We present a technique for determining the rheology of visco-elastic materials in the ultrasonic frequency range based on the measurement of the longitudinal and shear waves reflected and transmitted by a slab of material with a controlled thickness sandwiched between two delay lines. The high sensitivity of this Fabry-P erot resonator-based technique allows one to access characteristic relaxation times associated with visco-elasticity. As a useful application of the technique, we characterize the Olympus SWC-2 shear-wave couplant in the 0.7-5 MHz frequency range.