Seawater--Acoustic properties

A detailed study of a novel method for high-speed acoustic communications in ports and
shallow water is presented. A series of field experiments, coupled with simulated results
using an acoustic channel model have been conducted to outline the optimal modulation
schemes for use in the highly reverberant and Doppler dominated shallow water
acoustic channel. Field experiments were conducted in the vicinity of the SeaTech
marina and the Port Everglades turning basin in water depths of 2 to 15 meters and
ranges of between 25 and 75 meters. An automated FAU acoustic modem transmitted
BPSK and QPSK modulated messages centered at 300 kHz, with a source level of 173
dB re 1pPa and a symbol bandwidth of 25, 50 or 75 kHz. The coded rate varied from
25000 to 150000 bits per second. These high data rates are made possible using a high
resolution Decision Feedback Equalizer with an efficient Doppler compensation process.
The results of this study demonstrate the ability of such a system to transmit video
images in a shallow water environment.

Experiments have been performed which determined the fatigue crack growth rate (FCGR) of short cracks (a > 0.1mm) for five high strength steels (yield stress 370-570 MPa) in air and in natural seawater with and without cathodic protection. Attention was focused upon Regions I and Il of the classical FCGR-stress intensity range(Delta K) curve with particular consideration of the near-threshold behavior for short cracks. Single edge notch (SEN) three-point bend specimens and a direct current potential drop (DCPD) crack monitoring system were employed, and test parameters simulated offshore structure conditions. The results indicated enhanced FCGR for short cracks compared to macrocracks by 3-20 times in air and 2-6 in seawater free-corroding(FC). Also, the Delta Kth for short cracks was apparently lower than for long ones in both environments. The transition from short to long crack behavior occurred at constant $\Delta$K in each environment (15.6 MPa m in air and 10.0 MPa m in seawater(FC)) irrespective of initial Delta K (Delta K(0)). The transition crack length ranged from 0.25 to 1.6 mm and was inversely proportional to $\Delta$K(0). Scanning electron microscope fractography showed that the mechanism of enhanced crack growth rate was associated with secondary crack (SC) formation in air and SC or inter-granular cracking (or both) in seawater (FC). The enhanced FCGR for short cracks was minimized by polarization to -950 mV(SCE). Through an elastic-plastic fracture mechanics analysis using the J-integral parameter it was found that the influence of plastic deformation at the crack tip was approximately independent of crack length (short versus long), and the linear-elastic fracture mechanics analysis gave a realistic representation for fatigue behavior.

This research investigates the validity of an acoustic propagation model by comparing theoretical reflection coefficients, function of frequency, to FAU chirp sonar measurements (chirp sub-bottom profiler). An acoustic model has been implemented to estimate the spectrum of energy reflected from sandy sediments in the presence of surface scattering. The surface roughness being the dominant reverberation part, the volume scattering has been neglected in this model. A laser scanning system involving an image-processing algorithm has been designed to measure the seafloor bottom roughness using 1D Fourier transforms. In the case of anisotropic roughness, an estimation of the sand ripples dominant direction is provided involving 2D Fourier transforms. Measurements of acoustic data using a chirp sonar and estimation of bottom roughness from video data of the scanner over an artificial bottom are provided to compare the reflection coefficients obtained from the data actually measured with those from the acoustical model.