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Torresan, M.E., Hampton, M.A., Gowen, M.H., Barber, Jr., J.H., Zink, L.L., Chase, T.E., Wong, F.L., Gann, J.T., and Dartnell, P., 1995, Final report: acoustic mapping of dredged material disposal sites and deposits in Mamala Bay, Honolulu, Hawaii: U.S. Geological Survey Open-file Report 95-17.


Introduction 1, 2
  Study Area
  Previous Studies
  Seafloor Materials

K1-93 Survey
  Scope of Work
  Sidescan Sonar

  Sonar, 3.5kHz 1, 2, 3


  1   2   3   4   5
  6   7   8   9 10
11 12 13 14 15
16 17 18 19 20

Plate 1

Apx 1: Statistics 1
Apx 1: Statistics 2
Apx 2: Equipment 1
Apx 2: Equipment 2

References 1, 2, 3


A number of enigmatic features appear on the sonar mosaic, characterized as sets of high­backscatter, short, parallel lines or ³chatter marks², having distinct trends and spacing. The features, called trains in this report, are primarily located in the southern part of the study area, south of the South Oahu disposal site. One example is labeled ³train² on figure 6. The features comprise sets of en echelon, high backscatter lines about 50­100 m long, having spacings of about 25­50 m, that combine to form linear or looping trains ranging from less than 0.5 km to over 1 km long. Most but not all of the lines within a train have the same trend. The trains are not resolved on any of the 3.5 kHz acoustic profiles and their origin is unknown, but they may result from disposal activity. For example, a disposal action may have released enough sediment to create a train in response to the local current regime, but without a continuing source of sediment, the train becomes starved, similar to the bedforms described by Cacchione and others (1987) and Cacchione and Drake (1990). The trains appear as bedforms responding to the local current activity, owing to the similar trend and spacing shown by the high­backscatter, short, parallel lines associated with the differently oriented trains. If disposal activity in fact formed these trains, then the strange and curved geometry of the trains may be a function of the disposal vessel turning or drifting during a discrete disposal action. At this time there is no evidence to suggest that these trains or their shapes are influenced by seafloor topography or channels.

The 3.5­kHz subbottom profiles complement the sonar imagery and show characteristic features relative to the location of the specific profile. Note that the seafloor reflector on all profiles shown in this report is typically characterized by a purple color. An overlying green signal observed on many records is an artifact of the display software (ODEC, oral communication).

There are two types of seafloor as seen on the 3.5-kHz profiles that occur within each site and adjacent to the eastern edge of the former Honolulu Harbor site, and these are distinguished by amount of relief shown on the 3.5-kHz profiles. Generally, the disposal sites are characterized by an undulatory seafloor with or without subbottom diffractions and no subbottom reflectors (figures 10, 11, 12, 13, and 14). The diffractions seen on figures 12 and 13 correlated with bedforms seen on the sidescan sonar mosaic. The second type of seafloor seen on 3.5-kHz subbottom profiles is composed of relatively large, rounded bedforms such as those observed along lines 4, 5, and 11 ( figures 4, 6, 7, 8, and 9). The bedforms occur over the northern and eastern portions of the former Honolulu Harbor site and extend east about 2 km beyond the high-backscatter, plume-shape deposit and into the low-backscatter seafloor (figures 4, 6, 7, 8 ,9 and plate 1). The mounds are up to 2 m high and have crests spaced up to 150 m apart along ship's track. Some appear to correlate with the high-backscatter footprints visible on the sonar mosaic, such as those visible in figure 7 along line 4, and those visible along line 11 as seen in figure 7. These bedforms start within the eastern edged of the plume-shape Honolulu Harbor deposit and extend beyond the high-backscatter deposit and into the low backscatter native seafloor implying a natural rather than anthropogenic origin these features. The location of the bedformss east of the former Honolulu Harbor site and in a topographic low or trough that dips to the southeast (figure 5 and plate 1) suggests that native sediment may have formed bedforms in response to the local current regime. These bedforms may be migrating downslope to the southeast, although their shape and symmetry as seen in the 3.5-kHz profiles does not suggest or imply a specific trend or transport direction. At this time their origin is unknown.


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URL: https://pubs.usgs.gov/of/1995/of95-017/08bsidescan.html
Maintained by: Michael Diggles
Author: Florence L. Wong
Last modified: May 27, 2005 (mfd)

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