What makes our equipment and services different to our competitors? Let us talk you through the equipment and instruments we use to conduct passive seismic surveys and have used in several passive tomography projects. It can also be used as a helpful FAQ guide to questions that many of our clients have asked over the years.

 

1. Why do you use a wide band sensor?

The source of the energy for passive tomography exploration comes from micro-earthquakes that occur below the area of the installation of the micro-seismic network. These recorded seismic events usually have a magnitude from –2R, up to 3R and their frequency spectrum is between 2Hz to 20Hz. Therefore, we need to use a wide band seismometer that has a range of at least 1Hz to 30 Hz is necessary to record these seismic events.

In PST exploration, we are using sensors with a recording bandwidth of between 0.1Hz and 100Hz to ensure high quality recordings. The low frequency response gives us the ability to calculate moment tensors as well.

Figure 1. Seismic event recording.

Figure 2. Seismic event plot.

Figure 3. Seismic event frequency spectrum plot.
You may be wondering why do we not just use broadband sensors? The simple answer is that broadband sensors are significantly more expensive than the sensors we choose to use. It would make the survey to cost prohibitive. We do not obtain more data using broadband sensors in local micro-earthquakes recordings.

Figure 4. Seismic event recording from wide band and broadband sensors.

Figure 5. Seismic event frequency spectrum plot from the 40T BB sensor.

Figure 6. Seismic event frequency spectrum plot from the wide-band sensor.
Figure 4 shows a typical seismic event recorded from wide band and eight broadband sensors. Station 19 is one of the broadband sensors which could not be told apart from the 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 in figures 5 and 6 respectively. These spectrum plots represent frequencies of the recorded signals of the same earthquake.

Why not use geophones? Small dimension, commercial geophones usually have a natural frequency of 4.5Hz or 10Hz. When using these kind of geophones the lower part of the spectrum of the signal is lost. This is because the geophone cannot respond in a low frequency area of below 4.5 Hz and its sensitivity falls dramatically. There are also geophones on the market with a natural frequency of 2.0Hz or 2.5Hz, but they are too large to be used in small boreholes. To use them would require larger boreholes to be made thus impacting on the cost of the installation. The price of a 2.0Hz or 2.5Hz sensor is also is a several times more than the one with 4.5Hz geophone.

Another advantage of using a 4.5Hz geophone, is their sensitivity 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 for special leveling. An elastic packer is more than adequate to hold the sensor into the borehole. It can be leveled according to the casing verticality. It is even able to operate when there is a degree of tilt to the borehole casing.

So, the ideal solution for fast, accurate installation are to have the small, wide band seismometers with 4.5Hz geophones designed according to the force balance principle. Our wide band sensors are perfect for local microearthquake recording in passive tomography projects. Our INTEGRA24 sensor has all the electronics built-in to the instrument.

 

2. Why do you use a borehole type sensor?

Most seismological sensors are surface mounted. Seismic vaults usually have to be made for their installation, where the noise level is less than at the surface. Our instruments are mostly borehole type so they can be installed at a typical depth of 20 metres. The noise level there is much less than at the surface. The borehole can usually be made at low cost with just a drill and some man hours. Smaller boreholes can be drilled by hand in tougher areas where there is no vehicles access.

Figure 7. Seismic noise at the 20 metres depth borehole.

Figure 8. Seismic noise at the surface.
 

3. Why do you use a high sensitivity sensor?

The magnitude (Richter Scale) of the recorded events for a passive seismic tomography exploration, range from –2R to 3R. To be able to record such small events, the sensor must be very sensitive. Our instruments have can measure sensitivity at 1500V/m/sec.

 

4. Why do you make your sensors to self test?

The operator needs a way to apply a fast signal test to the equipment in order to verify proper operation. Since PST is a geophysical method, most clients demand that their instruments are capable of doing this themselves, much like the conventional seismic instruments do. Our PST seismographs come equipped with a built-in signal generator combined with a micro-controller and sends a signal to the sensor. The square wave or sinusoidal signal that is sent to the sensor with constant amplitude, forces the masses of the geophones to move. The geophones produce a seismic signal of their output proportional to their movement. The diagrams below depicts the shape of signal that the user should see in this scenario.

Figure 9. Sinusoidal signal calibration.

Figure 10. Pulse signal calibration.
Once the user is connected with the SRi32 through the DataMonitor, he can verify the sensor is functioning correctly. The digitzer’s processor uses this signal to perform an auto calibration. All seismic stations of the microseismic network can be calibrated during the recording period.

 

5. Why do you use high resolution digitizers?

The resolution of a digitizer is its most important output or function. 24bits digitizers are mostly used for seismic exploration. The SRi32 unit is based on a 4th generation 32bit digitizer, with the dynamic range at 138dB at 250sps, whilst the dynamic range is 142dB at 1000sps.

Figure 11. SRi32 noise spectrum and histogram plot.
 

6. Why do the instruments use high storage capacity media? 

Our seismic stations are standalone instruments, powered by just a standard 12V battery.

The seismic crew usually visits the stations once a week where they will change their batteries and retrieve any data. In areas of difficult topography the crew may not be able to visit the stations as frequently. The stations must therefore have enough memory to store data for longer periods of time.

The SRi32 recorder stores its data to a microSD flash card with a capacity of up to 64Gb. Using for example, a 2Gb microSD card, recording can only last up to a maximum of 26 days.We will use larger capacity Compact Flash cards where the project demands it.

 

7. Why do your units have a 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 consisting of 50 seismic stations can spread an area of 1000 – 2000 m². If the stations are situated in easily accessible terrain, it normally takes the seismic crew a week to ten days to visit them all. It is therefore imperative that each seismic station (recorder and sensor) can remain powered for least 10-12 days. It is possible to power our seismic stations for this period of time using a set of double 12V/7.2Ah lead acid batteries.

 

Figure 12. Our seismic recorders.
 

8. Why do you use smart, automatic event detection software?

One of the most challenges aspects of data processing is the detection of a seismic event. Things are straightforward when the data is free of noise. However, in some instances this is not possible when the micro-seismic network has to be installed in proximity of residential or industrial areas. Even when the sensors are placed at 20m, noise can be detected by the seismic band and will be recorded. In areas where the sensors are not installed into bedrock, noise is generally higher. For this reason a special event detection tool was designed based on a modified recursive STA/LTA and curtosis algorithm. Adaptive filtering of the data is applied from the software prior to processing in order to reject noise components outside of recording band.

The tool functions by processing the data from neighboring stations to see if they have recorded any seismic activity. By searching each station’s data, the event detection software can detect and extract all recorded seismic events, even ones of small magnitude and where data is noisy.

Figure 13. Event Detection Software.

Figure 14. Event Detection Software.
 

9. Conclusion

In order to make a successful passive tomography acquisition, the recording equipment should have possess special characteristics. The design of the SRi32 integrates fifteen years of technology in PST exploration.