MAPPING SEABED VARIABILITY USING
COMBINED ECHOSOUNDER AND XBPS
FOR SONAR PERFORMANCE PREDICTION
K.M. KELLY AND G.J. HEALD
QinetiQ, Winfrith Technology Centre, Dorchester, Dorset, UK
E-mail: [email protected], [email protected]
The acoustic experiment, Cerberus, took place in the SouthWest Approaches to the
English Channel in August 2001. During this experiment geoacoustic information was
obtained using a combination of Expendable Bottom Penetrometers (XBPs) and the
echosounder seabed classification systems, EchoPlus and Roxann. The results of
combining these geophysical techniques mapped both variations in sediment type and
the presence of large bathymetric features in the region. This improved understanding of
the seabed type and variability will lead to improved acoustic prediction. Combining
these techniques provides a simple and effective method for gathering seabed variability
data for input to acoustic models. This paper investigates the practicability of using
XBPs for ground truthing the echosounder systems and compares the results from the
three systems. The effect of the seabed variability on sonar performance prediction is
discussed.
1
Introduction
During August and September 2001 an Active Sonar experiment (Cerberus) took place
in the Southwest Approaches. Environmental data was obtained during the experiment in
support of the sonar operations. This included a geophysical dataset from the outer shelf
of the Southwest Approaches continental shelf region.
The geophysical dataset obtained included data from two sediment discrimination
systems, EchoPlus and Roxann, and from expendable Bottom Penetrometers (XBPs).
The two discrimination systems use the first and second echoes from the echosounder
which are related to roughness and hardness respectively. Background on these systems
can be found in Chivers [1] and the theory of the first and second echo for sediment
discrimination is given by Heald [2]. These systems give an indication of seabed spatial
variability. The XBPs were cone penetrometers deployed using the ship’s Expendable
bathythermograph (XBT) launcher. They measured deceleration (g) and seabed
penetration (cm). These parameters can be related to different sediment characteristices
and hence to seabed type [3].
The seabed in this region is composed mainly of Quaternary sands and gravels
which overlie the Pleistocene clayey sands of the Upper Little Sole Formation [4]. These
Quaternary deposits are formed into a series of large tidal sand ridges, which are the
main bathymetric feature of the outer shelf. They form ridges up to 60 m high, 200 m
long and 15 km spacing and are relict features which formed during the Late Devensian
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Sonar Performance, 99-106.
© 2002 All Rights Reserved. Printed in the Netherlands.
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K.M. KELLY AND G.J. HEALD
lowstand of the ice age, when sea levels were lower and tidal forces over the shelf were
stronger [5]. They are now in a state of decay.
After the Ice Age, as the sea levels rose, the ridges advanced landward until they
reached the part of the shelf where sediment supply was insufficient to maintain their
growth. Their landward extension stopped at about the 120 m isobath. Seabed reworking
during this period formed a sandy gravelly lag pavement. The present seabed consists of
partly mobile sediments which are swept by tides and wave induced currents into a
series of beforms which range in size from minor sand ripples to tidal sand ridges.
Figure 1 is the British Geological Survey (BGS) surface sediment map for the region.
Figure 1. BGS surface sediment map for the survey area.
2
2.1
Geophysical datasets
Echoplus
EchoPlus is a seabed discrimination system that can be installed on any vessel with a
single beam echosounder. It is hardwired into the ship’s echosounder and uses the
echosounder returns from the first and second echoes to determine the roughness and
hardness of the seabed. Mapping the variations in the roughness and hardness outputs
gives an indication of seabed variability and allows classification into discrete regions.
The first echo consists of a leading edge, which is caused by the initial reflection
and scattering from the seabed, and a trailing edge, which is dominated by scattering
from the seabed and possibly reflections from the sub-bottom. The analysis window
integrates the trailing edge of this first echo to determine a measure of the seabed
roughness. The rougher the seabed the more energy will be scattered back to the
transducer and the more energy will appear in the integrated analysis window. The
MAPPING SEABED VARIABILITY
101
second echo includes contributions from seabed, sea surface and sub-bottom. This echo
is used to determine the hardness of the seabed. The harder the seabed the stronger the
nearfield scattering will be and the more energy appears in the analysis window [1].
EchoPlus was hardwired into the MV BREMEN echosounder which was an Elac
LAZ 72 echosounder which was operating at 30 kHz. The system operated well
throughout the exercise and a good quality dataset was obtained.
2.2
Roxann
Roxann works on the same principle as that already described for EchoPlus, using the
tail of the first echo (E1) and the second echo (E2) to give an indication of roughness
and hardness [4]. In this case Roxann used a Simrad echosounder operating at 200 kHz.
Normally the echosounder transducer is deployed over the side of the survey ship
attached to a metal pole, but during Cerberus, due to the size of the ship, this could not
be achieved. There was also no suitable moon pool available on the ship which could be
used for tranducer deployment. Therefore a towed body arrangement was set up. This
was by no means ideal since the length of cable required resulted in a reduced signal to
noise ratio, meaning that the data obtained was very noisy. Also the system expects the
transducer to be just below the surface, but with a towed body it was deployed somewhat
deeper. This meant that the time delay until the second echo, calculated by the system
using the water depth was slightly inaccurate resulting in a poor return from the second
echo. This could be overcome by changing the timing based on the position of the
transducer in the water column.
2.3
XBPs
Expendable Bottom Pentrometers (XBPs) are expendable seabed probes for measuring
the in-situ physical properties of seabed sediments. They use the same technology as the
widely used expendable bathythermograph (XBT), except that an accelerometer replaces
the usual thermistor and special electronics and computer logic are employed to sense
and record several different aspects of impact and penetration into the seabed.
When the probe impacts with the seabed the measured deceleration is proportional
to the resistive force exerted by the sediment. This depends on the undrained shear
strength of the sediment. After the probes downward motion stops there is a period of
heavily damped oscillatory motion with no further net penetration, particularly in stiffer
granular sediments. Whilst this motion is occurring the probe is behaving in a way
analogous to a mass on an elastic foundation. This portion of the record can be analysed
to obtain an estimate off the dynamic shear modulus which in turn can be used to
calculate shear wave velocity [6].
There are much higher peak forces and rates of deceleration with very little
penetration in areas where the surface sediment is sand. In softer fine grained sediments
where porosites are typically greater than 50% the probe penetrates to a significant
depth. The time required for full penetration is larger and the maximum deceleration is
much smaller.
3
Comparison of geophysical datasets
Deploying the two echosounder systems simultaneously during Cerberus meant that a
comparison could be made between the two systems. They use essentially the same
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K.M. KELLY AND G.J. HEALD
processing techniques, the trailing edge of the first echo being used to give an indication
of seabed roughness, and the second echo being used to give seabed hardness. The main
differences between the two systems are their operating frequencies and the mode of
deployment used.
The Roxann E1 signal indicated several regions of increased seabed roughness but
the E2 signal remained consistently low, only indicating increased seabed hardness on
two occasions. Both coincided with Roxann transects across a large patch of gravelly
sand and sandy gravel.
Figure 2 shows both the Roxann and EchoPlus data for one of the transects of the
gravel patch, section A-A Fig. 1. The Roxann E1 signal is giving a strong return
associated with the gravelly sand patch. E2 shows a similar trend although the signal is
weaker. The EchoPlus hardness signal is very similar to the Roxann E1 signal. There is
no variation in roughness. This shows that both systems are detecting the same seabed
features.
Figure 2. EchoPlus, roughness and hardness and Roxann E1 and E2 for section A-A (see Fig. 1).
Both the EchoPlus and Roxann systems provide information on seabed variability,
but since the signal can be strongly affected by other environmental factors, such as the
interaction of the second echo with the sea surface, repeated tracks under different
environmental conditions can give different results. As a result the same
roughness/hardness relationships will not always apply and some form of ground truth
will be necessary in order to use these systems for actual seabed classification. They will
however consistently show relative discrimination levels. Normally the ground truthing
uses cores or grabs to identify the sediment present at the seabed. However this involves
the ship stopping which is not always practical. The possibility of using XBPs for
ground truth was therefore investigated.
Figure 3 is a plot for all the XBP data showing the deceleration curves. Although all
the XBP deployments were identified by the XBP software as seabed type I, sand and
gravel, there are three distinct curve types visible in the data. The first type shows a
maximum deceleration of greater than 200 g. This probe was deployed on the top of
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MAPPING SEABED VARIABILITY
Haddock Bank and is likely to be representative of coarse sand or gravel. The second
type corresponds to most of the remaining XBP deployments. Decelerations are between
100 and 200 g. These deceleration curves are representative of sands which makes up
most of the seabed in this area. The final type shows decelerations lower than 100 g and
taking place over a greater time spread. They also show a distinct double deceleration, a
small initial deceleration being followed by the main deceleration. This preliminary
deceleration is probably caused by a disturbance of the sediment by the downforce of
water just ahead of the probe. These probes were deployed in a region to the west of the
trials area where muddy sands are more prevalent on the seabed.
250
200
150
100
50
0
-50
100
105
110
115
120
125
130
135
140
145
150
Time (ms)
Figure 3. Deceleration curves for XBP dataset.
Figure 4 is a plot of penetration against deceleration for the XBP data. In general the
two are related, higher decelerations being associated with less penetration, although
there is some variability within the sands. The groupings of the deployments into the
three seabed types identified are shown.
The definitions of gravel, sand and muddy sand determined from the XBP data were
then used to identify the corresponding EchoPlus data distribution. Figure 5 shows
roughness/hardness plots of EchoPlus data from a 1-min window surrounding 3 typical
XBP deployment, one for each seabed type.
The data shows distinct variations in the roughness/hardness relationships for the
different seabed types. Sands appear to give lower hardness values that both muddy sand
and gravel. The main difference between the muddy sand and the gravel is that
roughness is lower for the muddy sands and the data forms a distinct cluster. The
roughness/hardness relationship for the gravels shows a greater spread of data values. As
a result, when an attempt was made to define seabed type using this raw data, it was
difficult to distinguish between the muddy sand and gravel as a result of the spread of
values associated with the gravel. However the regions of sand could be clearly
identified, see Fig. 6. This type of acoustic signal “overlay” is not uncommon in these
types of dataset and emphasizes the need for good ground truth.
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K.M. KELLY AND G.J. HEALD
Figure 4. Plot of penetration against deceleration for the XBP dataset showing the distribution of
the different seabed types.
12
10
8
muddy Sand
6
Gravel
Sand
4
2
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Roughness
Figure 5. Relationship of roughness and hardness for the three different seabed types.
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MAPPING SEABED VARIABILITY
Figure 6. Seabed type map based on the seabed type distributions defined in Fig. 5.
Range (km)
0
10
20
30
40
50
60
40
50
60
70
No Sandbars
Sandbars
80
90
100
Figure 7. Propagation loss at 3.5 kHz for the Cerberus environment, with and without sandbars
present.
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K.M. KELLY AND G.J. HEALD
Acoustic predictions
The geophysical data obtained during Cerberus indicated that the seabed in the survey
area was relatively consistent in composition, ranging from muddy sands to gravels with
most of the region consisting of sands and gravelly sands. Model predictions indicated
that these variations did not significantly affect propagation loss. Neither did the
inclusion of the sandbars, as is illustrated in Fig. 7. Propagation loss predictions for this
environment, both with and without sandbars present only differs by a few dB.
5
Conclusions
The two echosounder based seabed discriminators, EchoPlus and Roxann, are useful
survey tools for assessing seabed variability. In order to attempt to categorize the seabed
types present some form of ground truth is required. This paper investigates the potential
for using XBPs for such a purpose. The results would suggest that this is a potentially
viable technique although there are some problems with acoustic overlay when relating
the two types of data. The results indicate that the effects of seabed variability in the
Cerberus area are not significant for sonar performance prediction.
References
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acoustic bottom classification system performance for the Cairns area, Great Barrier Reef,
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