Studies of three dimensional lithospheric structure and tectonics using seismic tomographic methods: Applications to resource assessment

Abstract

New seismic techniques, utilizing signals from the large numbers of micro-earthquakes occurring in tectonically active areas, can now provide detailed 3-D seismic velocity structure images and 3-D maps describing on-going tectonic deformation. The deformation is expressed in terms of spatial variations in the magnitude of the stress released by micro-earthquake fracturing, the spatial variations in total strain energy release by brittle fracture and also in terms of the 3-D determination of the (time dependent) energy release levels from rapid creep deformation along faults. Taken together, the seismic velocity structure and the spatial definition of the tectonic state of the medium can be used to characterize the physical properties of the material and its deformational strain state and to define the large scale faulting dynamics within the medium. This material state and tectonic state information can then be used to infer the presence of mineral deposits based on correlations of imaged intrusive bodies and other structural features (faults, folds, etc.) with the intrinsic material properties inferred from seismic velocity and other material parameter determinations. An example from the Andes Mountains in South America, in an area with deep seated oil and gas deposits, is presented to illustrate the applicability of these seismic methods for resource exploration and the delineation of micro-tectonic features. In this application, a network of ten micro-arrays, each consisting of four sensors with about a half kilometer aperture, was designed to detect and locate earthquakes to very low magnitudes in order to define the tectonic activity and structure in an area of about 2,000 km 2 . All the event locations were used to define active fault zones in three dimensions and were analyzed to infer patterns of stress and energy release, as well as fault mechanisms. The recorded events were also used to obtain detailed tomographic images of the three dimensional P and S wave velocity structure to mid-crustal depths. In addition to monitoring discrete event activity to low magnitude levels, the network was also used to perform Array-based Seismic Emission Tomography which is used to determine the low level seismic energy emitted during rapid creep. The 3-D seismic velocity structure determined from the earthquake P and S wave signal data, showed the presence of major high angle thrust faults to mid-crustal depths and the locations of sedimentary units beneath the network that correlated with known structure from deep wells. The velocity structure was also correlated with both the large and small scale tectonic patterns. Variations of the Poisson's ratio, computed from the P and S wave velocity determinations, showed anomalously high values (above .35) in the area surrounding the major high angle thrust faults. This anomaly is interpreted as a consequence of high densities of fluid filled fractures within and near the faults. This interpretation is supported by the nature of the tectonic activity observed and, in more direct fashion, by the presence of a deep oil production zone centered within the shallowest of the zones identified. Consequently, these seismic methods are able to locate oil/gas deposits directly from Poisson ratio determinations, as well as indirectly through structural and lithological inferences. It is also concluded that the application of these methods, for the precise location of intrusive bodies and major crustal faults, should prove useful in the exploration for other minerals.