Abstract

A method for the reconstruction of hologram images by digital computer has been developed and its technical feasibility, demonstrated. The method for the detection of geological formations is applied to acoustic data from a series of two-dimensional models of the earth. The models are constructed from 4 ft × 8 ft acrylic sheets and a radially pulsed (wide-band) energy source is used. Values of elastic displacement scattered to the surface by the buried formations constitute the basic data from which the images are reconstructed. The images are displayed as computer plots of amplitude versus position. Narrow frequency bands ranging from 5 khz to 70 khz are selected from the wide-band signals to form images at several different frequencies. Range-gating in the time domain is demonstrated to be a useful technique for the removal of source-generated noise. At all frequencies, the images demonstrate lateral and depth resolution near the diffraction-limited values predicted by equations from classical optics. The method permits detection of objects with discontinuities that are sufficiently sharp to produce significant diffraction of the incident energy, such as a geological fault. Images of weakly diffracting objects, such as a triangular anticline, are more difficult to interpret because of strong ghost images of the source. The results show that the interference of diffracted and reflected energy can lead to erroneous interpretations of acoustic hologram images.

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