The Use of Phased-Array Doppler Sonars near Shore
Journal of Atmospheric and Oceanic Technology
Phased-Array Doppler Sonars (PADS) have been used to probe an area several hundred meters on a side with 8 m spatial resolution, sampling every second or less with under 2 cm/s rms velocity error per sample. Estimates from two systems were combined to produce horizontal velocity vectors. Here concerns specific to use in shallow water are addressed. In particular, the shallower the water is, the larger the fraction of bottom backscatter, so the stronger the bias is toward zero Doppler shift in
... Doppler shift in the estimates. First, direct comparisons are made with other current measurements made during the ONR-sponsored multi-investigator field experiment "SandyDuck," which took place in fall 1997 off the coast of Duck, NC. While the coherences between PADS and in situ current measurements are high, the amplitude of the sonar response is generally low. To explore this further, a simplified model of wave shoaling is developed, permitting estimates of wave-frequency velocity variances from point measurements to be extrapolated over the whole field of view of the PADS for comparison. The resulting time-space movies of sonar response are consistent with quasi-steady acoustic backscatter intensity from the bottom competing with a variable backscatter level from the water volume. The latter may arise (for example) from intermittent injection of bubbles by breaking waves, producing patches of high or low acoustic response that advect with the mean flow. Once this competition is calibrated via the surface wave variance comparison, instantaneous measured total backscatter intensities can be compared to an estimated bottom backscatter level (which is updated on a longer time-scale, appropriate to evolution of the water depth or bottom roughness) to provide corrected sonar estimates over the region. 2 "Blocking" by the bubble plumes left by plunging breaker was described for systems from 40 kHz to 200 kHz (Smith, 1993; Thorpe and Hall, 1993) , and appears to limit applicability to the region outside the break-point of incoming surf. More recently, influences of surf-generated bubbles on acoustic propagation and of bubble advection in the surf-zone (Dahl, 2001) and in rip currents (Caruthers et al., 1999; Vagle and Farmer, 2001) have been described. However, biasing of Doppler-based velocity estimates toward zero by bottom interference has not been quantitatively addressed previously. A companion paper (Smith, 2001) describes the general use of high-frequency "phased-array Doppler sonars" (PADS) to probe near-surface horizontal velocities over a continuous time-space segment extending hundreds of meters on a side and many hours in duration. Given the comprehensive measurement needs mentioned above, application of this technique in the nearshore is compelling. Continuous coverage of waves and currents in space and time appears feasible, permitting estimation of the vertical component of vorticity of the nearshore currents, and of the divergence of the waves' "radiation stress" and mass flux (for example). Smith (2001) describes the essential technique and algorithms yielding quantitative estimates of errors and biases, including an objective technique for combining information from two (or more) systems to estimate horizontal vector velocities. Here concerns specific to using high-frequency (~200 kHz) PADS in shallow water are addressed. There are several aspects of the nearshore environment that distinguish it acoustically from deep water: (1) The bottom backscatters sound that competes with the signal from scatterers in the water volume. The received signal attributable to bottom backscatter varies with water depth, and can also vary slowly in time (presumably as bottom roughness characteristics evolve). In general, this becomes significant when the wind and waves are weak, when few bubbles are generated. (2) Plunging breakers can produce a "wall" of bubbles so dense it is acoustically impenetrable (Smith, 1993; Thorpe and Hall, 1993) . This limits the shoreward extent of measurements, confining the sample area to outside the active surfzone. (3) There are large variations in the scatterer content of the water, on scales of meters to tens of meters (e.g., water advecting offshore in "rip currents" that is full of bubbles from the surfzone (Smith and Largier, 1995)). (4) Advection of water from inlets can also lead to variations in stratification and in particle content of the water. These issues are addressed through comparisons with data collected as part of "SandyDuck," a multiinvestigator field experiment sponsored by ONR with assistance from the Army Corps of Engineers and the USGS. Data in or near the field of view of a dual-PADS deployment were provided by P. Howd (vertical profiles at one location, 3 min. average currents) and by S. Elgar, R. Guza, T. Herbers, and W. O'Reilly (current meters at many locations, near bottom; 0.5 s samples). The selection of comparisons and interpretation of the results are guided by consideration of the underlying physics. In particular, the predictable behavior of surface waves propagating on a nearly planar beach permits the comparisons to be extended over the whole area, and to the full time-space behavior of the response. The results indicate a relatively straightforward competition between bottom and volume backscatter. Experimental Setup Two "Phased Array Doppler Sonars" (PADS) were deployed as part of "SandyDuck" in September and October, 1997 at the Field Research Facility (FRF) of the US Army Corps of Engineers. Looking shoreward from the 6-m depth contour, they probed a total area about 400 m alongshore by 350 m cross-shore (figure 1). Over the smaller region probed by both systems, perhaps 200 m by 300 m, horizontal velocity vectors are fully resolved. In the outer corners, only one component is resolved; however these 1-component estimates still provide useful information, particularly concerning wave propagation.