Passive Seismic Tomography

Parsan in association with Seismotech Geophysical Solutions Ltd. of Greece is provider of Passive Seismic Tomography.

Passive Seismic Tomography is a new revolutionary geophysical exploration technique which uses the natural seismicity (microearthquakes) as seismic sources and a portable seismological network as receivers in order to perform a detailed 3-D seismic velocity and Poisson ratio model of the upper few Km of the crust. This service provides both P and S wave velocity structure determinations in active tectonic areas using natural occurring microearthquakes as seismic sources. Such tomographic imaging methods are particularly attractive since they provide not only structural information as conventional 3-D reflection surveys but lithological as well, and do so at a small fraction of the cost of a comparable reflection survey.

This technique is particularly useful in complex geological and mountainous areas as it can provide information on hidden structures. Both reconnaissance and detailed 3-D surveys can be provided. Interpretations of velocity structure characteristics relative to oil/gas prospects are based on survey results integrated with other existing geologic and geophysical data. Furthermore, the assessment of the 3-D variations of Poisson’s ratio can indicate the presence of gas and fluids in sedimentary formations. Generally econnaissance surveys in tectonically active areas require only a few months for good results, considerable less time than that required during conventional high-resolution exploration surveys.

Knowledge of a subsurface fracture system is of vital importance for an accurate evaluation of the potential and day-to-day production of a reservoir. Seismotech Geophysical Solutions Ltd. has developed a methodology to assess 3D fracture patterns, their density (number of cracks per unit volume) and outline zones of high permeability in areas where has installed passive seismic networks and at a small extra cost. The method is based on the shear wave splitting phenomenon by analyzing the recorded during the passive survey microearthquake waveforms.

Seismotech Geophysical Solutions Ltd. have developed a method for mapping subsurface fracture densities using the time differences of split shear waves from microearthquakes. These measurements are then inverted using backprojection tomography to locate the spatial distribution of crack density. Standard tomographic display methods were then used to create maps of crack density for characterization and development of the reservoir.