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Survey Report Customer Funn i Hafrsfjord Sigbjorn Daasvatn Gymnasvegen 5 4737 Hornnes Norway

Project Number 2018/01/M Contractor Innomar Technologie GmbH Schutower Ringstr. 4 18069 Rostock System SES-2000 sixpack Serial Number 2017/02/C/01/L Vessel Freya Surveyor Peter Hümbs phuembs@innomar.com Survey period 06. – 08.02.2018 Survey Area Hafrsfjord Stavanger Norway

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Table of Contents 1. Introduction .......................................................................................................... 3

2. System description .............................................................................................. 4

3. Methods ............................................................................................................... 5

4. Processing ........................................................................................................... 7

5. Results ................................................................................................................. 9

6. Conclusion ......................................................................................................... 14

7. Appendix ............................................................................................................ 14

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1. Introduction The Hafrsfjord became famous for the battle in the year 872, when Harald Schönhaar (Harald Hårfagre) defeated his rivals and laid the foundation for the union of Norway. In the year 2018 Innomar Technology was asked to help with parametric echo sounder SES-2000 sixpack to investigate the sub-bottom for any artefacts which may proof the battle from the year 872. During the survey session two different areas were surveyed. The first area was approximately 1180m x 50m. The depth range was 31m – 40m of water. The second area was approximately 130m x 40m. The depth range was 1.5m – 8m of water.

Figure 1 Run line overview of the SES-2000 sixpack survey in the Hafrsfjord

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2. System description The parametric sediment echo sounder system SES-2000 sixpack from Innomar Technologie GmbH Germany was deployed using the parametric acoustic effect. Transmitted sound waves of two slightly different high frequencies interact in the water column forming a low frequency component. The SES-2000 system uses two primary frequencies near 100 kHz to generate different frequencies between 5 kHz and 15 kHz. The high frequencies can be used to determine the water depth and the low frequencies are able to penetrate the bottom. The reflected low frequency returns information about sub bottom structures, sediment layers and embedded objects as well. The acoustic penetration depth strongly depends on the sediment types. The penetration depth of low frequencies is greater than of high frequencies. However, the resolution increases with frequencies and decreases at lower frequencies used. A real advantage of the parametric SES-2000 system versus linear echo sounders is the achieved half power beam width of +/-1.8 degrees for the low frequencies with a small and portable transducer. Hence, the active sounding area (footprint) of the transducer is comparably small. The narrow beam width is valid for all generated difference frequencies and results in a very good horizontal resolution independent from the chosen frequencies (5 – 15 kHz). Additionally, the beam has virtually no side lobes decreasing lateral disturbances which is important for surveys in narrow areas, such as harbour basins. With an active sounding area of 0.2 x 0.2 m, which is the size of the SES-2000 transducer, the following beam widths and resolutions are possible:

Frequency Aperture Angle Footprint at 5 m depth

100 kHz (linear)

3.6° 0.31 x 0.31 m

4kHz - 15kHz (parametric)

3.6° 0.31 x 0.31 m

Table 1 Beam width and acoustical footprint (- 3dB) of the SES-2000 sixpack system

The possible transmission of very short pulses without ringing effects, for instance one sinus pulse of 15 kHz, results in very high resolution of layers and objects (layer to layer resolution of down to 5 cm thickness). The accuracy of the depth measurement is given with 0.02 m + 0.02 % of the water depth for the 100 kHz and the smallest range. The user has the possibility to adjust the signal length for the transmitted pulse. A longer pulse obtains more energy, but the achievable resolution decreases. SES-2000 has a pulse repetition rate of up to 60 pulses per second (depending on the range). At a water depth of 10 meters and a selected range of 10 meters the pulse repetition rate is 28. The higher the pulse repetition rate, the higher is the chance of multiple hits of embedded objects and their identification in the echo data.

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Figure 2 Comparison of beam width between parametric (left) and linear (right) echo sounder systems

High and the low frequency data are digitally stored. The information from a GPS receiver is stored too. Only the data of the online selected range can be stored together with the system parameters and then be processed. During the processing most of the signal processing parameters can be changed, excluding hardware dependent parameters like the operating range, the frequency or the amplifier settings. During the survey online echo plots are generated, so the survey can always be adapted to the survey conditions and requirements.

3. Methods Sub-bottom

During the survey the Innomar SES-2000 sixpack was used. The system was mounted on the Survey boat „Freya“. (Figure 3 )

The SES-2000 sixpack generates low secondary frequencies (selectable in the range of 5kHz-15kHz) to penetrate the sediment by using the parametric effect. This transmission method offers the following advantages in particular:

• very small opening angle of <+/-2° results in a small sounded area and thus good horizontal

• short transmit pulses without ringing achieve a very high vertical resolution of layer boundaries or objects down to 5 cm (depending on the frequency)

• A high transmission pulse rate allows the detection of small objects (pulse rate depending on the recording area)

The first area was covered with a set of 35 lines and the second area was covered with set of 33 run lines. The lines were recorded with a distance of 1.5 m (deep area) and 1.2m (shallow area), as well as some additional test lines. The recording range was set to 20 m for the deep area and to 7 m for the shallow area. The sediment echo sounder data were recorded with a low frequency of 10 kHz.

The SES-2000 sixpack system uses a transducer array with 6 single transducers. Five operating modes can be selected to suit the task:

• 6 beam mode (6BM) Arrangement of the 6 transducers in one line for highest data density (6 lines parallel at the selected transducer distance)

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• 5 beam mode (5BM): Combination of each 2-single transducer to achieve higher sound level with a little reduced data density (5 parallel lines)

• Triple beam mode (TBM): Two transducers are each operated together, which leads to a reduction of the data density at a higher sound level and thus to greater achievable penetration. (3 lines parallel)

• Dual beam mode (DBM): Each three single transducers are operating together, which leads to a halving of the data density at a higher sound level and thus greater achievable penetration. (2 lines parallel)

• Single beam mode (SBM): Rectangular arrangement of the 6 transducers and parallel operation for maximum penetration into the ground. (1 line)

During the survey the DBM has been used for the deeper area in order to increase the penetration into the bottom as well as to increase the number of transmitted pulses. That was more important than the penetration in order to achieve a maximum data density for water depths up to 40 meters. The sediments in this area are very soft and could easily penetrated. For the shallow part the 5BM was used in order to increase the density of the data.

The recording range was recorded with a sample rate of 96 kHz which results in a data sampling lower than 0.8 cm per pixel. The length of the transmit pulse was set to shortest possible value of 100 µs (one sinus cycle) in order to achieve the highest possible vertical resolution. In theory the best achievable vertical resolution at 10 kHz is 7.5 cm, where practically due to filtering and footprint the resolution results in 12 cm. The transmit pulse rate of the system was approximately 14 pings per second and the ping rate for each track were 7 pings per second in the deep area. The boat speed was 2m per second and the array width was 1.2 meter which results in a distance between the transducers (DBM) of 60 cm. In comparison the ping rate in shallow area was 66 pings per second which result in 12 pings for each track (5BM).

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Figure 3 Mounting the SES-2000 six pack transducer and additional sensor on the vessel "Freya"

Additional Sensors

The positioning was done with a Septentrio ALTUS APS-U RTK system using local Norwegian correction data service. The coordinates were recorded in WGS-84 format as LAT/LON and were converted into UTM zone 32. At the same time, the GPS system provided the true heading data (two antenna system), which is required for determining the position of the SES-2000 sixpack transducer arrays in three-dimensional space. The elevations of the GPS were recorded to feed the measurement data and used for water level correction during data processing.

A motion sensor (Seatex MRU-5) provided roll, pitch and heave data that allow echo data to be corrected for the ship's movements (heave and lever arm correction for the individual transducers in the array). For the time-to-depth conversion a sound velocity of 1430m/s has been used.

The navigation software Hypack 2017a was used for the line planning and for navigation (separated screen with left-right-indicator for the helmsman).

4. Processing The data were processed with the Innomar ISE-software and exported into 3D data. Those data were converted into a regular data grid by a tool called SESGridder made by Innomar and were visualized by the 3D volume renderer Avizo.

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Figure 4 Track plot view of the deep area

The echo data has already been recorded with limited bandwidth, i.e. the primary frequency (100 kHz) and secondary frequency (6 kHz) are separated by analog filters in the system and the secondary frequency is optimally filtered with a digital bandpass depending on the system setting (frequency and pulse length). The amplitude values are available as 16-bit values with a sampling rate of 96 kHz.

The following processing steps has been carried out:

a) Interpolation of the coordinates (the Update-Rate of the GPS is lower than the ping rate of the SES system Ping Rate des Systems, therefore each ping gets an individual coordinate by a linear interpolation)

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b) Array offset correction according to the used transmit mode c) Determination of water level table based on the recorded GPS-Z values d) Improvement of the signal to noise ratio by filtering

e) Applying a threshold table to convert the 16-bit amplitudes into a color table

f) Export of the 2D sections (*.gif)

g) Export of x,y,z,amplitude files for all channels (*.bin/*.txt)

h) Calculation of the regular 3D grid out of the irregularly distributed data

i) Volume rendering and 3D visualization

5. Results Due to the good steerability of the ship, the lines are generally very straight, parallel and evenly distributed. There are no big gaps. The data quality of the sub-bottom data is generally very good. No outliers are visible and intersection points fit very well in height and position. In both areas the sub-bottom data show an excellent penetration into the sediments. In the sediment of the deep area are many individual reflectors visible. Most of those very obvious reflectors appear with a hyperbolical shape. (Figure 5) This hyperbolic shape is created if the sound lobe hits a round object, a boulder or small patches of gas when moving over it with the vessel. The interpretation of the 2D sections can be difficult in terms of recognizing coherent structures of those single reflectors. Therefore, the entire sub-bottom data were used to calculate a 3D model (Figure 6). The size in of the example in figure 6 is approximately 200 x 150 meters in Y and in X. The size in Z-direction is 21 meters. The calculated cell size is 0.5 x 0.5 x 0.01 meters. The model can be rotated and moved in all directions. Furthermore, it is possible to apply so called clip planes to the model. In Figure 6 a clip plane has been applied in Z direction beginning from the top. (approximately 1m below the bottom) Now it is visible, that the hyperbolic reflectors seen in the 2D section can be recognized as almost round structures.

Another example can be seen in Figures 9 and 10 showing an elongated feature across the survey area. It seems there is a little depression like a trace. Normally those traces are created by fishery or icebergs but in this area the reason is probably a dragged anchor.

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Figure 5 Data example of the deep area with highlighted hyperbolic reflectors

Figure 6 3D data example of the deep area (200 x 150 x 21 meter)

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Figure 7 Cut thru the 3D model beginning from the top (clip plane orientation along Z). Clear visible the hyperbolic reflectors as more or less round structures.

Figure 8 3D model from figure 6 rotated to visualize the hyperbolic reflections from the side.

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Figure 9 3D data example shows a small depression across the survey area

Figure 10 data example 2D section (line number 101) of the little depression which is shown in figure 8

In the shallow area, plenty single reflectors can be found at different depths below the seabed. An example of a 2D section is shown in Figure 11. That can be confirmed by the 3D model, where also many single reflectors are visible (Figure 12). In the 3D model shown in Figure 13, a coherent structure of anomalies is visible.

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Figure 4 2D data example of the shallow area (line WS49) where sediment layer and hyperbolic reflection are visible

Figure 12 3D data example of the shallow area. Cut thru the 3D model beginning from the top (clip plane orientation along Z) with some anomalies in the sediment

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Figure 5 3D data example of the shallow area with horizontal clip plane through the 3D model (along Z axis) showing a distinct buried structure

6. Conclusion From our point of view the survey was successful. The system has worked without any failures. The bracket was well prepared and worked very well. The boat was suitable for navigating the closely spaced lines. Unfortunately, there was not enough time to survey a bigger area. Only a very small area in comparison to the entire site could be surveyed. Due to the morphology of the Hafrsfjord with its deeper waters, it should be considered to use the system in future and try to get the SES-2000 sixpack closer to the seabed in order to increase the resulting resolution. It is also worth to consider the usage of remotely or autonomously controlled surface vehicle for a more efficient survey of larger areas.

7. Appendix All data is provided digitally:

a) Footage of the deep and shallow area b) X,y,z, amplitude text files for each line c) GIF graphics for each line d) SEGY seismic data files for each line