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

So why chose a GEObit unit? 

1. Why wide band sensor?

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

Somebody may ask, why not broadband sensors? The answer is that the broadband sensors are much more expensive than our sensors and makes the survey inefficient in price terms. We do not get more information using broad-band sensors of local micro-earthquakes recording. At the next experiment we used wide band and broad-band sensors at the seismic network. Here is an example of a recorded earthquake.

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

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

2. Why force-balance sensor?

The S-100 sensor is based on force-balance principle. Special electronics provide feedback at 4.5Hz geophones, plus additional electronic bandwidth correction and provide a 10sec-98Hz sensor unit. The 4.5Hz geophones are not so sensiitive over 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 from us, is more than enough to hold the sensor into the borehole. It is leveled according the casing verticality and it is operational, ranging within its characteristics even there is some degrees of tilt at the borehole casing. So, the ideal solution for fast, accurate installation is the wide-band seismometers consisted of 4.5Hz geophones, having small dimensions and low cost, designed according the force-balance principle. Our wide-band sensors meet all the requirements for local micro-earthquake recording in the passive tomography projects and in our new GEOThree digitizer/recorder, the sensor electronics are build-in into the instrument.

Figure 7: seismic noise at 20m depth borehole

Figure 8: seismic noise at the surface

4. Why high sensitivity sensor?

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

5. Why in-build sensor self test?

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

Once the user connects  with the GEOThree unit through the DataMonitor, he can verify the proper operation of the sensor. In parallel, the processor of the digitizer uses this signal to perform auto-calibration function at the system. So, all the seismic stations of the microseismic network can always be calibrated during the recording period.

Figure 11: Digitizer noise spectrum

Figure 12: Digitizer noise histogram plot

8. Why very low power consumption?

 One other important point of the standalone seismic station is the power autonomy. Given that is it powered from a simple 12V lead acid battery, the cycle must be as many days as the seismic crew needs to visit the seismic station. Seismic networks consisted of 50 seismic stations, spread in an area of 1000 – 2000 Sqm2 placed in accessible terrain, usually takes a few weeks to the seismic crew for visiting them. So 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 12V/62Ah lead –acid batteriy.

Geobit & ISTI join together at the AGU Fall Meeting in Chicago, IL, 12 – 16 Dec 2022.

We are glad to announce that we are exhibiting live again at the AGU Fall Meeting, after an absence of two years, because of Covid19 restrictions.

With 65+ years of combined experience, GEObit and ISTI are joining forces to become the most competitive and fastest growing joint venture group in the seismic monitoring services industry.

Providing state-of-the-art seismic instruments, network design, installation techniques, data monitoring and processing services, our group is able to deliver a cost-effective total solution to the energy exploration & production industry, especially to the oil, gas and geothermal sector. Our key benefits are that we produce in-house hardware and software while installing and maintaining the microseismic monitoring network to ensure a high level of data quality and availability. In other words, we make the technology – we apply the technology – we offer the technology.

We have worldwide experience having installed and operated seismic networks all over the world, from Indonesia/Papua jungle to the Middle East desert, from Alaska glaciers to Himalayan Mountains. In the USA, we are currently running many dedicated microseismic monitoring projects with over 200 stations installed in the past 10 years.

Visit us at booth #1726, AGU 2022

Chicago, IL, USA  |  12 – 16 December

Meet our team and learn about the latest offerings. Dimitris Mourtzouchos & Effie Siaini (Geobit), Paul Friberg & Sid Hellman (ISTI), and others will be available to answer your questions.

More information about AGU Fall Meeting 2019 may be found here.

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