Mapping the Earth with Elastic Wave Holography

Remote sensing of the earth has been carried on for many years by the use of transient elastic waves, as in the seismic method. With the advent of optical holography and the development of high-power sonic sources and electromechanical vibrators, it is now possible to map the subsurface structure of the earth with steady-state coherent waves. This is the process of earth holography, which provides a three-dimensional optical view of the earth´s interior. The basic principles of wavefront reconstruction in optics are reviewed and their application to sonic and seismic wave holography discussed. The problems of scaling and field sampling are discussed in connection with the resolution of elastic wave systems. It is shown that the sampling density required is within practical field limits for adequate resolution. Experiments were carried out with a small water tank model using frequencies in the low MHz range and small metal objects. Data were recorded on magnetic tape and subsequently displayed. The resulting holograms and photographs of the reconstructed images are shown. Synthetic holograms and reconstructions are shown and the problems of recording and display are discussed. Possible extensions of this work to large area, deep mapping of the oceans and the earth are outlined.