Let there be light: illuminating physical models from the surface
David C. Henley, Joe Wong
A complex physical model (or a new seismic target) should be illuminated by seismic energy over as wide an observation aperture as possible, in order to adequately image all the details of the model. Ideally, this aperture would include 360deg of illumination of the model, and the resulting image will then accurately capture the model features for interpretation. In the field, however, we often cannot illuminate a target over more than a fraction of the ideal aperture. Hence, it is useful to explore how well we can detect and characterize the features of an unknown target using reflection data acquired over a restricted aperture, for example, data recorded on the exposed upper surface of a model, representing 90deg or less of angular range.
A complex physical model was installed at the CREWES physical modeling facility to explore illumination from various combinations of sources and receivers, including subsurface placements utilizing both vertical and horizontal boreholes. Several complete ultrasonic modeling surveys were conducted using this model. From the available data sets, we selected a multi-fold conventional 2D CMP survey and a high-spatial-resolution single-fold zero-offset survey to see how much we could observe or deduce about the physical model, using only sources and receivers located on the upper surface of the model.
Strong coherent noise appears in both data sets, and we demonstrate techniques for attenuating the noise, both on the single-fold data, and on source gathers and common-offset gathers of the conventional survey. We then form various images from each data set and discuss the model features seen on each image. Surprisingly, the single-fold high-resolution image shows more actual detail than the multi-fold stack for the shallow features, and the single-fold zero-offset image extracted from the multi-fold survey convincingly reveals the deeper model features, albeit at lower horizontal resolution.