Imaging from one-bit correlations of wideband diffuse wave fields

We present an imaging technique based on correlations of a multiply scattered wave field. Usually the Green's function h(AB) between two points (A,B) is determined by direct transmit/receive measurement. When this is impossible, one can exploit an other idea: if A and B are both passive sensors, h(AB) can be retrieved from the cross correlation of the fields received in A and B, the wave field being generated either by deterministic sources or by random noise. The validity of the technique is supported by a physical argument based on time-reversal invariance. Though the principle is applicable to all kinds of waves, it is illustrated here by experiments performed with ultrasound in the MHz range. A short ultrasonic pulse, sent through a highly scattering slab, generates a randomly scattered field. Behind the slab is the medium to image: it consists of four liquid layers with different sound speeds. The cross correlation of the field received on passive sensors located within the medium is used to estimate the speed of sound. The experimental results show that the sound-speed profile of the layered medium can be precisely imaged. We emphasize the role of wideband multiple scattering and of source averaging in the efficiency of the method, as well as the benefit of performing one-bit correlations. Applications to seismology are discussed. (C) 2004 American Institute of Physics.