Downhole method is performed for the detection of low velocity zone which is undetectable with conventional surface seismic. A method for determining soil dynamic modules by using P wave velocity and S wave velocity. Implementation is carried out within the borehole. Sledge hammer that is source of wave is used on the surface and receiver probe is moved in borehole. Travel time is measured, seismic wave is calculated. Measurements is executed below or above of groundwater table. Borehole is prepared before measurements. Casing is installed which has 3 inch or 6 inch and the back of the casing is filled with injection.