VARIABILITY EFFECTS DUE TO SHALLOW SEDIMENT GAS
IN ACOUSTIC PROPAGATION: A CASE STUDY FROM THE
MALTA PLATEAU
K.M. KELLY
QinetiQ, Winfrith Technology Centre, Dorchester, Dorset, UK
E-mail: kmkelly@qinetiq.com
A comparison was made between measured and modelled propagation loss data from the
Malta Plateau. The measured data was obtained during November 1999 at 3.5 kHz, with
both source and receiver beneath the thermocline. This maximised the interaction of
sound with the seabed. Propagation loss was modelled using the Synthetic Pulse
Reception Model (SPUR). Different sources of geoacoustic data for input to SPUR were
compared and gave very different predictions for propagation loss. The variability
observed in the data was attributed to the presence of shallow gas in the seabed
sediments. Once this had been taken into account, a good match between the measured
and modelled propagation loss was obtained. These results illustrate the importance of
good environmental characterisation for sonar performance prediction and highlight the
significant effect that shallow gas can have on the geoacoustic properties of sediments
and the resulting acoustic propagation.
1 Introduction
In order to predict propagation loss for sonar performance assessment, a number of
models are available. One of the most recent to be developed is the Synthetic Pulse
Reception Model (SPUR). This model allows complex range dependent environments to
be modelled.
To test the accuracy of the model predictions, and the effect of improved
geoacoustic input to these models, a comparison was made with measured sonar data.
The Low Frequency Active Sonar trial, Mercury’99, which took place on the Malta
Plateau in November 1999, provided an ideal propagation loss dataset for this study.
Good supporting environmental data was obtained during the trial and the trials area has
been geologically and geoacoustically well characterised by both seismic surveys and
core measurements.
GEOSEIS is a geoacoustic database which provides an alternative generic approach
for obtaining geoacoustic information from areas where other sources of data may be
absent. This study compared existing geophysical data available for the area, with
GEOSEIS predictions, to provide geoacoustic parameters for input to SPUR. The results
show a good match between predicted propagation loss using the best GEOSEIS
geoacoustic prediction available for the region and the measured propagation loss data.
The most significant factor affecting the geoacoustic properties of the Malta Plateau
sediment was the presence of shallow gas. Once this had been taken into account that the
best match between measured and modelled propagation loss was obtained.
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N.G. Pace and F.B. Jensen (eds.), Impact of Littoral Environmental Variability on Acoustic Predictions and
Sonar Performance, 263-270.
© 2002 All Rights Reserved. Printed in the Netherlands.
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K.M. KELLY
Geoacoustics of the Malta Plateau
The Malta Plateau is the shallow water region between Sicily and Malta. The central
portion of the plateau has an almost constant depth of about 140 m. To the east the 200
m contour marks the top of the Malta Escarpment which plunges steeply into the Ionian
basin. To the west the plateau deepens gently to a slope break at about 160 m.
The acoustic basement over the plateau is associated with the Messinian sea level
lowstand, when the Malta Plateau underwent sub-aerial erosion. This basement consists
of Miocene limestones and dolomites overlying an upper Cretaceous volcanic horizon at
the top of a Cretaceous dolomite succession [1].
The central Malta Plateau is an area of significant Plio-Quaternary sedimentation.
An 8 to 12 m thick layer of very soft fine grained sediments covers a horizontally
bedded succession of unconsolidated sediments. There are six seismo-stratigraphic units,
each representing a phase of sedimentation followed by a period of erosion, probably
associated with a fall in sea level. These units are thickest in the central plateau, thin
towards the basement highs and pinch out against the topographic high to the east, where
there is a rough region of exposed rockhead. The seabed is almost flat with occasional
pockmarks. These combined with the acoustic signature of the top unit in seismic
sections, indicate the presence of shallow gas in the uppermost Plio-Quaternary
sediments [1].
To the west of the plateau the base of the slope consists of slumped deposits derived
from the plateau region. This slumping would appear to be recent and the margin may
still be active [1].
The geoacoustic parameters for the Malta Plateau sediments, for propagation loss
modelling, were obtained from GEOSEIS [2]. This is a geoacoustic database containing
data on a wide variety of sediment and rock types. It can be used to provide generic
geoacoustic parameters for any given sediment. The more detailed sedimentological
information that can be provided the more representative the geoacoustic parameters are
likely to be. GEOSEIS has the potential to provide realistic geoacoustic parameters for
areas where there is little or no geoacoustic data available.
Sedimentological data was available for five cores from the Malta Plateau [3], listed
in Table 1. Of the five cores 254 and 255 were sedimentologically very similar,
composed mainly of silt with small fractions of clay and sand and 30–45% CaCO3. 256
was composed of fine silty sand and 258 dominated by silt and clay, both with a slightly
lower CaCO3 content. Core 257 has been excluded since it was from a region containing
debris deposits adjacent to one of the rocky outcrops and was therefore not
representative of the plateau region [3].
Table 1. Sedimentological analysis of core data [1].
Core
254
255
256
258
Average
% gravel
3.1
1.4
0.6
1.2
1.575
% sand
28
30.5
68.7
9.6
34.2
% silt
58.7
48.1
21.3
44.9
43.25
% clay
10.2
18.3
9.9
44.3
20.675
% CaCO3
34.9
38.6
30.1
29.6
33.3
SHALLOW SEDIMENT GAS AND ACOUSTIC PROPAGATION
265
The sedimentological analysis from the cores was used to derive geoacoustic
parameters using the GEOSEIS algorithms. Two sets of parameters were obtained
GEOSEIS 1, which used the average sedimentological composition for the cores, and
GEOSEIS 2, which also took porosity into account. This was slightly lower at 50–55%
for the cores, than the average porosity for similar sediments in GEOSEIS (63%). The
parameters obtained from GEOSEIS are average values for sediments of similar
composition within the database and have been corrected for frequency dependence
using the Kramers-Kronig relationship derived from Kolsky [4];

 f (kHz ) 
1
 (ms-1)
Vpf = Vpm 1 +
ln
 πQp  m(kHz ) 
(1)
where
f
m
Qp
Vp
is the required frequency,
is the measurement frequency,
is the quality factor,
is the p-wave velocity.
The final factor, which was taken into account, was the presence of gas in the PlioQuaternary sediments of the Malta Plateau, as indicated by the pockmarks and seismic
signature [1]. The presence of even only a small amount of gas in sediments can have a
very significant effect on sediment acoustic properties, with velocities being reduced by
as much as 15–50% [5]. Domenico [6] showed that a velocity decrease of 36% occurred
between 6% and 13% gas content, indicating that a small but critical amount of gas can
significantly alter the acoustic properties of the sediment.
Density and attenuation are also affected by the addition of gas. Density is reduced
as gas replaces the pore water so decreasing the density of the pore fluid. Since the
average porosity of the sediment is known it was possible to calculate a reduced density
for a 10% gas content. The equation used to calculate the density was:
ρsediment = ρgrain(1-φ) + ρwaterφ
(2)
where
ρsediment
ρgrain
ρwater
φ
is the density of the fluid saturated sediment,
is the density of the sediment grains,
is the density of the pore water,
is the fractional porosity.
GEOSEIS contains data on gassy sediments and this allowed predictions to be made for
the sedimentological analysis (GEOSEIS 1) data with gas contents of 2% and 5%.
Domenico [6] showed that after the initial velocity decrease as gas content increased to
13%, velocity then remained nearly constant regardless of how much more gas was
added. Additional predictions were therefore made for a 10% content in both the
sedimentological analysis prediction (GEOSEIS 1) and the prediction taking porosity
into account (GEOSEIS 2) by reducing velocity by 30%.
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K.M. KELLY
There is little information on the effects of gas on attenuation, however it does
appear that attenuation is significantly increased. Doubling the attenuation for 10% gas
content gave a reasonable approximation, which was consistent with GEOSEIS
For the purpose of this study the Malta Plateau was modelled as a range independent
environment. However, it should be noted that there is variability across the region in
both sediment composition, as indicated by the differences in the sediment compositions
of the cores, Table 1, and in the sediment gas content [1].
3 Acoustics Results
Propagation loss runs were carried out using the Parabolic Equation model, SPUR. This
is an improved version of the Range-dependent Acoustic Model (RAM). RAM is based
on the split-step Padé solution [7], which allows large range steps and is the most
efficient PE algorithm developed to date.
The Malta Plateau provided a range independent acoustic scenario. The seabed was
relatively flat and featureless with little geoacoustic variability. Expendable
Bathythermographs (XBTs) deployed during the acoustic experiment showed only slight
oceanographic variability with time. Wind speed and sea state remained constant and
low, true wind speed rarely exceeding 3 m/s. Figure 1 shows the sound speed profile
used in the modelling. This was an average for the XBT profiles obtained during the
experiment. For a source depth of 70 m this profile provides a strongly seabed
interactive environment.
Figure 1. Averaged XBT profile used in the propagation loss modelling.
SHALLOW SEDIMENT GAS AND ACOUSTIC PROPAGATION
267
Range (km)
Figure 2. Measured propagation loss from four runs during Mercury’99.
Measured energy level data were obtained from four propagation loss runs carried
out at 3.5 kHz during the Mercury’99 experiment. Propagation loss was calculated using
a source level of 184 dB re 1µPa@1m. Combining these four runs provided a set of
propagation loss data covering ranges of 7–16 km from the source, Fig. 2. This could be
done because the environmental conditions did not vary significantly either within or
between the different runs. The data could then be compared with the propagation loss
predictions made using SPUR.
SPUR runs were carried out for the different seabed types listed in Table 2. The
SPUR results and measured data were overlaid to allow a comparison to be made
between the model predictions and real propagation loss for the range independent
scenario on the Malta Plateau, see Figs. 3 and 4. Changing the geoacoustic properties of
the seabed made a significant difference to the propagation loss predictions obtained.
Table 2. Summary of geoacoustic predictions for the Malta Plateau.
Sediment
GEOSEIS 1 (detailed sedimentology)
2% Gas
5% Gas
10% Gas
GEOSEIS 2 (including porosity)
10% Gas
Vp
1433
1397
1408
1003
1511
1118
Density g/cm3
1.626
1.522
1.610
1.178
1.855
1.548
Attenuation dB/λ
1.01
1.26
1.40
2.00
0.35
0.70
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K.M. KELLY
The presence of gas in these sediments is a major factor affecting their geoacoustic
properties. Figures 3 and 4 show the effects of varying the amount of sediment gas on
the SPUR predictions for the GEOSEIS 1 and GEOSEIS 2 geoacoustics. For the
GEOSEIS 1 prediction, Fig. 3, the measured propagation loss data can be seen to fall
between the 5% and 10% sediment gas content predictions, with a difference of 20 dB
between the no-gas and 10% gas scenarios at 15 km.
Range (km)
Figure 3. Effect of increasing gas content on the GEOSEIS 1 prediction, based on the
sedimentological analysis of core data.
Range (km)
Figure 4. Effect of increasing the gas content on the GEOSEIS 2 prediction which takes porosity
into account.
SHALLOW SEDIMENT GAS AND ACOUSTIC PROPAGATION
269
For the GEOSEIS 2 geoacoustics, which takes sediment porosity into account the
measured data gives propagation loss values slightly higher than the SPUR predictions
for a 10% sediment gas content with a difference of about 30 dB between no gas and
10% gas, see Fig. 4. GEOSEIS 2 with 10% gas is the best GEOSEIS geoacoustic
prediction that could be made for the Malta Plateau, based on all the available
sedimentological and geological information. The SPUR prediction using these
parameters gives the closest match with the measured data.
Although it is often possible to recognise the presence of gas in seabed sediments,
as a results of pockmarks or seismic signature, it is not possible to give an accurate
estimate of how much gas is present. Since small quantities of gas can have a very
marked acoustic effect this presents a serious shortcoming. Even in an area as well
characterised as the Malta Plateau a quantitative estimate of gas content is purely
speculative. However, the evidence would suggest that sediment gas content is relatively
high, and the predictions for a 10% gas content, for the best GEOSEIS prediction,
GEOSEIS 2, gives the closest match to the measured data. This result is within 2–3 dB
of the measured data.
These results illustrate that for a well characterised, range independent environment,
such as the Malta Plateau, a good match between measured and modelled propagation
loss can be obtained. This is however dependent on good quality seabed information
being available. In this case the issue of gas content is particularly important. Since there
are no means of quantifying the sediment gas content the effects this has on the
geoacoustic properties of the seabed sediments can only be estimates.
4 Conclusions
These results demonstrate the potential usefulness of GEOSEIS as a generic tool for
providing geoacoustic data for input to acoustic models. Provided that a
sedimentological breakdown can be provided for a particular seabed sediment,
GEOSEIS should be able to provide sufficiently accurate geoacoustic parameters for use
in propagation loss modelling.
The propagation loss predictions provided by SPUR will only be as good as the
environmental data available for modelling. In shallow water regions, such as the Malta
Plateau, a good understanding of the seabed is essential for sonar performance
prediction. These results therefore highlight the importance of good geoacoustic
information for areas of operational importance.
One particular geoacoustic problem highlighted by this study is the problem of
modelling sediments that contain shallow gas. Shallow gas is a common phenomenon,
present in many seabed sediments throughout the world. Even a small quantity of gas
can have a dramatic effect on the geoacoustic properties of the seabed.
References
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UNESCO reports in Marine Science 58, 117–122 (1992).
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K.M. KELLY
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