Abstract: A two-dimensional array of a plurality of transducers comprising a first plurality of like sub-arrays (11, 11a, 11b) of transducers (10) in a circularly symmetric arrangement around a common centre (C), where the transducers in each sub-array of the first plurality have individual distances from the common centre that form a progressive series of distances with a first lower limit and a first upper limit. Each sub-array in the first plurality of sub-arrays comprises at least three transducers arranged on a first straight line (12), and the first straight line is offset laterally a first distance (d) from the common centre. The number of sub-arrays is odd, and the sub-arrays may be separate units that can be selectively assembled to form the two-dimensional array and selectively disassembled.