Standard calibration procedures for multibeam sonars currently only address the fidelity of the bathymetric data. Equivalent effort is needed to ensure that the acquired seabed backscatter strength measurements are referenced to a similarly precise level. This paper presents an operational method utilizing multiple pre-calibrated split-beam echo sounders covering a wide range (50-400 kHz) of frequencies. This is needed to cover the full range of frequencies utilized by multisector multibeams operating in continental shelf depths.