formed having a variable array depth elements 15 as illustrated in fig2 by forming the reflective array 14 that has various array depths elements 15 across the longitudinal axis of the array 14 with reflective elements 16 the reflectivity at points along the array 14 is increased as the distance of the points along the array 14 from the transducer 10 increases the depth of each element 15 in the reflective array 14 is such that the power reflectivity per unit length of the reflective array u03c3 x is given to first approximation neglecting diffraction and multiple reflections by the following equation equ1 where the ratio of the depth of the array at x to the depth of the first array element x 0 is given by the equation equ2 and the ratio of the depth of the last array element and the first array element is given by the following equation equ3 where u03b1 represents the power absorbtivity of the array per unit length x is a variable representing the distance from the start of the array and l represents the length of the array to design a variable depth array of length l a practical maximum value for the depth d l is assigned and substituted into equation 3 together with experimentally measured values for u03b1 and u03c3 l and array length l to determine d 0 thereafter these values etc are.