Abstract
The author considers the problem of determining the contour of a rigid target from a discrete set of measurements of the acoustic scattering amplitude in two dimensions. With ultimate application to medical ultrasound imaging in mind, he is interested in a situation where the number of directions from which the target may be viewed is limited. Thus instead of the usual analysis in which a single frequency of sound and many angles of incidence and scattering are used, he investigates the extent to which the number of viewing angles may be reduced by using several frequencies of ultrasound.