Abstract: In order to measure a layer in the volume of a reflective structure, a transducer matrix emits at least one sonic or ultrasonic wave packet which coherently impinges on the layer. The echo signals at each element of the transducer matrix are processed separately, on the basis of determined distances between transducer elements and volume elements, and signals corresponding to the echo of each volume element on all the transducer elements of the matrix are calculated by correlation. By emitting two or more wave packets and evaluating the resulting phase or frequency difference therebetween, a two-dimensional velocity field of the structure in the layer can also be determined.