Using Synthetic Aperture Sonar in Marine Archaeological Surveys

some first experiences

Øyvind Ødegård, Martin Ludvigsen Norwegian University of Science and Technology

(NTNU), 7491 Trondheim, Norway oyvind.odegard@ntnu.no

Petter A. Lågstad Norwegian Defence Research Establishment (FFI),

P O Box 25, NO-2027 Kjeller, Norway

Abstract - Acoustic sensors like side scan sonars and multibeam echosounders are important tools in marine archaeology. In this paper we will give a very brief explanation of the principles behind SAS compared to conventional SSS, and how these differences affect practical use for archaeological purposes. We will also describe requirements and procedures for deployment, operation and data processing. We will show examples of SAS output, both imagery and bathymetry, demonstrating resolution and range. We will present some first experiences from using SAS for marine archaeological purposes, pros and cons compared to traditional SSS, and discuss some perspectives for future applications.

Synthetic aperture sonar, Marine Archaeology, AUV, ROV, Sonar imagery, Bathymetry, AUR-Lab, FFI

I. INTRODUCTION There is a long tradition for the use of remote sensing technology in marine archaeology. Acoustic sensors like Side Scan Sonars (SSS), Sub Bottom Profilers (SBP) and Multi Beam Echosounders (MBES) have been common for decades. Especially SSS have been much used. They require little training as they produce data that are easy to interpret, and also are easy to deploy and operate. The principle behind conventional SSS is that acoustic pulses, often referred to as pings, are emitted sideways in fan shaped beams. The beams are narrow along track to attain high resolution, and wide across track to attain long range. An array then measures the strength of the ping’s echo over time. Measurements of all the successive echoes are then combined to create a sonar image [1]. In principle each pixel in a SSS image represents the strength of the echo of one ping at a certain distance. Since sound wave attenuation in water increases with frequency [2] there is a tradeoff between range and frequency. This means that high resolution imagery can only be attained at shorter ranges, and vice versa longer range equals lower resolution. Synthetic Aperture Sonar (SAS) is a technology based on principals derived from radar technology [3]. Departing from the conventional one ping one echo mapping that constitutes the traditional sonar image build up, SAS is based on a multiple ping multiple echo mapping process. By post processing the signals recorded by an array with multiple receiver elements moving along track it is possible to emulate

a synthetic array independent of its physical length. Each pixel in the resulting SAS image is thus calculated based on multiple echoes of multiple pings. In short this means that the along track resolution of SAS is independent of frequency and range [3]. The difference between SAS and SSS is effectively illustrated by comparing respective imagery of the submarine U735 lying on the seabed outside Horten, Norway [4]. Even though the distance to the submarine was over 20 times longer, the SAS image clearly yields a much higher resolution than the conventional SSS image (Fig.1).

Fig 1. German WWII submarine U735 outside Horten, Norway. Top: Synthetic Aperture Sonar (HiSAS 1030) at 205 – 245m range. Bottom: Side Scan Sonar (EdgeTech 4300-MPX) at 10-50m range.

In December 2012 NTNU Applied Underwater Robotics Laboratory (AUR-Lab) together with the Norwegian Defence Research Establishment (FFI) conducted a week long research cruise with the research vessel RV Gunnerus in Trondheimsfjorden, Norway. The research cruise had an interdisciplinary profile [5], and one of the missions was to survey historic shipwrecks in Trondheim harbor using the Autonomous Underwater Vehicle (AUV) Hugin HUS deploying a SAS system.

Trondheim was founded in AD 997 as the first capital of Norway, and has been an important shipping center for over 1000 years. Situated on the Nidaros peninsula, the sheltered riverbanks of the river Nidelva provided a good harbor for the city. For 500 years the river sufficed as harbor, but in the late 16th and early 17th century it became too shallow for the deeper drafted seagoing vessels of the period, and the harbor moved outside the river mouth letting ships anchor between the river and the small Island Munkholmen. The seabed consists mainly of gravel and sediments from the river, and constitutes poor anchoring conditions. We know of at least two ships losing anchor in northwestern storms to be wrecked against the rocky Lade shore to the east [6]. An area of interest was set centering on two known wreck sites, of which one had been investigated earlier [7]. Four survey lines in NS directions and four in EW direction in a classic lawn mower pattern were planned to give ample coverage of the wrecks at different ranges and aspect angles. In addition two survey lines were placed along the rocky Lade peninsula were several wreck sites have been reported by divers but never accurately positioned and mapped (Fig.2).

In March 2013 NTNU AUR-Lab returned to the survey area to perform a video survey of one of the two historic wreck sites using the ROV Minerva. The object of the survey was to

provide a visual ground truth for better assessing the quality of the SAS data.

II. METHODS

A. AUV The Hugin HUS is a scientific AUV, owned and operated

by FFI. Several other Norwegian scientific institutions contribute financially and it is used in a variety of scientific applications. It is a member of the Hugin family of AUVs, which has been developed as a collaborative effort between FFI and Kongsberg Maritime for 20 years. Hugin HUS is rated to 3000 m. depth and can be fitted with a variety of instruments and sensors. Main payload sensor for the archaeological mission was the HiSAS 1030 Interferometric Synthetic Aperture Sonar allowing for 17 hours running time between battery charges. This gives a maximum range of 128.5 km pr. dive with a maximum range of 200 m (400 m swath) at 2 m/s speed. A crew of four (two on deck and two in control room) was dedicated to operate Hugin HUS and post process data during the mission.

Hugin HUS can typically operate at different levels of autonomy depending on mission requirements, environmental conditions and safety parameters. In open waters Hugin HUS can execute a preprogrammed mission completely unsupervised; returning to be recovered after the survey is finished. In a harbor with much activity and ship traffic it is desirable to control and monitor the AUV at all times, and in

Fig 1 Trondheim harbor with a HiSAS imagery mosaic showing the investigated area.

Trondheim harbor RV Gunnerus followed Hugin HUS on the surface with a transducer in the water keeping an open communication link at all times.

B. RV Gunnerus RV Gunnerus is NTNU’s research vessel that for this

research cruise functioned as base for all AUV and ROV operations. RV Gunnerus is equipped with Kongsberg Simrad Dynamic Positioning system (SDP-11) combined with Kongsberg Seatex Differential Positioning Sensor (DPS-232), Kongsberg High Precision Acoustic Positioning system (HiPAP 500), and a Kongsberg Seatex Motion Reference Unit (MRU-5) integrated in a Kongsberg Seapath 300 system.

C. HiSAS 1030 The HiSAS 1030 is a multi-element widebeam wideband

Interferometric Synthetic Aperture Sonar developed by Kongsberg Maritime and FFI for mine detection [8]. It was specially designed for Hugin AUVs, and can acquire high resolution imagery and bathymetry at the same time with swaths typically 400 m. wide. HiSAS 1030 can produce imagery with a theoretical resolution of 2 cm both along- and cross-track for the whole swath. A number of factors such as inaccuracies in navigation, sound velocity, and topography can affect focus negatively and thereby reduce the image quality [9].

All data is stored locally on a hard drive and can be downloaded only after the mission is finished and the AUV is back on board the ship. As SAS technology is quite different

from SSS, it follows that data processing is more complex. Two software suites have been specially developed for processing and managing SAS data. The FOCUS Synthetic Aperture Sonar Signal Processing Toolbox developed by FFI and Reflection developed by Kongsberg Maritime [10]. Raw data must first be processed in Focus to produce data that can be exported in standardized formats such as Kongsberg (.all) or the Extended Triton Format (.xtf). The data can then be viewed and managed in Reflection which has much of the same functions as other sonar data processing software..

III. RESULTS

A. Historic wreck site After surveying the selected area of interest with multiple

passes, there were several datasets describing the same wreck at different ranges and angles.

The historic wreck was first discovered on a SSS test run in 1981 [11] and later investigated by Fredrik Søreide in the period 1997-99 [7]. The visible remains of the wreck are 20 meters long, and a sample of wood from the site has been carbon dated to 17th or 18th century [7]. The wreck site is representative for what typically can be found along the Norwegian coast. Wood eating organisms have destroyed most protruding features, and what is left is a flat flake of the hull partially covered by a thin layer of sediments with a keelson as the significant visible indicator.

Fig 3 HiSAS image of historic wreck draped on bathymetric model with bin size 0.5 m and

Comparing the HiSAS images of the wreck at different ranges and aspect angles shows a consistency in detail and resolution independent of range and AUV track direction (fig. 4). The images closely resemble conventional SSS imagery familiar to most marine archaeologists. The pronounced NW-SE oriented line in the middle of the ship shaped feature is a partially exposed keelson approximately 18 meters long and protruding up to 20 cm off the seabed. In images 5-8 it is possible to see some vague features running parallel with the keelson with some shorter features perpendicular on top of them. This was later identified as probable planking and frames

attached to the keelson with ROV using HD camera, demonstrating that structural elements or objects smaller than 10 cm could be recognized in the HiSAS imagery at all distances (fig. 5). The AUV kept a fixed altitude above seabed for all passes thus obtaining different grazing angles with corresponding differences in shadow length. In some images the shadows indicate that the wreck is a mound rising from the seabed. A profile created from the bathymetric data confirms a modest elevation at the wreck site (fig. 3), suggesting that more remains of the ship could be buried in the sediments.

Fig 4 Images 1-8 shows HiSAS imagery of historic wreck site in Trondheim harbor. At the bottom of every image is annotated ground range and travelling direction (e.g. WE means AUV is travelling from West to East).

Fig 5 Frame grab from HD video survey of historic wreck site using ROV Minerva. Detail of keel with frames. 10 cm distance between laser points.

B. Second wreck site In the lead in to one of the survey lines a previously

unknown wreck was discovered at 80 meters depth in a slight slope at 136 meters ground range from the AUV (fig. 6). The HiSAS imagery showed a typical hull shape measuring 26 long and 5 meters wide with concentrations of features at mid ship and at both ends (fig. 7). On inspection with ROV it turned out that this was a vessel probably dating to mid or late 19th century, and that the features seen on the HiSAS data were mainly metal constructions and probably parts of some kind of machinery – with for instance shafts corresponding with intense straight features on the HiSAS data (fig. 8). During this quick ROV inspection no hull structures could be seen, and any remains of the ship itself must be buried in the sediments.

IV. DISCUSSION The results from the mission in Trondheim harbor clearly

demonstrate that SAS can be a very useful tool in marine archaeology. We have seen that it can produce detailed images of wrecks at relatively wide swaths, and that high resolution is maintained independent of range. Manually referencing and rectifying data sets to combine SSS imagery and MBES

bathymetry is a time consuming task, and the ability to produce co-registered data is therefore of great value.

Since HiSAS utilizes a large transducer opening angle (30 degrees), it is able to “see behind” features and acquire data from areas normally appearing as shadows in SSS imagery. This is of particular benefit to bathymetric modeling, drastically reducing no-data areas.

SAS technology is relatively new, and is still developing in a fast fashion. Though similarities with familiar seabed imagery can entice archaeologists to see this as sophisticated and expensive SSS, juxtaposing the two technologies shows that they cannot be directly compared. The simple SSS technology is robust and user friendly, and as for data - what you see is what you get. SAS on the other hand requires a range of auxiliary data input, a trained crew for operations and not least post processing that involves special software and

complex computing – but the data output is correspondingly more advanced. The ability to acquire high resolution imagery and bathymetry independent of range is a very clear advantage of SAS. Experiments with different SSS systems on typical archaeological objects on the seabed suggests that a range of 10-15 m produces optimal image resolution, and that the effective range for marine archaeological purposes is circa 40 m [12]. SAS should under good conditions be able to acquire better resolution at four to

Fig 6 Full swath with wreck located outermost on starboard side.

Fig 7 Starboard channel with enlarged wreck inset. Fig 8 Detail of probable machine constructions in wreck.

five times that range, increasing effectiveness with at least 400 percent.

In addition to increased resolution and range SAS also has other capabilities that differ from conventional SSS. Since resolution is not only independent of range but also frequency, it is possible to produce high resolution imagery with low frequencies that enables penetration in to sediments and materials [13]. This combined with coregistration of data acquired at different times with sub centimeter accuracy [14] implies a huge potential for archaeological applications. For instance repeated surveying of vulnerable cultural heritage to monitor deterioration or establishing time series for research on site formation processes are obvious examples.

V. CONCLUSION We have described how a SAS system deployed on an

AUV was used for marine archaeological purposes. We have shown examples of data at different ranges demonstrating resolution and coverage. We have pointed out some main differences compared to conventional SSS technology, and highlighted some pros and cons. Finally we have discussed some future perspectives regarding the potential of using Synthetic Aperture Sonar technology on different archaeological operations.

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