Earth coordinate 3-D currents from a modular acoustic velocity sensor

MAVS, modular acoustic velocity sensor, measures 3D flow components along four acoustic paths using differential travel-time techniques, removes the offset from each acoustic axis due to individual characteristics of the cabling and any electronic offsets, and rotates the corrected components into Earth coordinates with a gimbaled two-axis magnetometer. In the present version of MAVS, the sensor is assumed to be aligned vertically to within 15°. Three requirements had to be met to allow the conversion of the three-axis current measurement into Earth coordinates in situ without introducing non-removable bias. First, flow distortion by the sensor had to be reduced to an acceptable level so that a cosine response in elevation as well as azimuth could be applied. Second, the determination of offsets for zero flow had to be easily measured and compensated, and these offsets had to be stable. Third, the measurement of sensor-frame heading had to be accurate and tilt tolerant. Fairing of the transducer support rings achieved the first requirement. Enabling the sensor to be auto-zeroed and rigidly capturing the cables inside the transducer supports achieved the second. Rotating the precisely measured sensor-frame components of velocity into Earth coordinates with the relatively imprecise magnetometer components and vector averaging the resultants achieved the third. That these can be done in a small instrument with low power permits a modular approach to current sensing. Direct reading, stand-alone logging, and modular component applications of a modular acoustic velocity sensor, MAVS, have been implemented