Shallow Survey 2005 4th International Conference, Plymouth 12-15 September 2005

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High-Resolution Sub-Bottom Profiling for the “Shallow Survey” Common Data Set using the

Parametric Echosounder SES-2000 Sabine Müller, Jens Wunderlich, Peter Hümbs, Stefan Erdmann

Innomar Technologie GmbH, Germany (www.innomar.com)

Abstract Innomar Technologie GmbH Germany was invited to provide data to the “Shallow Survey 2005” common data set and surveyed an area in the Plymouth Sound northwest of Drakes Island to Mountbatten Breakwater using the parametric sub-bottom profiler SES-2000 standard. The survey results proved the capabilities of the SES-2000 system to achieve high-resolution echo plots in shallow water areas. A detailed mapping of the seafloor was possible, even small outcrops and steep slopes can be seen clearly. In areas with clay/silt or medium sand a sediment penetration of nearly 5m at water depths of about 8m was achieved. The surface sediments of the entire survey area were classified according to sediment samples.

The Parametric Sub-bottom Profiler SES-2000 For the survey the parametric sub-bottom profiler SES-2000 standard of Innomar Technologie GmbH (www.innomar.com) was used. The compact design of this system – only a small trans-ducer and one 19-inch unit are required – allows very easy and mobile installations.

(a) (b) Nonlinear (parametric) echo sounders use the parametric acoustical effect. The transmission of sound waves under high sound pressure results in nonlinearities at the sound propagation. If two slightly different frequencies f1 and f2 (so called primary frequencies f1<f2; f2/f1≈1) are transmitted at high sound pressures simultaneously, the transmitted signals interact. There are new frequencies generated (so called secondary frequencies), for instance the sum and the difference frequencies of the transmitted (primary) waves [1, 2]. The difference frequency F=|f2-f1| (in the range of 4 to 15kHz for the SES-2000 standard) is low enough to penetrate the seafloor. The reflected primary-frequency signals (about 100kHz for the SES-2000 standard) may be used for exact determination of water depth even in difficult situations, e.g. soft sediments on top of the seafloor. The generated difference-frequency signal has some advantageous properties compared to conventional – by linear acoustics – generated signals, the most important one is the directivity of the sound beam. The directivity for the difference frequency is similar to the primary frequency’s directivity. Small transducers can be used to generate narrow beams even at low frequencies. Furthermore, there are no significant side lobes for the difference-frequency sound beam. Because of the high system-bandwidth of a parametric system, really short signals can be transmitted.

Fig. 1: SES-2000 standard (a) and transducer (b)

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Due to these advantageous properties the seabed echoes from parametric sub-bottom profilers have a steeper slope than echoes from linear ones and are better to detect at low signal to noise ratios. Detection of small changes in the acoustic impedance and a high resolution of layers become possible and a more realistic and more accurate picture from the seabed and sediment structures beneath will be produced. For parametric systems, short pulses, narrow beams and the absence of side lobes results in less volume reverberation as well as less reverberation from the bottom surface compared to linear systems. This makes parametric sub-bottom profilers particularly useful in shallow water areas. Table 1 shows the main parameters of the SES-2000 standard sub-bottom profiler. This system achieved very good results in shallow water areas as well as in water depths of more than 500m using moon-pool mounted as well as over the side mounted transducers [3].

Water depth range 1 … 500 m Vertical resolution up to 6 cm Penetration depth up to 50 m (depending on sediment type) Accuracy of the depth measurement 0.02 m + 0.02% of the water depth Primary transmitter frequency ca. 100 kHz Secondary transmitter frequency 4, 5, 6, 8, 10, 12, 15 kHz Transmitter pulse length 0.07 … 1 ms Repetition rate up to 50 s-1 Beam width ±1.8° @ 4…15 kHz Beam steering range ±16° roll Transducer dimensions ca. 20 cm × 20 cm

Table 1: SES-2000 standard Main Parameters

Survey Results Innomar’s test survey took place in an area northwest of Drakes Island and east of Mountbatten Breakwater on the 14th September 2004. The SES-2000 standard sub-bottom profiler and a pipe-mounted transducer was installed on board of the survey catamaran “Catfish”. During the survey data of more than 20 tracks at a survey speed of 3 to 5 knots were collected within about four hours. Figure 2a shows the track plot of the survey and the positions of sediment samples overlaid on the interpolated bathymetry. The sediment samples were taken by the University of Plymouth using a Day Grab at several locations across the survey area [4]. There are six sample locations within the area that was surveyed by Innomar. The main components of these grab samples are given for reference in table 2.

Grab Sample Major Constituents % of Total H1 medium and fine sand 73 (40 / 31) H2 sand (medium / coarse / fine) 55 (20 / 20 / 15) H3 pebble and granule 60 (40 / 20) H4 medium/fine sand and pebble 72 (46 / 26) H5 clay/silt and very fine sand 88 (57 / 31) H6 pebble and medium/fine sand 78 (45 / 33)

Table 2: Ground truthing samples and their major constituents [4] The sub-bottom profiling results at five of these sample positions (H1 – H5) are given in figures 3 to 6. These echo plots show the analyzed backscatter from the seafloor and sub-seafloor structures in a rainbow-like colour scale (blue for weak echoes and red representing strong reflections). All given values for the water depth or layer thickness in the echo plots are calculated with a sound speed of 1510 m/s. Changes of the sound speed due to temperature variations or other influences were not taken into account. As expected the echo prints at sandy positions (H1, fig. 3 and H2, fig. 4 and 5) and at clay/silt locations (H5, fig. 6) show a flat and very reflective seafloor. A sediment penetration of 3m at a water depth of 5m is visible in the sandy area around H1 and a penetration of nearly 5m at water depths of about 7m was achieved in clay/silt around sample H5.

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At the pebble area around H3 (fig. 5) the echo plot shows a rough surface and there is no penetration. Samples H4 and H6 are located at the slope of a channel. The echo plot around sample H4 (fig. 4) shows a signal similar to the sandy area around H2, but no penetration due to the seafloor’s gradient. The results around sample position H6 are similar to H4 and the echo plot is not shown separately.

(a)

(b) sand + clay/silt pebblesand / pebble

0 to 1 1 to 2 2 to 3 3 to 4 4 to 5 5 to 6 6 to 7 7 to 8 8 to 9 9 to 10 10 to 11 11 to 12

Layer Thickness [m]

417800 417900 418000 418100 418200 418300 418400 418500 418600 418700 418800 418900 419000 419100 419200 4193005578900

5579000

5579100

5579200

5579300

5579400

5579500

H1H2

H3

H4

H5H6

(c) Fig. 2: Track plot and interpolated water depths of the surveyed area, incl. grab sample positions (a), result of sediment classification

with overlaid sediment thickness (b) and echo plot example (profile x = 417800 – 419270, y ≈ 5579400) with overlaid water depth and rock layer (c)

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The seafloor classification map shown in figure 2b was created based on the analysis of the first return of the transmitted sound pulse. There are several parameters of the received echo analyzed in the time domain that are influenced by the seafloor like the amplitude, the pulse length, the rise and fall time. The seafloor samples from the surveyed area are rather homogeneous and there is no sample with a 60%+ constituent, see table 2. Therefore in spite of the six classes in the surface sample classification [4] only three classes were distinguished for this acoustic classification: sand/silt/clay (H1, H2, H5), sand/pebble (H4, H6) and pebble (H3). All received signals were classified based on similar acoustic returns compared to the reference signals at the sample positions. The sediment penetration (difference of digitized water depth and rock layer) along the profiles is overlaid on the classification map in fig. 2b and the obtained values agree very well with the classified sediment type. In all the echo plots the seafloor and the rock and sediment layers were digitized, an example is shown in figure 7. In this example a sediment penetration of nearly 7 m at a water depth of about 8 m was achieved.

Conclusions The survey results proved the capabilities of the parametric sub-bottom profiler SES-2000 to achieve high-resolution echo plots in shallow water areas. A detailed mapping of the seafloor was possible, even small outcrops and steep slopes can be seen clearly, see fig. 4. The sediment samples provided by Plymouth University made it possible to associate the echo plots to different sediment types and to classify the entire surveyed area. In areas around samples with a high fraction of sand or silt/clay there was good sediment penetration.

Bibliography [1] Hamilton, M.F.; Blackstock, D.T.: Nonlinear Acoustics, Academic Press, 1998. [2] Novikov, B.K.; Rudenko, O.V.; Timoshenko, V.I.: Nonlinear Underwater Acoustics,

AIP Press, 1987. [3] Merklin, L.R.; Levchenko, O.V.: Sub-bottom Profiling of the Caspian Sea,

International Ocean Systems 9(2005)4, p. 11-15. [4] Eames, J.; Jones, G.: Sediment Sampling Shallow Water Area One (Inshore Area),

University of Plymouth, 2005.

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Fig. 3: Echo plot example around sample position H1

(range 3 – 12m, frequency 6kHz)

Fig. 4: Echo plot example around sample positions H4 and H2 (range 2 – 30m, frequency 12kHz)

Fig. 5: Echo plot example around sample positions H2 and H3

(range 6 – 18m, frequency 12kHz)

Fig. 6: Echo plot example around sample position H5 (range 4 – 16m, frequency 12kHz)

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