Imaging and interpreting the shallow crystalline crust

In the past, many tests of high-resolution seismic imaging methods, utilizing 'off-the-shelf' technology from hydrocarbon exploration have had only limited success. Some problems associated with the application of seismic profiling in the shallow crystalline crust differ from those encountered in sedimentary basins. Perhaps the most fundamental distinction stems from differences in statistical properties of the reflection coefficient time series. Analysis of power spectra derived from borehole data indicates that the reflection coefficients in the crystalline crust are depleted in the seismic bandwidth relative to sedimentary basins. This fundamental difference must be considered in the design of shallow crustal seismic surveys in order to achieve success. To a first approximation, acoustic impedance variations in sedimentary basins are controlled by velocity changes, whereas velocity and density can vary independently in the crystalline crust and changes in density may be relatively more important in controlling reflectivity. In certain instances, density fluctuations outweigh velocity fluctuations. For example, physical rock property data from mining districts in the Canadian Shield suggest that massive Fe-oxide or sulphide bodies will generate strong seismic reflections. (C) 1998 Elsevier Science B.V. All rights reserved.