It is very important to investigate the strength of surface layers in order to mitigate the risk of shallow slope failures or debris flows. A synthesized approach to the geophysical survey method and sounding tests is developed in the present study. The surface wave method (SWM) is selected as the geophysical survey method, while dynamic cone penetration (DCP) is selected as the sounding test method. From SWM and DCP, the shear velocity, Vs, and the DCP blow count, Nd, respectively, as well as two kinds of results, need to be transformed to the standard penetration test blow count, NSPT, in order to synthesize the two methods. The indicator simulation (IS) method, which is a kind of geostatistical method, is employed to simulate the random field of N values by synthesizing two types of results. The proposed procedure is applied to evaluate the strength of the weak surface layer of a cut slope composed of weathered granite. The distribution of the weak layer could be evaluated by the spatial distribution of the probability of N <2.