frequency). Because the geophones cannot respond in the low frequency area (below 4.5 Hz), its sensitivity falls dramatically. There are also geophones in the market with a natural frequency of 2.0 Hz or 2.5 Hz, but their dimensions do not allow them to be used in small boreholes. Their bandwidth is enough for magnitude 1 or magnitude 2 local events, but not enough to cover low frequency (below 1 Hz).

2. Why a Force Balance Sensor?

The S-100 sensor is based on the force balance principle. Special electronics provide feedback on 4.5 Hz geophones, plus additional, electronic bandwidth correction, and provide a 10 sec – 98 Hz sensor unit. The 4.5 Hz geophones are not so overly sensitive to tilt. Usually they respond perfectly within 5 – 10 degrees of tilt. This means that the sensor can be installed into the borehole without any need of special leveling. An elastic packer, provided by us, is more than enough to hold the sensor in the borehole. It is leveled according to the casing verticality and it is operational even if there is some degree of tilt at the borehole casing. Thus, the ideal solution for a fast, accurate installation is the low cost, small, wide band seismometer designed according to the force balance principle and consisting of 4.5 Hz geophones. Our wide band sensors meet all the requirements for local microearthquake recording in passive tomography projects. In our new SRi32L digitizer/recorder, the sensor electronics are built into the instrument.

3. Why borehole type sensor?

Many seismic sensors are the surface mount type. For their installation, seismic vaults usually need to be opened where the noise level is less than the surface. Our instruments are mostly the borehole type, so they can be easily installed at a typical depth of 20 meters. The noise level at this depth is much less than at the surface. The borehole can easily be opened (in most cases) using a drilling machine at low cost, during working hours. Small diameter boreholes can be dug by hand in areas not accessible by vehicles.

Figure 7: Seismic Noise at a 20 m Depth Borehole

Figure 8: Seismic Noise at the Surface

4. Why a high sensitivity sensor?

The magnitude (Richter Scale) of recorded events for passive seismic tomography exploration, range from –2 R to 3 R. For the recording of such small events, the sensor must be very sensitive. Our instruments provide high sensitivity such as 1500 V/m/sec.

5. Why a built in sensor self test?

The operator needs a way to apply a fast signal test for the equipment in order to verify proper operation. Most of the clients are asking for these kind of tests to be performed by the instruments. The SRi32 recorder provides a built in signal generator combined with a micro-controller, and injects the signal into the sensor. The square wave or sinusoidal signal that is injected into the sensor with constant amplitude, forces the masses of the geophones to move. The geophones produce a seismic signal proportional to their movement. The shape of this signal is shown at the figures 9 and 10.

Once the user connects with the SRi32L unit through the data monitor, he can verify the proper operation of the sensor. In parallel, the digitizer’s processor uses this signal to perform auto calibration of the system. Therefore, all seismic stations in the microseismic network can be calibrated during the recording period.

Figure 9: Sinusoidal Signal for Sensor Calibration

Figure 10: Pulse Signal for Sensor Calibration

6. Why a high resolution digitizer?

The digitizer’s resolution is one of the most important parameters for seismic instrumentation. 24 bit digitizers are mostly used in seismic exploration. The SRi32 unit is based on a 4th generation, 32 bit digitizer, with a dynamic range of 136 dB at 250 SPS but the SRi32 unit actually has a dynamic range of 142 dB at 10 SPS.

7. Why large capacity storage media?

Our standalone seismic station units are powered from a typical 12 V battery which may be cycled by the seismic crew or charged by a solar panel. The seismic crew usually visits the stations about once a month for changing the battery and retrieving the data. For places where this routine visit is difficult, the storage media must have enough capacity to store the data for a longer period. Using a 2 Gb microSD, the recording duration for 3 channels at 100 SPS will be up to 26 days. A larger capacity, compact flash card will be used for a longer recording period. The units support up to a 64 G flash card.

Figure 11: Digitizer Noise Spectrum

Figure 12: Digitizer Noise Histogram Plot

8. Why very low power consumption?

One other important point about the standalone seismic station is the power autonomy. Given that is it powered from a simple 12 V lead acid battery, the cycle must be as many days as the seismic crew needs to visit the seismic station. Seismic networks consist of 50 seismic stations, spread over an area of 1000 – 2000 square miles, and placed in accessible terrain. It usually takes a few weeks for the seismic crew to visit them. Therefore, the power autonomy of each seismic station (recorder + sensor) must be enough for at least one month of operation. Our seismic stations can operate for this time period, powered from a set of double 12 V/62 Ah lead acid batteries.

Figure 13: Portable Seismic Station