In elastic wave computerized tomography (CT) detection, PVC cases are always arranged in boreholes to protect the borehole wall as well as the transmitting and receiving transducers. Refracted wave would be generated when the elastic wave velocity in casing is larger than that in surrounding medium, hence reducing the accuracy of inversion result (dislocated or even false anomalies). In this article, we took a subway line investigation as a research case, and analyzed the complicated detection of high-velocity anomalies (boulders) in low-velocity medium (clay). According to two principles of identifying refracted wave, namely, first arrival travelling time being linear and take-off phase being positive, we could differentiate between refracted wave and direct wave, and pick up the correct direct wave for inversion. The proposed method is efficient and accurate in identifying the location and distribution of anomalies.