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ACOUSTIC AND IN-SITU TECHNIQUES FOR MEASURING THE
SPATIAL VARIABILITY OF SEABED GEOACOUSTIC
PARAMETERS IN LITTORAL ENVIRONMENTS
JOHN C. OSLER, PAUL C. HINES AND MARK V. TREVORROW
Defence R&D Canada – Atlantic, P.O. Box 1012, Dartmouth, Nova Scotia B2Y 3Z7, Canada
E-mail: [email protected]
Geoacoustic properties of the seabed are required for accurate modeling of acoustic
propagation, and hence sonar performance prediction. Characterizing acoustic
interaction with the seabed is particularly important in shallower water environments as
the propagation typically involves extensive interaction with the sea surface and seabed.
DRDC Atlantic is developing acoustic and in-situ techniques for seabed classification
and measuring geoacoustic parameters. The acoustic technique uses normal incidence
acoustic returns, in the 1 to 10 kHz band, from the seabed and sub-bottom. The
transition from interface to volume scattering depends upon frequency and sediment
type and can be used to distinguish the composition of near surface marine sediments.
An experimental methodology has been developed using a vertical line array of
receivers and a downward-looking superdirective projector array. The technique is also
being adapted for use with commercial normal incidence sub-bottom profilers. In-situ
measurements are being made using the DRDC Atlantic free fall cone penetrometer
probe (FFCPT). It has been developed to measure acceleration and dynamic sediment
porewater pressure as a function of depth of penetration into the seafloor. It also records
hydrostatic pressure and optical backscatter for detection of the mudline. This
combination of sensors permits the direct application of geotechnical analysis methods
and parametric-based correlations already long established in engineering practice. The
FFCPT provides two independent means of calculating the undrained shear strength, as
well as other engineering variables that are used to identify the sediment grain size
characteristics. The probe has a modular design allowing additional sensor payloads to
be integrated, the first of which uses resistivity as a means to determine sediment bulk
density. Experimental results, using the acoustic and in-situ techniques, will be
presented from two joint US-SACLANTCEN-CAN sea-trials in 2001 at the New Jersey
Strataform area and the Scotian Shelf.
1
Introduction
DRDC Atlantic is developing acoustic and in situ techniques to determine geoacoustic
properties of the seabed for accurate modeling of acoustic propagation. The emphasis of
this research is on continental shelf water depths at frequencies that are relevant to
tactical and low frequency active sonars. The surficial geology of the Eastern Canadian
and American continental margins is tied to the geological processes associated with sealevel changes during periods of glaciation, and in the case of the Scotian Shelf, with the
direct effects of glaciers themselves. This has led to high spatial variability in the
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N.G. Pace and F.B. Jensen (eds.), Impact of Littoral Environmental Variability on Acoustic Predictions and
Sonar Performance, 83-90.
© 2002 All Rights Reserved. Printed in the Netherlands.
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composition and physical properties of the surficial sediments and is the motivation to
develop the techniques described in this paper.
The acoustic approach extends the capabilities of an existing classification technique
to lower frequencies, into the band of interest for ASW sonars. It is being adapted for use
on vessels with commercial seismic profiling systems and for active sonobuoys. These
could transit or drift through an area of operational interest as part of a rapid
environmental assessment scenario. The in-situ approach uses a probe to measure
geotechnical and geoacoustic properties of the seabed. These mechanical properties are
useful for marine engineering, mine burial prediction, and permit a sediment classification
based on grain size. One geoacoustic sensor module has been developed for the in-situ
method and it measures resistvity as a means to determine porosity. It has been noted
[1,2] that porosity is an effective parameter, for first order geoacoustic characterization
of the seabed, to which the more traditional quantities of sound speed, density, and
attenuation may be directly or empirically related.
2
Acoustic technique
Heald [3] has developed a surficial sediment classification technique using the first and
second normal incidence acoustic seabed returns. It is typically used with mono-static
echo sounder systems that operate at several 10s of kHz. At these frequencies, bottom
roughness at the seabed interface is the dominant scattering mechanism. Following Pace
et al. [4], the first seabed return is treated as a far-field, mono-static, arrival whose
intensity is inversely proportional to the mean square slope—a roughness parameter. The
second seabed return is treated as a near-field bi-static arrival, with a virtual source above
the water surface, whose intensity is proportional to the Rayleigh reflection coefficient.
The intensity of the first and second seabed returns are integrated over time to yield
energy, E1 and E 2 respectively, and the ratio E 1 E 2 is used to classify different
sediment types [3].
Figure 1. Schematic diagram of the DRDC Atlantic Underwater Acoustic Target deployment.
TECHNIQUES FOR MEASURING GEOACOUSTIC PARAMETERS
85
The technique in [3] is based on a Helmholtz-Kirchhoff model with gaussian
roughness and ignores any coherent reflections and seabed penetration. Hines and Heald
[5] have extended this theory to lower frequency (1 to 10 kHz range) by including
volume scatter from coherent energy penetrating into the seabed. Scatter from the
interface initially dominates the intensity of the seabed return, however because of its
rapid decay, the return is subsequently dominated by volume scatter. The transition
between surface and volume scatter, and their respective rates of decay, form the basis of
the technique to classify near surface sediments. The intensity time series are a function
of roughness parameters, frequency, and the composition of the surficial sediment layer.
Wide bandwidth, or multi-frequency systems, may exploit the frequency dependence and
provide further discrimination regarding seabed composition.
Experiments were conducted using the DRDC Atlantic Underwater Acoustic Target
(UAT). The UAT is a ship-launched echo-repeater (Fig. 1) with a 15 hydrophone vertical
line array (VLA). Immediately above the VLA, there is a pair of ITC 1007B 16 cm
diameter spherical projectors whose acoustic centers are separated by 62.5 cm. The
projectors are controlled individually but may be used in tandem with user specified time
and phase delays. The power and electronics modules for the UAT are housed in two
space frames that were displaced as far as possible horizontally from the VLA to
minimize their influence on the scattering measurements.
Figure 2. Theoretical (solid line) and measured (magenta dots) beam patterns for the steady state
radiation from a pair of transducers separated by 5λ 4 and driven in phase quadrature.
Acoustic energy that propagates from the source and is reflected by the sea surface
gives rise to a series of arrivals that are unwanted and can mask the seabed reflected
arrivals that are of interest. Their amplitude was reduced by creating a directional source,
with the main lobe pointed at the seabed, by time delaying and phase inverting the output
from the upper projector such that cancellation occurs above the transducers and
reinforcement occurs below (Fig. 2). Frequencies were selected for transducer separations
of 5λ 4 , 9λ 4 , and 13λ 4 (approximately 3, 5.3, and 7.7 kHz) respectively. In open
ocean experimentation, this two element projector array has reduced the amplitude of the
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surface reflection by approximately 10 dB. The reduction was less than that achieved
during calibration tests, but further refinements should improve the performance.
Figure 3. (a) Median intensity time series of the first seabed reflection of a 1 ms pulse arriving on
the uppermost hydrophone; and model predictions for (b) sand and (c) silt seabeds of interface
scatter, I sb1 , and of volume scatter, I vb1 , after [5].
Measured data and model predictions [5] for the intensity versus time decay of the
first seabed return are presented in Fig. 3 for a pulse vertically incident on the seabed.
The surface scatter intensity, I sb1 , is calculated for three values of surface roughness,
3.75, 7.5, and 15 cm, and a fixed correlation length of 1.5 m. The volume scatter
intensity, I vb1 , is calculated at 1, 5, and 10 kHz using the roughness parameters specified
in [5] and physical parameters in [6]. The model intensity curves begin (time zero) when
the pulse that is incident on the seabed transitions to an annulus from a spot. This is also
when the peak in I sb1 and I vb1 is expected to occur. The predicted intensity of the
volume and surface scatter have been superimposed to illustrate that the initial level and
its decay are dominated by surface scatter. At some later point in time, the intensity
becomes dominated by the decay of the volume scatter. This transition between interface
and volume scatter is a function of seabed type, seabed roughness, and frequency and
may be exploited to classify near surface sediments. It should be possible to invert for the
limited number of geoacoustic parameters in this simple yet effective seabed model and
compare the results with normal incidence [7] and wide angle inversion techniques that
yield high resolution geoacoustic models [8].
The median intensity of two hundred first seabed returns received on the uppermost
hydrophone of the UAT at three discrete frequencies are displayed in Fig. 3a. Though the
decay of the experimental data has a considerable amount of structure, perhaps due to
some layering in the seabed, the overall behaviour compares favourably with the model
predictions. That is, the initial decay is most rapid at the lowest frequency followed by a
sharp transition to a slowly decaying volume scattering regime. At the two higher
frequencies, the intensity levels at the onset of volume scattering are progressively higher
and decay more rapidly than the lowest frequency. The measured intensity curves are
consistent with theoretical predictions for a seabed composed of material that is
intermediate between the sand and silt cases presented. (Note that the data have yet to be
calibrated so its vertical axis is relative). These results were collected in the vicinity of the
AMCOR 6010 site [9] on the New Jersey Strataform area. The geoacoustic seabed
properties have been measured by independent means and are summarized in [9]. The
TECHNIQUES FOR MEASURING GEOACOUSTIC PARAMETERS
87
seabed is described as having “near surface layering” with the first twenty five metres
being a “sandy-silty-clay” layer. The average compressional sound speed in the first 5 m
is 1560 m/s and reaches 1830 m/s at 30 m depth.
3
In-situ technique
The free fall cone penetrometer test (FFCPT) is a free fall probe that has been developed
to measure mechanical (or geotechnical) and geoacoustic properties of the seabed. A
diverse range of military and civilian applications for the probe are envisioned.
Commercial applications may include pipeline survey work and support of dredging or
marine contaminant disposal operations. The FFCPT requires less logistical support than
the direct-pushed cone penetrometer test (CPT) and there is no hydraulic platform on the
seabed to potentially disturb the seabed material being measured. It can conduct surveys
quickly and may ultimately be integrated with a fully automated free fall winch that
operates on a moving vessel. Military applications include support for: mine
countermeasures by providing ground truth for seabed classification systems that are used
to predict mine burial and the effectiveness of mine hunting sonars; and for shallow water
active sonar by making measurements from which the geo-acoustic properties of the
seabed, as required for propagation and reverberation modeling, may be derived.
Figure 4. Schematic diagram of the DRDC Atlantic FFCPT. Optional sensor modules and ballast
may be inserted between the nose cone and the electronics module.
The basic FFCPT consists of a nose cone instrumented with geotechnical sensors,
power supply, electronics, and tail pressure sensor (Fig. 4). As the probe penetrates into
the seafloor nose first, it measures acceleration and dynamic sediment porewater pressure
as a function of depth. It also records hydrostatic pressure in the water, has an optical
backscatter sensor to detect the mudline, and allows additional ballast and geoacoustic
sensor modules to be integrated. This combination of sensors permit the direct
application of geotechnical analysis methods and parametric-based correlations
established in engineering practice [10]. The DRDC Atlantic FFCPT has been developed
in collaboration with Brooke Ocean Technology (BOT) Ltd. and Christian Situ
Geoscience (CSG) Inc. (both in Dartmouth, Nova Scotia). It incorporates the basic sensor
suite from their earlier 11.43 cm (4.5 inch) O.D. prototype into a modular 8.89 cm (3.5
inch) O.D. design (Fig. 4). The first geoacoustic module being developed measures
resistivity as a means to determine sediment bulk density. Future modules under
consideration include: a linear actuator to measure shear rigidity (when the probe is at
rest); and transducers for the direct measurement of compressional sound speed. The
ability to combine geotechnical and geoacoustic sensors in a single probe, as
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demonstrated in [11], allows a more complete characterization of the seabed than that
provided by acceleration-based penetrometers [12,13].
Figure 5. Material parameters versus depth for a deployment of the BOT FFCPT near NJ-1 on the
ONR Strataform. Analysis provided by Christian Situ Geoscience Inc.
The resistivity module has been developed by BOT and ConeTec (Vancouver,
British Columbia) using the design principles of a static resistivity CPT system [14]. The
excitation signal is typically set to generate a current-switched AC sinusoidal wave, at a
frequency of about 1 kHz. The primary controlling factor for resistivity is the
conductivity of the pore water phase; the conductivity of the mineral grains is a secondary
factor. Given a knowledge of the pore water chemistry, it is possible to make a secondorder evaluation of bulk density or porosity. If the sediment profile composition is known
in detail through an independent test (e.g. Fig. 6) then it is possible to account for vertical
variability in the grain conductivity. Extending the capabilities of the resistivity module to
permit measurements during penetration is under consideration [15]. The high rate of
initial penetration poses several technical challenges, such as an excitation rate of several
hundred kHz, that must be addressed.
The penetration displacement history is obtained from double integration of the
recorded acceleration and uses the optical sensor to define the moment of impact
(particularly helpful on softer bottoms). The FFCPT provides two independent means of
calculating the undrained shear strength. One using the dynamic penetration resistance as
measured using the accelerometer sensors and another using the dynamic porewater
pressure response as measured by the sediment porewater pressure sensor (Fig. 5). The
FFCPT utilizes a standard CPT-based sediment classification chart [10] as part of the
interpretation algorithm. There are accepted empirical relationships between the dynamic
pore pressure parameter, the normalized dynamic penetration resistance and sediment
grain size characteristics (Fig. 6).
TECHNIQUES FOR MEASURING GEOACOUSTIC PARAMETERS
89
Figure 6. Sediment classification chart [10] using the penetration resistance and pore pressure
parameter data in Fig. 5. Dots are color coded from violet at the surface to red at depth.
The 11.43 cm (4.5 inch) prototype was deployed at several locations near NJ-1
(39°14.949N, 72°51.798W) on the ONR Strataform area during the USSACLANTCEN-CAN Boundary Characterization 2001 sea-trial. It has measured the
geotechnical properties of a pervasive near surface sand layer that is approximately 10
cm thick and lies between clay and silt layers (Fig. 5). In several instances, the 11.43 cm
O.D. prototype was not able to pierce through this sand layer. This limitation in its
performance supported the decision to adopt a smaller diameter for the DRDC Atlantic
FFCPT. An O.D. of 8.89 cm (3.5 inch) was selected as it represents the smallest diameter
in which it is presently feasible to house the electronics and power supply. The 40%
reduction in the cross-sectional area of the DRDC Atlantic FFPCT has led to higher
terminal velocities for its descent through the water column, up to 10.5 m/s versus 6 m/s
for the prototype, and deeper penetration into the seabed, in excess of 2 m, in soft
sediments that include thin layers of shells or sand.
4
Conclusions
DRDC Atlantic is developing acoustic and in-situ techniques for seabed classification
and to determine the geoacoustic parameters required by models that predict the
performance of mine hunting and shallow water active sonars. Particular emphasis is
being placed on techniques that can be employed by moving vessels and/or air deployed
buoys to characterize the spatial variability of the seabed. An experiment to test the
predictions of a seabed and sub-seabed sediment scattering model has been conducted.
Initial results are consistent with theoretical predictions, demonstrating the expected
frequency dependent transition between interface and volume scattering, and form the
basis of a classification and inversion technique in the 1 to 10 kHz band. The in-situ
technique employs a free fall probe that combines geotechnical and geoacoustic sensors.
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Sediment grain size is determined using accepted empirical relationships between the
dynamic pore pressure parameter and the normalized dynamic penetration resistance.
Porosity is measured using electrical resistivity and can be related to the more traditional
acoustic properties of sound speed, density, and attenuation.
Acknowledgements
The authors would like to thank the officers and crew of CFAV Quest for their assistance
in the conduct of sea-trials, and Brooke Ocean Technology and Christian Situ
Geosciences Inc for analysis of FFCPT data.
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