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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


Most 3.5-kHz profiles collected over the disposal sites are characterized by a laterally discontinuous seafloor reflector, implying a change in acoustic impedance. This discontinuity is shown on profiles by color changes, e.g., changes in the seafloor reflector from dark purple to lighter purple or to green, or from greens to yellows. In places, the discontiniuties are quite subtle, but nonetheless are visible. These discontinuities typify most of the deposits that blanket both the former and the active disposal sites, and are shown in figures 7, and 10, 11, 12, 13 and 14. Typically, the substrate below the discontinuous seafloor reflector is featureless, lacking subbottom reflectors. An exception exists in the deeper area of the southeast portion of the former Honolulu Harbor site, which is characterized by relatively large mound-like bedforms, and an undulating, continuous seafloor reflector that has one discontinuous subbottom reflector (figures 8 and 9). A possible explanation for the discontinuous seafloor reflector and the structureless substrate likely resides in the nature and composition of the dredged material. Since the dredged material is composed of a cohesive gray mud admixed by dredging with sand- to cobble-size carbonate and basaltic debris (Torresan and others, 1994b), the nature of the dredged material can attenuate and/or scatter the acoustic signal such that the resultant profile lacks resolution and appears structureless in most profiles.

Contrasting with the laterally discontinuous seafloor reflector and the featureless dredged material substrate, subbottom profiles collected along line 17 (figure 15), located between the former Pearl and Honolulu Harbor sites shows subbottom reflector(s). Also, the deeper areas of Mamala Bay south of the disposal sites are characterized by a smooth, continuous seafloor reflector that has one or more subbottom reflectors (figures 16, 17 and 18). This echo character may be characteristic of the seafloor prior to dredge material disposal. Finally, tightly-spaced diffractions are common on many profiles (especially west of the disposal sites) and are always associated with bedforms visible on the sidescan mosaic (figures 4, 6, 19, 20, and plate 1). Some diffractions are probably associated with coarse debris visible on seafloor photographs taken during 1977 (Chave and Miller, 1977a, 1977b), and some are likely related to buried reef deposits, in addition to bedforms and surficial debris.


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URL: https://pubs.usgs.gov/of/1995/of95-017/09sidescan.html
Maintained by: Michael Diggles
Author: Florence L. Wong
Last modified: May 27, 2005 (mfd)

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