The 3D passive method is a seismic tomography method using micro earthquakes, down to some 10 km depth, producing pressure and shear waves which are recorded in a widely spaced grid of single three component geophones placed in an area of e.g. 1000 km2.
The passive seismic method may prove to be a breakthrough in seismic exploration and production in (macro earthquake active) areas with active micro-seismicity, as it provides 3 dimensional structural and lithological information at a fraction of conventional seismic survey cost albeit at a vertical resolution of hundreds rather than tens of meters, which is sufficient for prospect delineation in e.g. thrust belt areas.
Note that the method has been developed through the 10 years of earthquake experience build up by the Patras University. Specific field procedures, equipment and software have been developed to handle the massive data of say 60*3*5000 micro earthquakes acquired over 6 months.
The tomography principles are known in the industry and are exploiting the seismic frequencies between 1 and 10 Hz just below the conventional reflectivity seismic band. This method is used:
- By earthquake scientist to accurately locate hypocenters
- By petrophysicist in cross-hole well studies to detect subsurface seismic features around producing zones
- Only very recently seismologists started to record structural reservoir and especially fluid movements of oil and gas fields using the microcrack energy thought to be produced by stress relief caused by the production of reservoirs. However, the frequency range is in the KHz while the passive seismic is up to 50 Hz
- The presented 3D seismic passive method as offered by e.g.Landtech is unique that it employs earth seismic activity as present in geologic active areas down to 15 kilometre and provides a 3D view of the sub-surface geologic structure, expressed through the formations' velocity with a vertical resolution of a few hundred meters over relatively large areas for a fraction of 3D cost, e.g. a 10 month survey over 1000 km2 would cost around $ 800.000 US or $600-1600 US/km2
- To get a compatible detailed 3D sub-surface image, one would require a 3D survey in e.g. complex thrust belt area, which for exploration purposes would be prohibitively expensive(30,000-70,000 US/km2). A 2D regional survey would prove to be very difficult to interpret 2D structural data, neglecting three dimensional complexity and at a cost of at least 4 times the passive method
- The required software for automatic onset picking of the first and second arrival (i.e. P and S phases) of each event and the tomographic ray trace tracking has been developed by the geophysical department of Patras over the last 4 years. The picking of the shear events is a non-trivial process
This is the chance to get structural and lithologic geological information in tectonically complex and (often) thus poor quality-conventional-seismic-data areas for a fraction of the conventional cost.