1. Is your region appropriate for passive surveys?

Microearthquakes occur almost everywhere. In most cases micro seismicity is not detected by existing regional networks. Prior to any passive seismic tomography survey, our Landtech engineers perform a background micro seismicity feasibility study to verify the seismic activity, estimate how many seismographs are needed and how long the activity will be recorded for.

Below is an example of a feasibility study we carried out over a 400 km² area, located in the Zagros Mountains, Iran. During the pre-acquisition stage, a seismicity assessment was attempted, followed by a designation of the optimum parameters for network configuration. A checker board test was performed to assess the resolution of the designed seismological network.

We created an artificial model beneath the entire exploration block by introducing various cubes with different seismic velocities within a homogeneous velocity media (checker board.) We tried to recover the initial model inverting arrivals from synthetic seismographs to a varying number of seismological stations. For more information please see FAQ #6.

 

2. What is the cost of a passive tomography investigation?

The average cost to investigate a region of 1,000 km² with a rough topography is less than the cost of a conventional gravity survey. It would cost in the region of $2 million depending on conditions (e.g. topography, accessibility) and would cost considerably less if the total area to be investigated is larger. For example, the average cost to investigate an 8,000 km² mountainous area can drop down to $4-5 million depending on the number of the stations to be installed.

3. Is the data obtained less accurate than those of a conventional seismic reflection survey?

In many cases the data is even better. For example, in regions of rough topography or complicated geology they are much more accurate. In fact, in regions which encounter seismic energy penetration problems either because of heavy weathered surface layers or because of the existence of evaporitic formations, Passive Seismic Tomography (PST) is the only technique which can provide reliable results. Furthermore, in regions where the existing seismic sections show nothing due to the aforementioned problems, we can reprocess them with the PST velocity model and improve resolution as shown below.

4. Why Passive surveys should be carried out at the initial exploration stages?

Exploration costs can be reduced significantly by doing so. Passive surveys will help to reveal sub-regions of interest so the exploration can be focused on those locations only. In certain cases it may be the best possible solution in regions of difficult topography or where seismic penetration is not feasible.

5. How long does it take to conduct a 3D passive investigation?

It is dependent on the background seismicity of the region. We can typically supply detailed 3D Vp,Vs, Poisson’s ratio maps and 3D crustal property videos within 6 months. Prior to the survey being undertaken, we perform a complimentary pre-feasibility desk research which considers the seismicity of the region and its geotectonic structure. From this we are able to tell you the optimum number of stations required and the time frame of the project.

6. How sure can we be about the accuracy of the results?

PST results are very accurate. They have been crosschecked with results obtained from VSP’s and differed less than 5% in their readings. Furthermore, at Landtech we perform a series of quality control tests which undergo synthetic and inverse modelling to ensure the accuracy of the models we provide.

7. What kind of equipment do you use?

Landtech designs and manufactures its own seismic recording instruments. We have 24 and 32 bit digitizers as well as a great range of 3-component seismometers, particularly for passive seismic tomography surveys. We do not use off the shelf seismological instruments favoured by our competitors which are only suitable for regional academic seismotectonic investigations. For more information on some of our equipment please Click Here.

We can mobilise hundreds of instruments, globally and at short notice, all from our large data base of equipment.


8. How many earthquakes need to be recorded to obtain a sufficient resolution?

With an average background seismicity (recording more than 4 microearthquakes per day) the PST survey could last between 5 to 8 months. If the seismicity is less than average then we can either increase the number of recording stations or extend the survey period by 2 to 3 months. In the figure below we present a simple setup showing the ray paths from only 6 microearthquakes to 11 surface stations. Imagine the resolution that could be obtained if we used the all of the recorded microearthquakes (usually 600-1000) and an array of many surface stations (70-100 in average.) In this case many thousands of seismic rays would cross in a dense formation in the space below the exploration block.

For example, let’s suppose that we have installed a network of 50, 3-component seismographs all over the region of interest. In a tectonically active area we usually record more than 5-10 events per day on average. If we record 5 events in 60% of the seismographs (that is we can clearly recognise seismic waves in 30 instruments) we would get 5 events x 3 components x 30 seismographs = 450 seismic rays per day crossing the area. If we operate the network for 6 months we can end up with more than 80,000 seismic rays crossing our target.

9. How do you manage so many waveforms?

Landtech’s software engineers have developed a suite of algorithms to process the continuous seismological recordings, isolate microseismic events, determine the arrival times of the seismic phases and invert them for hypocentral determination and velocity models. A description of these software are described here: Click

10. What would you do in the case that the microearthquakes originate from a few, single active faults?

If this was to happen, after a few months we would move the seismographs to new positions in order to allow a complete ray coverage of the region of interest.

11. Do all the microearthquakes have to originate only within the area of interest?

We usually also put a small number of seismographs outside of the area of interest in order to locate accurately the earthquakes occurring nearby. Some of which cross the targeted area at various angles and can therefore help to improve the resolution. See the example below where most of the recorded seismicity has been recorded outside of the region of interest (white rectangle.) By installing a few stations around this region we were able to locate accurately any nearby events and use them for the seismic tomography. In fact, the most shallow events that occurred were located away from the target area. They hit the investigation site at a low angle and increased the resolution at the upper layers.

12. How long can a seismograph be operated for in regions with no electricity?

Landtech’s LTSR-24 digitizer has been designed with that very question in mind. It is a particularly low power consumption seismic recorder as it writes directly from the from the A/D converter to the flash card. It can be operated with just a single car battery for almost a month! Furthermore, Landtech engineers have also been able to keep their seismographs powered even in very isolated regions through the use of solar panels, micro wind generators and Hydrogen and Methanol cells.

13. Can you apply this methodology in areas of very rough topography?

PST methodology is not affected by rough topography as we do not have to deploy many geophones or lay out seismic lines and repeaters. It is therefore easy for us to operate in areas of rough terrain. All we have to do is to install single stations at a spacing of about 5 km, use a GPS to measure accurately the coordinates and leave nature to provide the seismic sources! We have operated in thrust belts with topography ranging from a few meters to a few kilometres above sea level.

If required, we use helicopters to deliver recording equipment to remote sites to reduce setup and data retrieval time.

14. What about environmentally protected areas such as swamps, tropical forests etc?

PST methodology is ideal for these areas. It is the only technique of this kind to have no environmental impact. We have successfully installed passive networks in extremely environmentally sensitive regions such as the Amazon and in national parks.
Click here for more details

At Landtech we ensure we take extra precautions on all our job sites to protect the environment and return it to how it originally was prior to survey.

15. Can you operate offshore?
Yes! We have designed and tested a unique ocean bottom seismograph which is able to operate up to depths of 10 km and is synchronised via an atomic clock.
Click here to download brochure

16. Can PST methodology be applied to regions where we already have poor quality conventional seismic (2D or 3D) data in order to improve them?

Yes this is indeed possible. Many of our clients have used PST results to increase the resolution of existing seismic sections. Click here.

17. Can we apply the PST methodology to areas with high noise (industrial etc?)

Yes Landtech uses special installation procedures and noise filtering techniques to increase considerably the S/N ratio of the recorded data.

Landtech has developed advanced algorithms to reveal the P- and S- wave phases from very noisy data. Click here to read more about these procedures.

18. Can you combine PST results with the results of Gravity and MT surveys?

Landtech was the first exploration company to apply joint inversion methodologies of Passive Seismic Tomography (PST), Gravity and MT data. This approach has significantly improved the resolution of the obtained geological models since PST, Gravity and MT data are dominated by different physical parameters (seismic velocity, density and resistivity). Various combinations of [PST, MT] ; [PST, Gravity] or [PST, MT, Gravity] data sets can be jointly inverted. Recently Landtech has developed a neural network joint inversion algorithm (GRAMAPAS) for the combined optimum inversion of PST, MT and Gravity measurements.

Below is an integrated representation of a characterisation of an entire block using data from Passive Seismic Tomography, MT and Gravity.

And below is the corresponding interpretation proposing new drilling sites as possible hydrocarbon targets.

19. Can PST methodology differentiate oil and gas reservoirs?

The Vp/Vs ratio is especially sensitive to the pore fluid found in sedimentary rocks. In particular, the Vp/Vs value is much lower (10-20%) for gas saturation than for liquid saturation.
In the figure below we depict 2 cross plot diagrams between [Vp/Vs], Vp and [Vp/Vs,Vs] for various depths. The values were obtained from a high resolution passive seismic tomography survey in a gas production field in Albania. For more details CLICK Here to see a complete field case.

20. Can the PST methodology be used for shale fracking and reservoir monitoring?

Landtech has developed special instrumentation and surface recording methodologies that actually requires no expensive boreholes to be made or down hole instrumentation to be used. As such it is far less intrusive and environmentally sound. Landtech’s LT-FS surface sensitive seismometer (click here to its specifications) can record microseisms of negative magnitudes many kilometres down.

LT-FS ultra-sensitive surface sensor
Landtech has also developed a procedure which is based on the shear wave splitting phenomenon, to delineate fracture zone directions within the entire reservoir. Click here to read more.

Landtech uses specially designed surface seismic networks based on VHF or WiFly telemetry for real time reservoir monitoring. Click here to see more.

21. Can we perform a passive seismic tomography survey over a very restricted area?

The minimum area that a PST survey can be performed depends on the background seismicity. However, it is not advisable to concentrate the survey just over a very restricted area because the number of microearthquakes recorded will not be adequate for the required seismic ray density. It is also not more cost effective to do so. It is better therefore to install the network over a larger area.

22. How easy is it to interpret PST results?

The interpretation of PST results is very easy as we have two different kinds of data i.e. Vp and Vp/Vs sections. In the diagram below, we present some examples of interpreted sections superimposed on some geological structures.

Interpreted Vp section obtained by a PST survey.

Interpreted Vp/Vs section obtained by a PST survey.

23. How sure can Landtech be that the technology works?

Landtech has tested the technology on several different data sets from different basins in the world. The results have been verified with numerous wells and seismic data. The results have been accurate in all cases.

24. Is there any relation between the so called Low Frequency Passive Seismic (LFP) method (or direct hydrocarbon indication) and Landtech’s Passive Seismic Tomography (PST) method?

Not at all! Some companies state that a natural low frequency can be detected upon a reservoir. Landtech’s engineers and other researchers have thoroughly experimented this methodology and found no indication of the existence of such a frequency. As such Landtech does not deliver this type of service. Landtech’s PST method is based on the seismology seismic tomography methodology and it is proven to be effective and accurate.

Should you be interested in reading more about the inadequacies of Low Frequency Methodology, we encourage you to read the paper published in the Bulletin of the Seismological Science of America click here.

25. Can the Passive Seismic Tomography (PST) method be used for pinpointing hydrocarbon reservoirs and drilling sites with reduced well risk?

Although a Passive Seismic Tomography (PST) survey provides mainly, structural and lithological information below the entire block of interest, it cannot be considered as a direct hydrocarbon indicator. In certain cases we can provide maps which depict sub regions within the investigated block possessing high probability for a successful well. This is due to the fact that one of the products of PST is the Poisson’s ratio which strongly depends on the fluid content within the pore space.

Furthermore, the mapping of the Q parameter all over an exploration block could reveal regions of high porosity, fracture zones and differentiate gas and oil in the pore space.

The decrease of Q (increase of attenuation) is obvious at the oil and gas depths.
By combining the results of MT and Gravity surveys (if they exist) and imposing additional data limits corresponding to the hydrocarbon targets the probability of drilling a producing well is vastly increased. See FAQ #18.

26. What are your guarantees?

We have many satisfied companies that have used our services and can provide numerous letters of recommendation upon request. Click Here.

27. How do you manage to keep a seismological network consisting of so many recording stations in perfect working order 100% of the time?

As we have designed and manufactured all our own equipment we have the resources, knowledge and parts available to service any faulty stations immediately.