Total performance check of quantitative echo sounders by using echoes from sea bottom

Even if a quantitative echo sounder is calibrated accurately before installing its transducer on a ship huh or on a towed body, it is necessary to calibrate it by using a standard sphere during a survey at sea. The sphere calibration is not necessarily easy, however, because the ship must be settled by anchors and the standard sphere should be suspended on the beam axis of the transducer; further it is not easy to calibrate the equivalent beam width. Therefore, another complimentary method is expected to check the quantitative echo sounder performance. We developed a method to check total performance, including the equivalent beam width, of quantitative echo sounders. In the method, we only measure averaged echo intensity from the flat sea bottom by the echo integrator. If the surface backscattering strength of the sea bottom is already known, an absolute check is possible, but if not, a relative check can be done. First, a relationship between the echo integrator output and surface backscattering strength of the sea bottom is derived and examined by measurements. Next, we conducted an actual performance check of quantitative echo sounders using this method and confirmed the effectiveness of the proposed method.