POL ABS calibrations 2005-2007
Moate, B. D.; Thorne, P. D.; Cooke, R. D.; Betteridge, K. F. E.. 2007 POL ABS calibrations 2005-2007. Liverpool, Proudman Oceanographic Laboratory, 36pp. (Proudman Oceanographic Laboratory, Report No. 183)Before downloading, please read NORA policies.
Suspended inorganic particles (such as sands) strongly scatter incident acoustic pressure waves at MHz frequencies, with the amplitude of the backscattered pressure being related to the mass concentration and nominal size of the particles in suspension. Over the past two decades, this physical premise has led to the development of transducer based Acoustic Backscatter Systems (ABS), usually designed to collect profile measurements at centimetric resolution over a metre or so of the near-bed region when implemented in sediment transport studies (Thorne and Hanes, 2002). Modern ABSs typically operate (simultaneously) at multiple frequencies, to enable derivation of the nominal particle radius (a0) from ratios of the backscattering form function at the different frequencies (Thorne and Hardcastle, 1997). The backscattering form function represents the backscattering characteristics of a given particle relative to its geometrical size, and is often expressed in terms of a parameter, x, where x = 2πa0/λ (Thorne and Meral, 2007), with λ the acoustic wavelength in water. In order to relate backscatter data derived from an ABS to particle concentration and size, the ABS requires calibration to determine the system constant at each of its measuring frequencies. An ABS system constant represents the electronic response of the ABS, its transmit and receive sensitivity, and the beam pattern of the acoustic transducer (Betteridge et al, 2007). In this way, a three frequency ABS will thus have three system constants, with one system constant for each of its component transducers. Whilst an ABS system constant is specific to a given ABS-transducer assemblage, it is independent of the concentration and size of the suspended particle populations the ABS is used to probe. By measuring the backscattered signal from suspensions of particles with known backscattering characteristics (such as glass spheres), over a number of different concentrations and sizes, ABS system constants can be determined and their size and concentration independency confirmed. In addition, comparison of system constants obtained before and after field deployments enables any calibration change during the deployment period to be discerned. In order to replace a number of aging systems, a new generation of three frequency ABSs was developed and constructed at the Proudman Oceanographic Laboratory (POL) over the period 2003 – 2005, and are hitherto referred to as ABS-1, 2 and 3. To date, these ABSs have mainly been deployed in the Dee estuary experiments and at Sea Palling during LEACOAST2. ABS-2 failed during its first deployment at Sea Palling (March 2006), due to leakage of its O-ring seals, and was subsequently replaced by a commercial 4 frequency AQUAscat ABS (AQUAtec Group Ltd.) in the next (October 2006) deployment. During recovery of the AQUAscat (January 2007), two of its cable connectors were damaged, and the instrument was returned to the manufacturer for repair and upgrade to twisted pair cables. In addition, also due to a damaged connector, the original 4 MHz transducer for ABS-1 (serial number 110) was replaced with the 4 MHz transducer from the failed ABS-2 (serial number 111), immediately prior to the October 2006 LEACOAST2 deployment. This report provides a chronological record of the calibrations of ABS-1, ABS-3, and the AQUAscat, carried out at POL over the period 2005 – 2007. Whilst this report does not aim to re-iterate either the experimental or calculation procedures detailed in Betteridge et al (2007), certain aspects of these procedures are elaborated upon here to ensure a complete record of the calibrations is achieved.