The relation between seismic P and S-wave velocities, standard penetration test blow counts (SPT-N) and relative density are the subject of our research. We focused especially on variations of SPT-N versus seismic velocities and relative density versus seismic velocities. Contrary to the traditional approach, the scattered nature of the graphs led us to define the empirical relations as upper and lower bounds instead of single average curves. The values, calculated from the upper and lower limits, have large ranges and, therefore, span whole observable values of the site and geological unit. The wideness of the range between the upper and lower limit recommends that if a geotechnical parameter is required for the assessment of the soil condition; direct gathering from the field or core sample must be employed for a realistic evaluation.