A circumferential acoustic logging instrument that operates at a radial mode resonance of 120 kHz utilizes opposing pairs of acoustic transmitters and receivers mounted on pad assemblies, which are pressed against the borehole wall. An important feature of the instrument is a baffle system which effectively minimizes the propagation and reception of the direct fluid wave. The incorporation of the baffle allows both types of boundary waves, the Rayleigh and the guided fluid wave, to be more easily recognized.