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SEASONAL VARIATIONS IN THE ACOUSTICAL
PROPERTIES OF BLACK SILT IN THE William Murphy, W. Bruce Ward, Robert Fleming, Stephanie Beda, Gary Fleming, Beckett Boyd Earthworks
LLC, and Theoretical
and Applied Geology phone:
203-270-8100 e-mail
contact: murphy@earthworks2020.com and ward@earthworks2020.com and Ben A. Baker US Army
Corps of Engineers, NY District A black sediment, variously called anthropogenic sediment,
harbor mud, ooze, “black mayonnaise,” or black silt, is found in many
industrial harbors and urban estuaries throughout the world. An example of this sediment from This 'black silt"
material grades from a suspension at the water-sediment interface to an ooze to interbedded laminae with sands and muds. Black silt is widespread throughout the
harbor, but patchy in distribution and varies greatly in thickness. Within
the deeper channels the black silt is geographically separate from coarser
sediments and commonly occurs as an ooze. On the shallow flats black silts are more
commonly interbedded with coarser sediments. Because the black silt is unsuitable for disposal at sea, harbor dredging, remediation and restoration projects depend on accurate mapping of the distribution and thickness of the black silt. We have used distinctive
acoustic properties to map the distribution of black silt in selected areas
for production geophysical projects for harbor improvement in In the summer, ultrasonic
measurements indicate that some samples of black silt (ooze) have acoustic
velocities significantly lower than seawater. In the winter and early spring the slowest
velocities are in the range of seawater.
In seismic profiles, the black silt appears almost a strong reflector
with a negative polarity in the summer.
The black silt is almost acoustically transparent with a positive
polarity in winter. Our measurements to date
suggest a seasonal variation that is hypothesized to be controlled by
temperature variations. In A working hypothesis is
that a primary component of the pore fluid is liquid hydrocarbons deposited
from industrial processes over the past 200 years. The modulus of the
hydrocarbon liquid in the pores varies differently with temperature than the
harbor water. The effect of
temperature on the sound velocity of water contrasts with the effect on
hydrocarbon. Wood’s equation models the
effective bulk modulus of a suspension as
in
which K is bulk modulus, the subscripts c, s, w, and o refer to composite,
solid grains, water and oil, respectively, Sw is the ratio of oil to water in
the pores or water saturation, and
We have plotted the velocity of the suspension as a function of water
saturation and temperature. We find
that the black silt as a function of temperature would be acoustically transparent in
February and reflective in July. Alternative hypotheses
for the seasonal variation in behavior may be (a) gas/liquid phase from
industrial hydrocarbons, (b) biologically generated gas, (c) seasonal runoff
and water column effects, and (d) changes in sediment concentration in the
suspension. The hypotheses are testable and separable by frequency dependence
and other critical experiments. These measurements also imply that the colder months are acoustically the best time to image through and below the black silt. The warmer months offer the best time to image the geographic distribution black silt when there is the strongest contrast between black silt and other sediment. |
, is estimated as