3-D Seismic Imaging of Complex Structures in Near-Surface Deposits

Abstract

One of the most important methods of investigating the geological structure of sedimentary rocks is controlled source reflection seismology. Reflection seismology has traditionally focussed largely on petroleum exploration problems, i.e., imaging formations in consolidated sedimentary rocks at depths in the range 0.1-10 km. However, hydrogeological investigations and large engineering enterprises also may require detailed pictures of subsurface structure, but in relatively unconsolidated surficial sediments at typical depths of 3-100 m. If subsurface structures are not too complex, relatively simple two dimensional (2-D) seismic profiling methods can provide detailed cross-sections of the lithological stratigraphy and structure. But, as has been well demonstrated in petroleum exploration, three dimensional (3-D) surveying methods are required to obtain an accurate picture of complex structures. Nowadays, especially in areas of former glaciation, increasing efforts are being placed on using 2-D seismology to obtain detailed images of near-surface geology in connection with groundwater, waste disposal, and engineering projects. But, although results are very gratifying in some respects, the resolution of very local structures by 2-D seismic surveys has been insufficient to identify possible hydrogeological, and engineering problems. It is natural to consider whether 3-D seismic surveying in shallow environment might yield beneficial results similar to those it provides in petroleum seismology (always presuming that survey cost can be restrained). To address this question, I have investigated the imaging capability of 3-D multi-fold high-resolution reflection seismology in near-surface complex deposits and designed a form of three-dimensional seismology suitable for hydrogeological, geotechnical and other similar surficial exploration purposes. A test 3-D survey was carried out successfully and its results surpassed expectations in several ways. The survey was relatively easy to carry out using standard engineering scale seismic equipment and a 3-4 person crew. It provided a cube of stacked data with fairly high dominant frequency ($\sim$300 Hz) covering a subsurface volume of 220 x 220 x 200 meters with a trace bin of 3 x 3 meters. In addition to the reflectivity image, the survey provided a velocity model to $<$100 m depth that assisted greatly in geological identification of the reflectors and was good enough to permit post stack depth migration. Details of the stratigraphy are much more clearly revealed in the 3-D stacked sections than in nearby 2-D sections. Generally, tills are expected to be massive units. However, the 3-D seismic sections show strong, highly continuous, reflectors within the till deposits which are of much greater continuity and extent than has previously been realized. Foreset bedding can be recognized in one of the sedimentary strata. Results indicate that high resolution 3-D seismic surveying can contribute effectively to the detailed hydrogeological and engineering assessment of sites with complex geology.