Interpretation of Sea Beam backscatter data collected at the Laurentian fan off Nova Scotia using acoustic backscatter theory

Abstract

The purpose is to use acoustic scattering theory and Sea Beam measurements to estimate seafloor roughness parameters. The Sea Beam backscatter data are from a test area in the Laurentian fan, a relatively flat region. Sidescan sonarlike images were reconstructed from the multibeam data. These images in the test area show two distinctly different types of areas (A) and (B). The backscatter model uses the Helmholtz-Kirchhoff formulation of scattering theory and correlation function C(r)=exp[-lr/lln], where r is the displacement, l is the correlation length, and n is the exponent. A single rough interface model fits the backscatter data in (A). The root-mean-square roughness was 6-8 cm and the correlation lengths were 140-270 cm. The exponent n ranged from 0.95 to 1.5. The type (B) areas required a two-layer model: the interfaces in the first type of areas (A) is covered by a sediment having a smoother surface. The rms roughness of the covering sediments were about 3 cm, the exponent n was nearly 2 and correlation lengths I were 90-120 cm. These acoustic models are consistent with the geological setting and processes. A insonified area P a • =l/2X•q_k2X• cos 40•/2R 2 P(s) • 1 a e --= 1/2Y• q-k2Y•/2R• 2 b power lawinP(s)=P•s -b orA(s)=A•s -•/2 Ps B • scattering function constant r B • --=k2R•22/8,r cos 2 B sf 1 and B sf2 scattering function constants for interfaces 1 and 2 C(r) C C a and Cs d ds DR DrDR i(o) G h Iref R • and R 2 =exp[--Ir/ll n] spatial correlation function, radial symmetry R 12 spatial correlation function sound speed in water average and surface sound velocity depth to the seafloor from ship's transducers depth of sub-bottom interface beneath the sea floor •'•T receiver directivity function W transmitter directivity function Ax •exp[-(x2/X•+y2/y•)] and Xo•dAqb, x• 2 Yo • dA •b for 0• =0 a = 1/cos 0•, slope corrected adB ----exp(-a•2-av•72+i2k• sin 0•) fi rms (root-mean-square) relief of the surface y mean backscattered intensity at the receiver • source intensity at the reference distance R re f (usually 1 m) wave number "correlation length" 0• and • Oe(i) exponent of the correlation function vector of parameters "power" spectrum is the spectral density at s = s • scattered sound pressure radial distance from origin to a scattering element on the scattering surface in the (r,½) coordinates are the distances of the source and receiver from the origin is the pressure reflection coefficient at the 1-2 interface spectral frequency -•(k, 0• ,R • ,R 2 ,A) scattering function backscattering strengths from the surface and subbottom total backscattering strength -=exp[-y2h2(1-C)] • rc / (2 sin 0) =(k/R•)cos 2 0•, yf 2=k/R• =k sin 0• attenuation in sediment in dB/m =0 =-k cos 0• elevation of an element of area above the mean level = arcsin{(c a/C s) sin[ Oa(i) +/x]}, effective angle of arrival incident angle 1545 d. Acoust. Soc. Am. 97 (3),

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High-frequency multibeam echo-sounder measurements of seafloor backscatter in shallow water: Part 1 – Data acquisition and processing

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