Multibeam

velocity of (acoustic) waves in common earth materials, but also on a well-defined

set of reflection patterns (Mitchum et al., 1977). Like GPR records, most seismic

reflection patterns are not uniquely tied to a single sedimentary environment, cre-

ating the need for comprehensive sets of seismic facies and interpretations for each

depositional setting.

Along France's Atlantic coast, Bertin et al. (in revision) used a detailed network

of shallow seismic profiles to determine the depositional history of Maumusson Inlet.

They showed that a recent increase in sedimentation in the backbarrier led to a

smaller bay tidal prism. The inlet channel responded to the reduced tidal prism by

shoaling and shifting southward. The seismic records not only established the pattern

of channel infilling, but also demonstrated how the shoaling of the channel facilitated

migration of the thalweg out of its bedrock-cut valley. Similar geophysical work by

Buynevich et al (1999) along the coast of Maine traced the evolution of Kennebec

River inlet using high resolution seismic profiles. These investigators speculated

that a slowing of sea-level rise and a stabilization of the inlet tidal prism resulted

in closure and significant shoaling of secondary entrance channels. Seismic data

documented that two of the former estuarine channels were more than 50 meters

deep (Fig. 6; Buynevich et al., 1999).

Shallow seismic surveys provide a means of studying the development of tidal

inlets prior to historical documentation. The pattern of subsurface reflectors ob-

served at tidal inlets can be related to channel cut and fill, inlet migration, erosional

and depositional trends, bedrock controls, and other inlet and geological processes.

At most tidal inlets surveys can be completed in a day or two and sophisticated

post-processing programs greatly facilitate data analysis and interpretations.

2.5. Multibeam

Multibeam technology was originally developed in the 1960s by the US Navy for

deep water bathymetric mapping (USACE 2001). This technology was extended

and has been available for use in shallow water environments since the early 1990s

(USACE 2001). Detailed coverage is helpful for ensuring navigation safety, mor-

phologic analysis, and evaluation of underwater structures (USACE 2001, Prickett

1996). Multibeam surveying is based on the return time of many acoustic signals

transmitted simultaneously. The timing, phase, and amplitude of the return signal

will depend on the depth of the seafloor, bottom reflectivity, and slope angle of the

incident beam (USACE 2001). Surveys collected by a multibeam system provide es-

sentially continuous coverage of the seafloor, although the exact coverage will depend

on the configuration of the system and water depth. Spacing of individual beams in

the transducer head range from 0.5 degrees to 3.0 degrees (USACE 2001). Potential

errors, such as vessel roll, pitch, and yaw, and time lag between the positioning