Passive Seismic Tomography (PST) is a revolutionary geophysical exploration technique. Through our networks of seisometers installed at the surface we can harness natural microseismicity (microearthquakes with magnitudes of -1 up to 2.0 Richter) continuously for a period of a few months.
In 1998 we became the first company to use a method of high resolution passive seismic tomography for hydrocarbon exploration. In addition to designing specific hardware for the purpose we also created new inversion algorithms and a neural network of fuzzy procedures for the reliable processing of the acquired passive data.
We use the hypocenters of the microearthquakes as seismic sources embedded within or below the target of interest and perform 3D seismic tomographic inversion of the P- and S-wave travel times to the surface recorders.
In conventional seismic, the seismic wave has to travel down and then up since the source is at the surface. With our method, the seismic sources are below or within the target of interest. We will, as a result, not register a two wave travel of the seismic wave. Our technique is therefore especially suited for regions where penetration is difficult e.g. thrust belts, heavy weathered surface layers etc.
We are able to obtain a highly accurate 3D velocity model in this manner and through modification of the seismic ray paths and minimising the difference between theoretical and measured travel times at the surface.
We can also measure the arrival times of S-waves, as well as other parameters too, that determine the attenuation of the medium (i.e. seismic amplitudes or first seismic pulse widths.) Landtech has developed special algorithms to automatically detect the P- and S-wave arriving phases.
Passive Seismic Tomography can give us 3D images of P- waves, S-waves, Poisson’s ratio and Quality factor (Q) or attenuation of the medium. Passive Seismic Tomography is the only technique which can provide 3D volumes of Poisson’s ratio, a parameter which can indicate the presence of gas and fluids in a reservoir.
The diagram below breaks down the steps of a high resolution Passive Seismic Tomography survey.
- Step 1 is data selection. It is important to use microearthquakes who’s hypocenter locations have been determined by the available data gathered. This is a straightforward exercise where using a well designed seismic network.
- Next, is to determine the initial velocity model for the 3D inversion. In order to construct the initial model, we will make use of all existing information we have. We may decide use a 1D model or a two sided model depending on the nature of the target area (i.e. the presence of strong lateral velocity discontinuities.) Geologic and seismotectonic maps, seismic profiles (if available) and the initial 3D models will lead us in this choice.
- We then need to gather all data relative to the sources. That is we separate the measured arrival times per earthquake and per station.
- We run the 3D inversion of the initial data set and carry out the initial and first tuning of parameters.
- We prepare all the routes to plot to ensure an adequate capture of the model required. We generally use the public domain GMT package to do this.
- We then run the QC sensitivity tests with synthetics. These include, for example, the checkerboard test.
- We run Landtech’s specially designed tomographic inversion algorithm with the final data set. This run, is in fact, a series of runs where the initial model and data set can be changed from run to run to ensure the caliber of the tomographic images.
- We apply Landtech’s neural network based lithological assessment algorithm.
- We plot the results. This involves numerous steps including planning view maps at different depths, cross-sections, 3D plots, plots of the residual times before and after the 3D inversion. We also plot the mean station residuals and their standard deviation.
- Lastly, we create a 3D animated video for visual analysis.
Click here… to see an example of a 3D Vp velocity structure below an exploration block we obtained using our PST method.
The benefits of Passive Seismic Tomography are substantial:
- It can be applied to all terrains including mountainous regions, rain forests, swamps etc. We only have to install single seismographs at intervals of a few kilometres and there is no need for dense geophone layouts.
- There is no need to use explosives, vibroseis etc as we use the natural microseismicity as our seismic sources.
- No day-to-day operations etc.
- No permits required.
- Provides 3D geological detail beneath the entire exploration block regardless of how vast and inaccessible it is. We can survey many thousands of km².
- Ideal for regions where seismic penetration has not been possible for example in heavily weathered surface layers, basalt barriers, etc. In Passive Seismic Tomography microearthquakes play the role of seismic sources within or below the target, so we have only 1-way ray paths, crossing the medium of interest, from the source to the sensor at the surface.
- In many cases, such as in regions of difficult geology, high topographic reliefs or strongly attenuating media, we can provide better data than conventional seismic.
- Provides not only 3D Vp but also and Vs velocity variations beneath the entire exploration block.
- Provides 3D distribution of Poisson’s ratio which depends on the type of fluids found in the pore space (i.e. water, gas, oil.)
- We can easily obtain 2D horizontal and/or vertical sections whenever we want throughout the exploration block.
- We can create a 3D animated video of Vp, Vs and Poisson’s ratio parameters below the entire exploration block.
- We can use the 3D velocity data to reprocess conventional seismic data (post stack depth migration) and increase their resolution.
- We can get a 3D variation of the seismic attenuation (or rock quality factor) below the entire exploration block. A parameter that is dependent on fracturing and the type of fluids found in the pore space.
- We can explore large areas of many thousands of Km² at a fraction of the cost of conventional 2D seismic.
- There is no need for explosives, vibroseis or expensive seismic equipment (e.g. cables, telemetry nodes, geophones etc.)
- It requires only a fraction of the personnel (seismic crew) required to conduct a conventional seismic survey.
- 100% safe to the environment.
- No explosives, road blocking for vibroseis etc.
- Uses natural seismicity as a source.
- Ideal for environmentally sensitive regions (rain forests, national parks, tropical forests, swamps etc.)
- No special permits required, even in environmentally sensitive areas.
- Can be used in frontier, geopolitical exploration areas to build a pre-project plan at minimum cost.
- Can be used to investigate large areas and determine points where to apply conventional seismic (better layout of seismic lines) and thus reduce the exploration cost.
- Helps to provide confidence in exploration activities not only by defining an accurate geological setting but also minimising the investment risk.
The following example shows how a PST survey using natural microearthquake seismicity can be compared to conventional seismic as far as seismic ray coverage and accuracy is concerned. In the figure below we present a simple case showing the ray paths from only 6 microearthquakes to only 11 surface stations. Imagine the resolution we could obtain if we used the whole 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 will cross the space below the exploration block.
3D mapping of geological formations even below blocks with high topographic relief.
The following figure shows the mapping of an evaporitic layer below a mountainous limestone block. This result was obtained from a 3D PST data volume by isolating velocities corresponding to the evaporites.
3D Velocity structure below an exploration block.
3D Vp/Vs structure below an exploration block.
In the following example we show a 3D view of all Vp/Vs values >1.785 between an exploration block delineating possible hydrocarbon traps.
The figure below shows how Vp/Vs values can differentiate water saturated carbonates from carbonates and flysch.
3D basement mapping below a block