30m may be the extent of their coring equipment and standard for geological cores.  If course if you hit bedrock first, that may be all you can do.  When you are putting in dams and other expensive infrastructure, geologists and soil mechanics, engineers need to know what is present, there may be lives and huge property loss at stake.  They cannot afford to make mistakes.  Perhaps there are other reasons too, such as mentioned sediment in a floodplain, they might want the depth so the layers can be evaluated as to their particle size structure, depths, colors, potential for piping erosion, dating of layers, etc.

There are two possible approaches: The first approach is to utilize ground motion attenuation relationships that have been developed for subsurface conditions of the type that prevails at the site. The second approach is to conduct a site response analysis compatible with the geotechnical and dynamic characteristics of the soil and rock layers beneath the site. In the practice of site investigation and evaluation for nuclear installation the probability criterion for design basis is 10-4/a (or 10-5/a in some countries) and free-fied response spectra has to be defined accounting the site response that can be nonlinear in case of soft soil sites.