VARIABILITY OF SHEAR WAVE SPEED AND ATTENUATION
IN SURFICIAL MARINE SEDIMENTS
MICHAEL D. RICHARDSON
Marine Geosciences Division, Naval Research Laboratory, SSC, MS 39529-5004, USA
E-mail: [email protected]
Values of shear wave speed and attenuation in surficial marine sediments (upper 30 cm)
are summarized and then correlated with easily measured sediment physical properties
(porosity, bulk density, and mean grain size). Shear wave speed ranges from a low of
5 m/s in soft silty-clays to a high of 150 m/s in hard pack fine sands. Shear wave speed
increases with decreasing porosity, increasing bulk density, and increasing mean grain
size. Strong gradients in shear wave speed in the upper meter of sediment, related to
increased effective stress (overburden pressure) and vertical gradients in sediment
properties complicate theses predictive relationships. Shear wave attenuation follows
the opposite trends as shear wave speed, increasing with increasing porosity, decreasing
mean grain size and decreasing bulk density.
1
Introduction
Knowledge of sediment geoacoustic properties is of fundamental importance to marine
environmental, military, and engineering applications. For instance, geoacoustic
properties are used to predict the stability of marine slopes, sediment consolidation
behavior, strength of marine foundations, liquefaction potential, mine burial, and
scattering of acoustic energy into and from the seafloor. In situ measurement of
compressional and shear wave speed and attenuation in marine sediments is a welldeveloped technology [1,2]. Values of shear wave speed and attenuation measured in a
variety of silicilastic and carbonate sediments over the past 14 years are compiled and
new regressions developed between sediment physical and geoacoustic properties are
presented.
2
Methods
Shear wave speed and attenuation were measured using several different versions of the
In situ Sediment geoAcoustic Measurement System (ISSAMS) [1,2]. The compiled
data include previously published studies near La Speizia, Italy during 1988 [1,3,13], in
the Adriatic Sea during 1989 [1,8], in Eckernförde Bay, Baltic Sea during 1993, 1995,
and 1997 [7,12], off Boca Raton, Florida in 1994 [8], in the West Sound off Orcacs
Island in Puget Sound, Washington during 1995 [10], offshore the Eel River, along the
Northern California Coast in 1996 [4,5], in the northeastern Gulf of Mexico, near
Panama City during 1989, 1993, 1998 [7,8] and near Ft Walton Beach during 1998
[9,11], and in the lower Florida Keys during 1994–1997 [6]. The data base also
includes unpublished data from the North Sea (1997) and unpublished data collected
107
N.G. Pace and F.B. Jensen (eds.), Impact of Littoral Environmental Variability on Acoustic Predictions and
Sonar Performance, 107-114.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
108
M.D. RICHARDSON
along the lower Florida Peninsula during 1998. Sediments range from very soft, lowporosity mud in Eckernförde Bay to fine-to-medium well-sorted sands in the
northeastern Gulf of Mexico. A few sites near La Spezia and in the northeastern Gulf of
Mexico were poorly sorted coarse sands. Carbonate sediments in the Florida Keys and
along the lower Florida Peninsula, were generally poorly sorted with a variety of coarser
reef debris and shell fragments mixed with finer grained calcareous algae. Calcareous
sediments have been shown to slightly different values of sediment geoacoustic
properties than silicilastic sediment of the same physical properties [6].
2.1
Geoacoustic Measurements
Values of shear and compressional wave speed and attenuation were measured using
several different versions of the In Situ sediment geoAcoustic Measurement System
(ISSAMS) [1,2,8,13]. Pulse techniques that utilize time-of-flight and amplitude
measurements between pairs of compressional and shear wave probes are used to
measure speed and attenuation. The geoacoustic probes are driven into the sediment
using a variety of platforms including diver-deployed, mechanically-operated, and
hydraulically-operated systems [8]. Although electro-mechanical methodologies of data
acquisition and probe deployment have evolved over the years, actual geoacoustic probe
design and signal processing has changed little. Compressional wave transmit and
receive probes are identical radial-poled ceramic cylinders mounted on a hollow
stainless steel tubes. The ceramic cylinders are potted in polyurethane resin to
electrically isolate the probes from seawater and for protection during insertion. Shear
wave transmit and receive probes are bimorph ceramic benders, potted in a stainless
steel ring with silicone rubber to allow unrestricted bender movement. A thin coating of
much harder polyurethane resin holds the ceramics in place and provides a tough
coating to protect the ceramics during insertion.
Compressional and shear wave speed and attenuation are typically measured over
pathlengths ranging from of 30 to 100 cm at depths of 5–30 cm below the sedimentwater interface. For compressional wave measurements, transmit pulses are driven
utilizing 38 to 58 kHz pulsed sine waves and time delays and voltages are used to
determine values of speed and attenuation. Actual values of compressional speed and
attenuation are calculated by comparison of received signals transmitted through the
sediment with those transmitted through seawater overlying the sediments. Shear speed
is measured as time of flight between probes driven at 100 to 2000 Hz. Recently;
techniques based on transposition have been developed to measure compressional and
shear attenuation without standards [5]. Even more recent improvements to
compressional wave measurement techniques, based on comparison of signals of paired
receivers [8], have not yet been applied to these data sets but would probably change
values of shear wave speed and attenuation very little.
2.2
Sediments
Sediment cores were carefully collected by either by divers in shallow water or from
box core samples in deeper water. After cores are acoustically logged, sediments were
sectioned at 2-cm intervals and water content was determined by water loss of samples
dried in an oven at 105°C for 24 h. Porosity and bulk density were calculated from
SEDIMENT SHEAR WAVE SPEED AND ATTENUATION
109
values of water content and values of grain density measured with a Quantachrome
Ultrapycnometer. The size distribution of gravel- and sand-sized particles was
determined by dry-sieving and a Micromeritics sedigraph or pipette analysis was used to
determine the silt- and clay-sized fractions. Grain size distributions are described by the
graphical methods of Folk [7]. Great care was exercised to minimize disturbance on
sediment cores and physical properties are measured soon as physically possible. The
original data for most of the data sets analyzed here can be found in the references
[1–13].
3
Data compilation
Shear wave speeds together with sediment physical properties (porosity, bulk density
and mean grain size) were measured at approximately 100 sites during that last 14 years.
In most cases the original data has been published as part of regional and local
geological and geophysical studies, high-frequency acoustic experiments, site surveys,
or papers on development and/or improvements to ISSAMS or data analysis techniques.
Several papers have provided summaries of past in situ measurements [6,8]. This paper
is confined to comparisons of values of shear wave speed and attenuation to sediment
physical properties.
In most cases shear wave speed was measured on multiple deployments of ISSAMS
within 25-m of the same location. Shear wave speed and attenuation were compiled
from multiple depths (10–30 cm) below the sediment water interface and between
multiple pairs of transducers. A typical deployment would yield 12 independent values
shear wave speed, and 3 independent values of shear wave attenuation. The
measurement frequency (70–2000 Hz) is dependent of transducer loading with lower
frequencies used in softer muddy sediments and higher frequencies used in sandy
sediments. Typically, 1000 Hz is used for most fine-to-medium sands and 100–300 Hz
is used in soft muddy sediments. Shear wave speed was measured at 100 sites but shear
wave attenuation has been only been compiled for 18 silicilastic sites. Work proceeds
slowly on the reanalysis waveforms to determine shear wave attenuation. Usually
multiple cores were collected are each site and sediment physical properties measured at
2-cm interval down cores.
4
Regressions
Near surface gradients of shear wave speed are common, especially in sandy sediment
(Fig.1), These gradients are primarily in response to increases in mean effective stress,
rather than changes in sediment physical properties [3]. The empirical relationships
developed in this paper do not account for the effects of these gradients and can best be
considered averages for the upper 30 cm of sediments. Corrections can be easily
applied to the relationship between shear wave speed and physical properties which will
account for these gradients. Scatter diagrams with empirical fits for shear wave speed
and attenuation compared to sediment physical properties are presented in Figs. 2 and 3.
As expected from past compilations of shear wave speed [6,8,13] shear wave speed
is highest in sandy sediments with high density and low porosity. The regressions for
shear wave speeds with porosity (R2 = 0.73) and density (R2 = 0.85) were improved
when restricted to silicilastic sediments (R2 = 0.897 and 0.92 respectively). Regressions
110
M.D. RICHARDSON
of shear wave speed with mean grain size were not improved with the deletion of
carbonate sediments. Shear wave speeds for the poorly-sorted carbonate sediment
found in the lower Florida Keys were higher than shear wave speeds in silicilastic
sediments, given the same porosity and density, and accounted for most of the
differences between sediment types. Very little evidence of cementation was evident in
TEM micrographs of these sediments suggesting other causes for the higher rigidity or
shear wave speed [6]. It was suggested that high intraparticle porosity (10–15%) in
lower Florida Keys sediments is sufficient to account for the difference higher shear
wave speeds given porosity or bulk density and yet preserve the shear wave mean grain
size relationships. For now it is suggested that the two different predictive regressions
be used for silicilastic and carbonate sediments or grain size be used to predict shear
wave speed. The first extensive compilation of shear wave attenuation and sediment
physical properties appears to yield some useful relationships. Shear wave attenuation
follows the opposite trends as shear wave speed, increasing with increasing porosity,
decreasing mean grain size and decreasing bulk density. Although the percentage of the
data accounted for by these linear regressions in not high (Fig. 3) the trends appear
convincing.
Shear Wave Speed (m/s)
0
25
50
75
100
125
150
-1
4
Depth (cm)
9
14
19
24
29
Figure 1. Gradients in shear wave speed at a well-sorted fine sand site (C1) in the North Sea.
Depth regressions include a linear fit (Vs = 72.9 = 2.3D) and the traditional power law fit
(Vs = 56.8D0.25).
111
SEDIMENT SHEAR WAVE SPEED AND ATTENUATION
160
Shear Wave Speed (m/s)
140
120
100
y = 163896x-1.937
R2 = 0.6513
80
60
40
y = 2E+06x-2.7087
2
R = 0.8712
20
0
20
30
40
50
60
70
80
90
Porosity (%)
160
Shear Wave Speed (m/s)
140
y = -12.195x + 134.24
R2 = 0.806
120
100
80
60
40
20
0
0
2
4
6
8
10
12
Mean Grain SIze (Phi)
160
Shear Wave Speed (m/s)
140
120
0.0021x
y = 1.4819e
2
R = 0.6198
100
80
60
40
0.0030x
y = 0.2340e
2
R = 0.9236
20
0
1000
1200
1400
1600
1800
2000
2200
2400
Bulk Density (kg/m3)
Figure 2. Empirical relationships between shear wave speed (m s-1) and sediment physical
properties (porosity, mean grain size and bulk density).
112
M.D. RICHARDSON
Vs Attenuation (dB/m/kHz)
250
200
y = 2.27x - 42.16
R2 = 0.64
150
100
50
0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
8.00
10.00
12.00
Porosity (%)
Vs Attenuation (dB/m/kHz)
250
200
150
y = 9.2271x + 20.236
2
R = 0.4956
100
50
0
-2.00
0.00
2.00
4.00
6.00
Mean Grain Size (Phi)
Vs Attenuation (dB/m/kHz)
250
200
y = -0.1364x + 321.82
2
R = 0.6521
150
100
50
0
1000
1200
1400
1600
1800
2000
2200
2400
Bulk Density (Kg/m3)
Figure 3. Empirical relationships between shear wave attenuation (dB m-1 kHz-1) and sediment
physical properties (porosity, mean grain size and bulk density).
SEDIMENT SHEAR WAVE SPEED AND ATTENUATION
113
Acknowledgements
ISSAMS benefited from the electro-mechanical expertise of Enrico Muzi and Bruno
Miaschi (SACLANTCEN) during early development and from engineering expertise of
Sean Griffin and Frances Grosz (Omni Technologies) for its current remotely-operated,
hydraulic configuration. Kevin Briggs (NRL), Briano Tonarelli (SACLANTCEN) and
Fedra Turgutcan (SACLANTCEN) provided most of the sediment characterization. This
work was supported by NRL Program Element 601153N, Herb Eppert, Program
Manager and is NRL contribution NRL/PP/7430-02-0003.
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