Reflection of Low-Frequency Sonar Signals from a Smooth Ocean Bottom

Abstract

Measurements of the bottom-reflection losses for a low-frequency sonar signal (0.7–3.0 kc/sec) are reported for two areas off the California coast and for a third area in the Bering Sea. At all three areas, the bottom losses were large at small grazing angles. Comparisons are made between the measured values of bottom loss and those calculated for a plane sound wave reflecting from a layered model of the ocean sediments. The sediment model consists of a number of absorbing solid layers. Reasonable agreement is shown between the experimental and calculated values of bottom loss.

Measurement of the viscoelastic and related mass-physical properties of some continental terrace sediments

“…This model shows good correlation with actual results in studies done by Bucker et al (2) and Anderson and Latham (1) in which several sets of viscoelastic constants are assumed and solutions are correlated with observed data. Although it has been demonstrated to be feasible and accurate, the viscoelastic model presents the difficulty of evaluating the Lame' constants for various oceanic areas and soft sediments.…”

“…According to Morris (16), this is primarily due to neglect of the conversion of compressional wave energy into shear waves at the sediment interface and layer boundaries. For this reason a viscoelastic model is favored which uses complex Lame* constants and provides for the generation of shear waves upon reflection and for absorption of the waves (2).…”

“…Although it has been demonstrated to be feasible and accurate, the viscoelastic model presents the difficulty of evaluating the Lame' constants for various oceanic areas and soft sediments. The only three direct methods known to be in use for evaluating them are by calculation from Stoneley wave velocities (10) or by use of torsional wave vibration (7) or a torsional wave viscoelastometer (4,11 becomes complex (13) and ni -j n2 [2] where n-, is the real part and n ? the imaginary part of the shear viscosity coefficient.…”

The use of surface wave techniques for verification of dynamic rigidity measurements in a kaolinite-water artificial sediment

“…Substituting [4] into [1] we obtain: It is possible to write solutions to the equations of motion, [5], [6], and [7] for surface waves whose displace- (8) ik (x+r,Z) If c = 3, the left-hand side of [10] is equal to 1, while 2 2 for c = it becomes -16(1-3 /a ) which is less than 0.…”

Measurement of the complex dynamic rigidity of recent marine sediments

“…(1) the Stonely wave technique [10], which is limited to low frequencies; (2) use of torsional wave vibration [7]; or (3) a torsional wave viscoelastometer [11,5,2]. 8 Research conducted by Hutchins [11] at the Naval Postgraduate School demonstrated the feasibility of measuring the complex shear modulus or dynamic rigidity of simulated ocean sediments.…”