We present in this special article the instrumentation that Landtech proposes for passive seismic and has been used in several passive tomography projects by the company. It is also a guide to questions that many of our clients asked over the years.
1. Why wide band sensor?
The source of the energy for the 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 into the band 2Hz to 20Hz. So, a wide band seismometer with the range of at least 1Hz up to 30 Hz is necessary to be used for the recording of such seismic events.
In PST exploration, we are using sensors with recording bandwidth from 0.1Hz to 100Hz in order to record these seismic events with the maximum quality. The low frequency response gives us the ability to calculate moment tensors as well.
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, because the geophone 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. Larger boreholes will be needed increasing the cost of the installation. The price also of a sensor made by 2.0Hz or 2.5Hz is a few times more than the one with 4.5Hz geophone.
Another advantage of the 4.5Hz geophones is their sensitivity over tilt. Usually they respond perfect 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 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, with small dimensions and 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 INTEGRA24, the sensor electronics are build-in to the instrument.
2. Why borehole type sensor?
Most seismological sensors are surface mounted. For their installation, usually seismic vaults have to be made, where the noise level is less than the surface. Our instruments are mostly borehole type, so they can be installed in a typical depth 20 meters. The noise level there is much less than the surface. The borehole can be easily (not always!) opened with a drilling machine with low cost, within some working hours. Small diameter boreholes can be opened by hand in rough areas where no vehicles can access.
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 have sensitivity as 1500V/m/sec.
4. 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. Since PST is a geophysical method, most of the clients are asking for such kind of tests to be performed from the instruments themselves, like the conventional seismic instruments do. The SRi32 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.
5. Why high resolution digitiser?
The resolution of the digitiser is one of the most important parameters of the seismic instrumentation. 24bits digitizers are mostly used in seismic exploration. The SRi32 unit is based on a 4rd generation 32bit digitizer, with dynamic range 138dB at 250sps, while dynamic range is 142dB at 1000sps
Our seismic stations are standalone instruments, powered from a typical 12V battery.
The seismic crew usually visits the stations once per week or later, to change battery and retrieve the data. For places that this routine visit is difficult to be done, because the access of some stations is difficult, the storage media must have enough capacity to store the data for longer period. The SRi32 recorder
when acquires its data to a microSD flash card with capacity up to 64Gbytes. Using a 2Gb microSD the recording duration will be up to 26days. Larger capacity Compact Flash Card will be used for larger recording period.7. 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 one week to ten days from the seismic crew to visit them. So the power autonomy of each seismic station (recorder + sensor) must be enough for at least 10-12 days. Our seismic stations can operate for this time period, powered from a set of double 12V/7.2Ah lead –acid batteries.
One of the most critical problems of the data processing is the seismic event detection. Things are simple, when the data are free of noise, but there are some cases that the micro-seismic network has to be installed very close to residential regions, like cities, or industrial areas. There, even the sensors are installed in a depth of 20m, noise which is inside the seismic band will be propagated into the seismic signal. In places also that the sensors are not installed within the bedrock, noise is generally higher. For this reason, GEObit engineers have design and develop a very special event detection tool, 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 the recording band.
The way it operates is to process the data from neighboring stations and search if any seismic event has been recorded. Searching all the station’s data, one by one into their neighborhood, the event detection software can detect and extract all the recorded seismic events, even its magnitude is very small and even the data are noisy. This software has been tested and evaluated in the noisiest environments, where seismic networks had to be installed around cities and industrial areas.
In order to make a successful passive tomography acquisition, the recording equipment should have the special characteristics. The design of the SRi32 integrates fifteen years of technology in PST exploration.