System and method for determining directional and non-directional fluid wave and current measurements

A subsurface buoy mounted sonar system for determining the directional properties of surface waves or internal waves of a fluid medium, the subsurface buoy mounted sonar system mounted on or in a sub surface buoy that can move or rotate during measurement, comprising:

• a sonar system having a plurality of transducers for generating respective, separate acoustic beams and receiving echoes from one or more range cells located substantially within the beams, the plurality of transducers including an Acoustic Surface Tracking (AST) transducer for generating acoustic beams and receiving echoes for making AST measurements;
• a motion sensor including at least a three-axis accelerometer measuring acceleration of the subsurface buoy mounted sonar system along three axes (AX, AY, AZ) and a three axes tilt sensor (MX, MY, MZ) to account for horizontal velocity and vertical displacement in the subsurface buoy mounted sonar system;
• a computer program executed by a processor for
• a) determining, based on the AST measurements, the distance from the system in the fluid medium to a free surface of the fluid medium and an ideal horizontal plane near the free surface of the fluid medium from which to determine the directional properties of surface waves or internal waves of the fluid medium;
• b) determining the directional properties associated with the surface waves or internal waves from the received echoes;
• c) determining along-beam velocities along the separate acoustic beams;
• and
• d) combining the along-beam velocities to form an equivalent orbital velocity vector at a central location over the AST transducer in the ideal horizontal plane determined by the AST measurements.