Optics, Acoustics and Stress in Situ (OASIS)
Figure 1. Stresses estimated from the 2007 measurements at the Martha’s Vineyard Coastal Observatory. The directly measured hour-averaged Reynolds shear stress agrees well with the drag-law estimate inferred from the measured burst-averaged velocity by means of an empirically determined drag coefficient . The standard deviation of the oscillatory bottom shear stress produced by surface waves is inferred from a wave-current interaction model and is much larger than the hour-averaged stress , which is associated with wind-driven and tidal currents. The wave-induced stress is primarily responsible for suspending particles and controlling the size of water-borne flocs. ()
|Figure 2. Acoustic backscatter profiles form the 2010 Mud MURI field program showing scattering from suspended sediment (15 to 80 cm), a fluid mud layer (1 to 15 cm) and the stationary seafloor (0 cm). b through e) Along beam velocity profiles for each of the 4 individual frequencies that are used in the multi-frequency inverse showing two cycles of a 5 s period wave. The velocity aliasing can be seen as the sharp transition from blue to red (discontinuity of 0.24 m/s ) at t=178 s in panel b, and t = 174, 176 and 178 of the higher frequencies. f) The result of the multi-frequency inverse showing the de-aliased velocity profiles. ()|
|Figure 3. Schematic diagram of the pulse-coherent Doppler sonar system developed at WHOI by Austin, Jaffre, Terray & Trakovski. The red shaded area represents the region that would be profiled at turbulence resoving rates (~10 hz) and the extended yellow region would be profiled at a slower rate (~2 hz). An additional pair of angled transducers is located in the plane perpendicular to the pair shown ()|
|Photograph of instrumentation just before deployment at MVCO in September 2011, showing Pulse Coherent Doppler Profiler (PCDP), Imagenex rotary sidescan sonar, Imagenex 2-axis rotary pencil beam sonar, Aquatec ABS and Nortek Vector ADVs. ()|
|Figure 5. Data from two prototype 881L tilted head rotary fanbeam sonars. The data in the right panel uses a wider beamwidth transducer to increase the ability to image under the sonar. The size of the imaged box is 2.5 m on a side. ()|
John H. Trowbridge and Peter Traykovski
To quantify and understand the effects of aggregation dynamics on the distribution of particles in the bottomboundary layer, and to understand how the properties of particles (composition, shape, and internal structure) affect their optical and acoustical properties.
- Obtain direct measurements of the turbulent Reynolds stress within the centimeters-thick wave boundary layer (WBL)
- Obtain concurrent velocity, turbulence and seafloor microtopography (bedforms) measurements to constrain the fluid dynamical environment within which the particle size distribution evolves.
- Improve state-of-the-technology for measuring boundary layer velocity and turbulence profiles (Pulse Coherent Doppler Profiler) and seafloor bedform measurements (Rotary Sidescan and Pencil-beam sonars)
The approach is to obtain direct measurements of the turbulent Reynolds stress within the centimeters-thick wave boundary layer (WBL), which has not been possible in previous field studies. This is motivated by the order-of-magnitude difference between relatively small stresses that have been measured above the WBL and the much larger stresses that have been inferred within the WBL, the discrepancy between measured and modeled stresses above the WBL, which has been attributed to processes within the WBL, and the dominant role inferred by Boss, Hill and Milligan of large stresses within the WBL in controlling particle size and thus strongly influencing particle settling velocities, concentrations, and optical properties (Figure 1).
The measurements will capitalize on multi-frequency pulse-coherent Doppler sonars developed recently at WHOI by Peter Traykovski and Fred Jaffre (2011) based on a high frequency sonar board designed by Gene Terray and Tom Austin and Peter Traykovski. The multi-frequency methodology (Hay an Zedel, 2008) removes the ambiguity associated with conventional pulse-coherent Doppler sonar measurements (Figure 2). The combination of range-gated vertical (transmit-receive) and slanted (receive-only) acoustical beams produces vertically resolved measurements of the horizontal and vertical velocity throughout the wave boundary layer and the lower part of the overlying current boundary layer (Figure 3). The effective sample volume is the product of the beam patterns along the acoustical transmission and reception paths, and is small because of the narrow vertical beam. The new sensors produce high-precision velocity measurements along with order-one-meter range, sub-centimeter spatial resolution, and a sample rate of approximately 18 Hz. With proper configuration, these sensor characteristics are sufficient for measurements of both the oscillatory velocity and the turbulent velocity fluctuations in the WBL at field scales.
Improvements to our Rotary sonar system were implemented by purchasing two new 881 L rotary sidescan sonars from Imagenex. We worked the manufacturer to develop a downward tilted head fan-beam transducer within an oil filled cap with sufficient vertical beam width to permit imaging of ripples from 1 m horizontal range from the transducer to 7 m based on an mounting height of 1m. The data acquisition system was also upgraded from a Persistor CF2 with only serial communications capabilities to a TS-7260 ARM9 based Linux embedded computer which supports Ethernet, USB and serial communications, while maintaining low power consumption.
The multi-frequency pulse coherent Doppler system first deployed in multi-frequency mode in the Skagit tidal flat program was upgraded to focused beam geometry. Analysis combined with the evaluation of several prototypes was used to determine to optimum geometry to maximize overlapping range of the fan beam with the vertical beam while not sacrificing performance of the horizontal velocity estimates due to a low incidence angle. The circuitry was also upgraded with a more powerful analog front end to increase the SNR of the system. Through extensive laboratory testing we were able to increase output power while ensuring the system did produce excessive non-linearity’s resulting in acoustic streaming. At maximum power output, due to non-linear momentum transfer to the water (acoustic streaming), the system produced 5 cm/s mean velocities away from the central transducer. By lowering the output power these were reduced to below 1 mm/s.
Since the instrumentation was deployed just before the submission of this report (Figure 4) results are limited to laboratory testing of the systems. Figure 5 shows data from two prototype 881L tilted head rotary fan-beam sonars in which we tested different transducer heads, one with a wider beamwidth transducer to increase the ability to image under the sonar.