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ADRAMAR Hangar à tabac – Chaussée des corsaires
35400 Saint‐Malo
Arch‐MancheProjectIntermediateReport
May2013
Geophysicalprospectionandsurveydivesonthefish‐weiroftheDavierrocks
(35)
LaetitiaLeRu
CONTENTS
Information sheet ................................................................................................................................................... 1
Financing .................................................................................................................................................................... 1
Team and acknowledgements.......................................................................................................................... 1
Introduction .............................................................................................................................................................. 3
I. Strategy and method of prospection ................................................................................................... 5
1. The site and its environment ............................................................................................................. 5
2. The equipment .......................................................................................................................................... 6
a. Hermine-Bretagne ......................................................................................................................... 6
b. Side-scan sonar............................................................................................................................... 6
i. Principle ........................................................................................................................................ 6
ii. The StarFish 450F .................................................................................................................... 7
c. Sub-bottom profiler ..................................................................................................................... 8
i. Principle ........................................................................................................................................ 8
ii. The Stratabox ............................................................................................................................. 9
3. The surveys ............................................................................................................................................. 10
a. Side-scan sonar surveys .......................................................................................................... 11
b. The sub-bottom profiler.......................................................................................................... 12
4. The results ............................................................................................................................................... 13
a. Side-scan sonar............................................................................................................................ 13
b. The sub-bottom profiler.......................................................................................................... 14
II. Survey dives ............................................................................................................................................ 15
1. Methods..................................................................................................................................................... 15
a. Determining where to dive .................................................................................................... 15
b. Diving conditions ........................................................................................................................ 15
2. Underwater observations ................................................................................................................ 16
Conclusion .............................................................................................................................................................. 18
Bibliography .......................................................................................................................................................... 19
Table of illustrations .......................................................................................................................................... 20
Documentation ..................................................................................................................................................... 20
1
Information sheet
Project title: Geophysical prospection and survey dives on the Daviers Rocks
Region: Brittany
Department: Ille-et-Vilaine
Commune: Saint-Malo
Toponym: Les roches du Davier (Davier Rocks), off the coast of Saint-Malo
Coordinates: 48° 40.6667’ N - 001°59.6500’ W
Depth: 12 metres
Chart No. SHOM 7130
Reference and owner: DPM – DRASSM
Nature of the operation: Geophysical prospection and survey dives
Operation No. OA 2003
Operator: ADRAMAR (Association for the Development of Research into Marine
Archaeology). Address: ADRAMAR, Hangar à tabac, Chaussée des corsaires, 35400
SAINT-MALO France.
Managers:
Laetitia LE RU, Underwater archaeologist & Project manager, Adramar.
Loïc LANGOUET, AMARAI.
Email:
Date: 25/02 – 01/03/2013
Financing This operation was funded by the ERDF as part of the Interreg IVA European
programme, Arch-Manche project. Adramar’s involvement was that of a service
provider.
Team and acknowledgements The team in the field comprised:
- Hervé BLANCHET: Geophysical systems operator (DRSM)
- Anne HOYAU-BERRY: Underwater archaeologist & Researcher (Adramar)
2
- Nicolas JOB: Photographer
- Franck JOUET: Captain
- Georges LE PELLETIER: Engineer (Adramar)
- Laetitia LE RU: Underwater archaeologist & Project manager (Adramar)
- Marine SADANIA: Underwater archaeologist (PhD student at Nantes University,
research unit UMR 6566 / Adramar)
We would like to thank all the members of Adramar who contributed to the success
of the operation.
We would also like to thank the staff of the ENP (Saint-Malo) for their welcome.
Finally, our heartfelt thanks go to Marie-Yvane Daire her team, as well as Loïc
Langouët, for making us a part of this project.
3
Introduction
The geophysical prospection and underwater surveys carried out by Adramar, at
the behest of Marie-Yvane Daire of the Centre de Recherche en Archéologie,
Archéoscience, Histoire (CReAAH, CNRS – UMR 6566) of the University of Rennes 1,
were part of the European project known as ‘Arch-Manche: Archaeology, art and coastal
heritage’.
This project is itself part of the European Interreg IVA 2 Seas Cooperation
Programme which brings together the coastal regions of four member states: France,
England, Belgium (Flanders) and The Netherlands. Its purpose is to encourage cross-
border cooperation between these coastal regions. The project was approved by the
European Commission and received funding of 167 million euros from the European
Regional Development Fund (ERDF) for the period 2007-2013. Of the four priority
themes, the Arch-Manche project falls into the fourth, the common priority with the
France (Channel) – England programme. The ambition of the participants is to formulate
a solution within the scope of a common endeavour in line with the objectives of the
programme priorities.
The partners of the CReAAH (France) are Hampshire and Wight Trust for
Maritime Archaeology (England), the Department of Geology and Soil Science, Ghent
University (Belgium), the Deltares Research Institute (The Netherlands). The focus of
the project is on integrated coastal zone management (ICZM) and in particular coastal
change since prehistory.
Selected through a tendering process, Adramar’s involvement is that of a service
provider working within the framework of the European Interreg IVA programme and
its twin objectives. The first of these involves acquiring archaeological data in order to
inform, complete and illustrate our knowledge of the coastal changes at the heart of the
project by studying the archaeological sites that highlight the evolution of the coastline
and sea levels. The second objective of this European programme, within the framework
of the Arch-Manche project, is to carry out methodological tests and implement various
techniques, especially geophysical techniques, in order to determine their pertinence for
the study of fish-weir sites.
Two distinct and different geographic areas have been defined for the surveys:
- an area known as ‘les roches du Davier’ (Davier Rocks) off Saint-Malo (department
35), and the subject of this intermediate report,
- a wide area around the Quiberon Peninsula (department 56).
The area around the Davier Rocks was prospected and surveyed from 25
February to 1 March 2013 and the week-long operation involved three actions to:
4
- calibrate and implement the detection devices: side-scan sonar and a sub-bottom
profiler,
- determine the dive points,
- document the fish-weir (geophysical, direct observation, photography).
The purpose of the operation was to resolve two distinct issues.
The first of these related to methodology because we wanted to see if underwater
geophysical surveys are a pertinent method for studying fish-weirs that no longer dry
out at low tide and add to the information already available, notably information
collected during aerial surveys.
On the scientific side, M. L. Langouët estimated that the weir at the Davier
Rocks—situated under chart datum like 14.5 % of the 574 stone fish-weirs so far
inventoried—would show a rise in the water level of 7 metres. The direct study of the
feature was therefore impossible without recourse to diving.
Figure 1: Le Petit Davier and Le Grand Davier. Enlargement of marine chart SHOM 7130.
5
I. Strategy and method of prospection
1. The site and its environment
The bay of Saint-Malo has one of the greatest tidal ranges in Europe. On average
the range is greater than twelve metres and, consequently, any surface or underwater
survey requires a special approach.
The Davier Rocks are situated north-north-east of Saint-Malo1. Le Petit Davier lies
to the north and Le Grand Davier, to the south. The fish-weir is situated between two
rocks and never dries out at low tide. It is classed type Ac in the classification system
devised by Loïc Langouët and Marie-Yvane Daire2. Despite the position of the feature
between the Davier Rocks, which required special manoeuvring on the part of the
captain, good sea conditions meant that the approach was relatively easy for the time of
year.
The works were carried out according to the state of the tide. The geophysical
devices were deployed toward the end of the flood, at high slack water and at the start of
the ebb in order to ensure a sufficient body of water for the equipment to work properly
without risk of damage and to guarantee the quality of the data.
Figure 2: Position of the fish-weir between the Davier Rocks.
1 Cf. Appendix 1, fig. 1, p. i. 2 DAIRE Marie-Yvane, LANGOUET Loïc, Les anciens pièges à poissons des côtes de Bretagne, un
patrimoine au rythme des marées…, Rennes, Coédition Ce.R.A.A. – A.M.A.R.A.I., Les dossiers du Centre
Régional d’Archéologie d’Alet, AG, 2010, p. 12.
6
2. The equipment
a. Hermine-Bretagne Hermine-Bretagne is Adramar’s support ship. Eighteen metres long, she was
originally an oyster dragger before Adramar acquired her and converted her into a
research vessel specializing in underwater archaeology. Her shallow draught (1 m)
allowed us to sail extremely close to the rocks during the survey.
The vessel’s role in the operation was to deploy and manoeuvre the geophysical
devices (side-scan sonar, sub-bottom profiler). She was also used as a platform for the
survey dives.
b. Side-scan sonar
i. Principle
The side-scan sonar is a transmitter and receiver of sound waves (approximately
450 kHz) and comprises several components.
One of these, the towfish, is towed through the water and transmits fan-shaped
acoustic pulses, perpendicular to the direction of travel, and covers an area of varying
size depending on the desired resolution of the image. The area covered either side of
the centre-line, or nadir, is called the range. The centre-line appears on the sonar image
as a blind spot but this is not the case3. In fact it represents the insonification of the
water column and the operator can simply, during post-processing, remove the black
strip which appears in the middle of the scan swath to constitute a continuous image of
the bottom.
The towfish is connected to the vessel by means of a coaxial electric cable which,
in real time, carries the data to the topbox for processing. The cable also serves as the
towing cable.
The working principle behind the side-scan sonar is simple. The acoustic pulse
transmitted by the towfish is reflected when it meets a surface, the bottom or any other
element present in the insonified area. Transducers situated on the towfish capture
these reflected, or specular, waves which travel along the same trajectory as the waves
initially transmitted by the device. Travel time is recorded together with intensity. As
sounds travels at a known velocity through water, the echo, once processed, allows the
system to produce an acoustic image of the seabed from which can be determined the
lengths, breadths and heights of the objects scanned.
Indeed, any obstacle of sufficient size will intercept part of the transmitted signal
and prevent it from being reflected by the seabed. This creates an acoustic shadow
which the operator can use to estimate the height of the obstacle.
3 Cf. infra, fig. 10, p. 14.
7
The frequency of the transmitted pulse determines the penetration depth of the
wave. Thus, the higher the frequency, the smaller the penetration and vice versa.
However higher frequencies provide greater resolutions.
The towfish must be towed at a depth equal to one tenth of the desired maximum
range. For example, for a range of 50 metres, the depth at which the towfish must be
towed is 5 metres.
Figure 3: Diagram showing the working principle of a side-scan sonar (Ballard, 2008, p. 7).
ii. The StarFish 450F
The sonar used during the survey was a StarFish 450F. Its frequency was 450 kHz
for a maximum range of 200 metres. Towfish are usually torpedo-shaped, but the
StarFish is shaped like a star to improve its stability. It has two transducers, mounted in
the lower fins, which are angled 30° downwards from the horizontal. It transmits
narrow horizontal beams of 1.7° but wide vertical beams of 100°, for the most part
focusing on the 60° range.
Figure 4: Front view of the StarFish and its transmission sector4.
4 http://www.starfishsonar.com/support/imagery/side-scan-sonar.htm
8
Deployment was easy because the device weighs less than 2 kg and is ‘plug and
play’. The towfish connects through a Kevlar-reinforced coaxial cable to a StarFish 450
electronics module (located in the vessel) which runs on 220V or battery and which is
connected to a PC through a USB port. Hypack software was used to process the raw
data.
Another special feature of this side-scan sonar is its use of ‘CHIRP’ (Compressed
High Intensity Radar Pulse) digital technology.
This means that, instead of using a single carrier frequency per burst, the acoustic
pulse uses several. This produces a ‘swept’ effect which provides denser cover and
improved processing of the acoustic return, and therefore greater image resolution.
c. Sub-bottom profiler
i. Principle
The sub-bottom profiler is also a transmitter and receiver of acoustic waves. It is
a seismic tool which uses frequencies in the tens of Hz and provides a cross section
image of the seabed, or sub-bottom profiling. Unlike side-scan sonar, it uses much lower
frequencies to map the features of the bottom where the waves transmitted by the
sonars do not reflect or penetrate.
Its angle of cover, or insonification, does not vary according to the desired
resolution but according to the aperture angle of the transducer, which is fixed for each
model of the sub-bottom profiler, and the depth, which can be expressed by the
following formula:
where P is depth and ao the aperture angle of the transducer cone.
The sub-bottom profiler measures variations in the density of the medium by
interpreting the variations in the returns of the low frequency as it passes through the
various layers of sediment on the seabed. These returns are recorded in order to create
seismic stratigraphic profiles. The lower the frequency, the deeper it penetrates into the
layers of sediment.
Towed behind a vessel or fitted to its hull, the system transmits a pulse vertically
through the water column. Part of the acoustic signal is reflected by the bottom while
the rest penetrates the layers of sediment and is reflected when it encounters a
boundary between two layers that have different acoustic impedances.
Instrument performance depends on several physical factors linked to the
transmission of the acoustic signal, such as power output, length and frequency, but also
to the salinity of the water and its temperature. The usefulness of the instrument
9
depends on the marine environment and the nature of the terrain, and in every case it
requires careful adjustment.
The sub-bottom profiler requires extensive post-processing in order to remove
artificial noise and information inherent to the navigation of both the vessel and the
towfish.
Figure 5: Diagram of sub-bottom profiling (Ballard, 2008, p. 8).
Combining the data collected through these two methods produces a complete
image of the archaeological landscape because the sonar provides information on height,
width and length while the sub-bottom profiler adds a fourth vector: depth under the
sediment.
However, the two methods cannot be directly linked because in theory the
collected information involves a single particular object that responds either to the
working principles of the side-scan sonar or those of the sub-bottom profiler. In practice,
therefore, we cannot claim to produce a given piece of information in four dimensions.
ii. The Stratabox
The sub-bottom profiler used for the survey was a StrataBox manufactured by
the SyQwest Company. The system is connected to a PC through a special interface
module and therefore simple to install. The StrataBox transducer was fitted to a pole to
ensure it was deployed horizontally and could provide adequate insonification of the
bottom. Fitted to the after section of the vessel, the system was coupled to a DGPS
Hemisphere Crescent A100 in order to obtain the exact position of the data during the
acquisition phase.
Transmission frequency is 10 kHz, which produces high definition data on the
sediment layers directly under the surface of the seabed. Lower frequencies would
provide greater penetration but lower resolution. With a resolution of 6 cm, the system
can penetrate the bottom by up to 40 metres.
10
As explained above, the performance of the device is directly linked to the nature
of the bottom and its physiognomy. For this reason, our surveying operations were
restricted to times when the water was at least 5 metres deep in order to avoid
irretrievably polluting the data with echo and noise. Likewise, choppy or rough seas
prevent the capture of exploitable data because the transducer cannot be maintained in
a horizontal position.
Figure 6: Stratabox in position and coupled to a DGPS © Adramar.
3. The surveys
The first day of the campaign, 25 February, was taken up with connecting the
geophysical devices and carrying out various tests to ensure that the whole system
worked properly. Work at sea took place on 26 and 27 February 2013 aboard the
Hermine-Bretagne, Adramar’s research vessel.
The objectives were to:
- create a sonar mosaic of the area,
- check and confirm the precise GPS location of the fish-weir,
- generate geophysical data,
11
- determine whether there are features near the weir that are not visible from the air.
a. Side-scan sonar surveys
Prospection began with side-scan sonar surveys which provided us with a
comprehensive view of the zone and enabled us to delimit it correctly and identify any
potential dangers.
The sonar was towed behind the Hermine-Bretagne at an average speed of 2.5
knots and we chose a range of 30 metres with a distance of 15 metres between profiles.
The swath, therefore, amounted to a total of 60 metres. In this fashion, and by
insonifying the zone several times, we overlapped the profiles and this allowed us to
better characterize the points we were looking for.
Figure 7: Routes of the profiles made by side-scan sonar. (Chart SHOM 7130) © Adramar.
The Hermine-Bretagne sailed close to the rocks in order to build as complete a
picture as possible of the zone. In all, 10 profiles were made including 3 perpendiculars
to the weir. The total length of the profiles was 1966 metres.
The headings for the profiles which are perpendicular to the feature were
115°/295°; for the second series of profiles, perpendicular to the first and parallel to the
feature, the headings were 30°/210°.
12
Figure 8: The Hermine-Bretagne manoeuvring close to the Davier Rocks during sonar profiling © Adramar.
b. The sub-bottom profiler
The theoretical profiles were identical for the sonar and the sub-bottom profiler.
Thus, once the side-scan sonar profiles were complete, we proceeded with the surveys
using the sub-bottom profiler and kept to the same heading across the feature and the
same average speed.
The distance between the profiles was initially 15 metres. Then, we carried out
intermediate profiles at a distance of 7.5 metres. The total length of the 19 profiles
amounted to 4,662 metres. Conditions at sea prevented us from making profiles parallel
to the feature.
13
Figure 9: In black, the routes of the profiles made with the sub-bottom profiler; in blue the dive points and the ends of the weir. (Chart SHOM 7130) © Adramar.
4. The results
a. Side-scan sonar
The side-scan sonar fulfilled its role perfectly in the sense that it enabled us to
delimit the study zone by pinpointing the location of the fish-weir between the Davier
Rocks. We observed that most of the zone is covered in sand.
The acoustic image allowed us to determine the existence of a line between the
two Davier Rocks. However the line was quite faint. The low reflectiveness of the
feature, its closeness to its environment, indicated that it was considerably worn down
and not very legible in situ.
The line runs from 48°40.569’ N – 001°59.656’ W to 48°40.600’ N –
001° 59.641’ W and was divided into 11 ‘targets’ which told us its exact position, its
breadth, the altitude of the towfish in relation to the bottom and, in theory, the height of
the components of the feature in relation to the seabed5. Here, however, certain heights
could not be determined by the operator because the acoustic shadows were
insignificant.
5 Cf. Appendix 2, fig. 2, pp. vi and ss., fig. 3, pp xviii and ss.
14
Figure 10: Side-scan sonar profile parallel to the fish-weir © Adramar.
The total length measured between the two targets was approximately 62 metres
and the width of the weir was between 4.5 metres and 5 metres depending on which
‘targets’ were used.
The general data acquired through sonar turned out to be essential for reading
and analysing the sub-bottom profiles that followed.
b. The sub-bottom profiler
The sub-bottom profiler confirmed the presence of rocks and, extremely faintly,
the line of the fish-weir mainly near the Grand Davier, to the south6.
However, it told us nothing about buried depth which seemed to indicate that
there was nothing but sediment under the weir. On the contrary, information provided
by the device showed only structures above the sediment. At first glance this seemed to
indicate that the rocks were those of the feature’s foundations.
Lastly, the surveys using the sub-bottom profiler did not confirm the existence of worn-down structures on either side of the weir.
From a methodological point of view, it is interesting to note that in this
particular case the sub-bottom profiler served to ‘validate’ the data collected during the
sonar profiling. Indeed, as explained above, the information captured by the sub-bottom
profiler on its own would not have been as pertinent and would have been much harder
to analyse without recourse to the preliminary side-scan sonar data.
6 Cf. Annexes 2, fig. 4, p. xxiii.
15
To illustrate this point, you only have to look at the position of the ‘targets’
identified during the processing of the data from the sub-bottom profiler which were
then transferred to the sonar image. Many of them are outside the fish-weir zone proper
and only the combining of the data from the two devices allows us to shed light on the
information obtained during sub-bottom profiling7.
II. Survey dives
1. Methods
a. Determining where to dive
The dives were made possible by determining precisely where to dive after the
side-scan surveys. Two pertinent points were marked with buoys at either end of the
line.
Figure 11: Position of the dive points © Adramar.
b. Diving conditions
The dives were carried out on 28 February 2013 at high and low water. During
the first dive, the depth was 13.5 metres at 09:53 and only 4.6 metres at 15:42.
7 Id., fig. 5, p. xxiv.
16
Visibility was greater at low water. However, despite the improved visibility, the
shallowness of the water prevented us from having a comprehensive view of the site.
Diving later in the year, in May for example, would not have been the ideal
solution either because it is a period when algal bloom is in full swing and the water is
laden with phytoplankton and zooplankton. Also, underwater plant-life is denser and
this makes observation more difficult, even impossible.
2. Underwater observations
The dives confirmed the presence of blocks in the places identified by the
geophysical data. However the poor diving conditions prevented us from observing the
linearity of the feature as it appears on the sonar image. Also, the blocks that were
present did not constitute, in the strictest sense, an architectural structure.
Instead, initial observations provided no information that would allow us to
categorize them as components of a fish-weir without recourse to the results of the
geophysical profiles and, in particular, the side-scan sonar data.
Some blocks were only visible because of the seaweed on or around them. Others
projected out of the bottom no more than 20 or so centimetres. A third group comprised
more impressive items and these were easily distinguished. The dimensions of the
blocks ranged from 40–50 cm to 80–120 cm. They showed no particular architectural
characteristic.
Nevertheless, one worn-down assembly seemed to differ from the other
components of the weir. Additional studies on this assembly would be required to
determine whether it is indeed a sluice.
During the dives we observed, as we staked the bottom, that the layer of sand was
thin (10 or so centimetres) but we were unable to determine whether the stakes came
up against natural rock or blocks that belonged to the structure that had got buried in
the sand. As mentioned above, the sub-bottom profiler did not indicate the presence of
any blocks under the sand nor did it allow us to determine the presence of a rocky
bottom just under the surface. A section of the weir would have to be uncovered if we
are to determine the exact nature of the assembly.
All the photos were taken with the scale placed in the south.
17
Figure 12: Rocks of the feature indicated by the seaweed attached to them. © N. Job/Adramar.
Figure 13: Rocks of the feature showing a special layout (sluice?) © N. Job/Adramar.
18
Conclusion
The operation on the weir between the Davier Rocks was essential in helping us
calibrate our observations both in terms of geophysical data and direct observation.
At the end of this initial phase of the operation we can formulate several
observations relating to method.
Firstly, it is very clear that the devices we used were complementary. Indeed,
because of the geographic position of the survey zone, subject to a great tidal range, the
use of the sub-bottom profiler in support of the side-scan sonar proved to be essential as
it allowed us to infirm or confirm the presence of vestiges buried by natural processes. It
allowed us to compare two sets of data and eliminate irrelevant information.
Secondly, the complementarity of the detection equipment extended to the phase
of direct observation. The limits of the latter, directly linked to weather conditions,
diving conditions and/or the state of conservation of the site, were such that in this
particular case without recourse to the side-scan sonar, the fish-weir, a structure that is
already difficult to observe and characterize by divers, would not have been located so
quickly and with as much certitude if a classic underwater prospection method had been
used. Underwater observation would never have been, in the conditions described
above, the only approach employed because it is so susceptible to the vagaries of nature.
Nevertheless, even if underwater observation is subject to the conditions
mentioned above, it must be remembered that is the only method available to us to
determine the presence or absence of items that geophysical devices cannot perceive.
In addition, the recourse to direct observation allows us to assess the site. In the
particular case of the Davier Rocks, we have to admit that the sonar image gives a
somewhat erroneous view of reality in the sense that it suggests the existence of a better
preserved fish-weir, a feature more readily legible by the diver. Recourse to direct
observation allows us to refine geophysical results.
19
Bibliography
BALLARD R. D., Archaeological Oceanography, Princeton University Press, Princeton and
Oxford, 2008.
BILLARD C. (dir.) Terre de pêcheries 4000 ans d’archéologie et d’histoire sur le littoral de
la Manche, Exhibition catalogue (Musée du Vieux Granville, 22 July – 30 September
2012), Bayeux, Coédition CRéCET de Basse-Normandie, Éditions OREP, Collections Les
Carnets d’Ici, 2012.
DAIRE M.-Y., LANGOUET L., Les pêcheries de Bretagne, Archéologie et Histoire des
pêcheries d’estran, Rennes, Coédition Ce.R.A.A. – A.M.A.R.A.I., Les dossiers du Centre
Régional d’Archéologie d’Alet, AE, 2008.
DAIRE M.-Y., LANGOUET L., Les anciens pièges à poissons des côtes de Bretagne, un
patrimoine au rythme des marées…, Rennes, Coédition Ce.R.A.A. – A.M.A.R.A.I., Les
dossiers du Centre Régional d’Archéologie d’Alet, AG, 2010, p. 20.
20
Table of illustrations Figure 1: Le Petit Davier and Le Grand Davier. Enlargement of marine chart SHOM 7130. _________________ 4
Figure 2: Position of the fish-weir between the Davier Rocks. _______________________________________ 5
Figure 3: Diagram showing the working principle of a side-scan sonar (Ballard, 2008, p. 7). _______________ 7
Figure 4: Front view of the StarFish and its transmission sector. _____________________________________ 7
Figure 5: Diagram of sub-bottom profiling (Ballard, 2008, p. 8). _____________________________________ 9
Figure 6: Stratabox in position and coupled to a DGPS © Adramar. _________________________________ 10
Figure 7: Routes of the profiles made by side-scan sonar. (Chart SHOM 7130) © Adramar. _______________ 11
Figure 8: The Hermine-Bretagne manoeuvring close to the Davier Rocks during sonar profiling © Adramar. _ 12
Figure 9: In black, the routes of the profiles made with the sub-bottom profiler; in blue the dive points and the
ends of the weir. (Chart SHOM 7130) © Adramar. _______________________________________________ 13
Figure 10: Side-scan sonar profile parallel to the fish-weir © Adramar. _______________________________ 14
Figure 11: Position of the dive points © Adramar. _______________________________________________ 15
Figure 12: Rocks of the feature indicated by the seaweed attached to them. © N. Job/Adramar. __________ 17
Figure 13: Rocks of the feature showing a special layout (sluice?) © N. Job/Adramar. ___________________ 17
Documentation Appendix 1: Administrative documentation Figure 1: Chart SHOM 7130 Cartographic document showing position (1/15 000).
Permission granted by the maritime prefect of France’s Atlantic coast No. 2-8420-2013.
Decision of the DRASSM No. 2013 – 07 / OA 2003. Appendix 2: Geophysical documentation (paper + CD) Figure 2: Location of the targets on the feature © Adramar.
Contact inventory of fig. 2
Figure 3: Location of the targets on the feature © Adramar.
Contact inventory of fig. 3
Figure 4: Stratabox profile (27_SBP 10_21_11.seg) illustrating the start of the weir at its
southern end in SBP 15 © Adramar.
Figure 5: Stratabox targets, including SBP 15, on sonar mosaic © Adramar.
Data sheet: Starfish 450F side-scan sonar.
Data sheet: Stratabox sub-bottom profiler.
Appendix 3: Archaeological documentation Photographic documentation (CD).
Appendix1
i
Figure1:ChartSHOM7130.Theredovalmarksthesurveyzone.
ii
PermissiongrantedbythemaritimeprefectofFrance’sAtlanticcoastNo.2‐8420‐2013.
iii
DecisionoftheDRASSMNo.2013–07/OA2003.
iv
DecisionoftheDRASSMNo.2013–07/OA2003.
v
DecisionoftheDRASSMNo.2013–07/OA2003.
Appendix2
Figure2:Locationofthetargetsonthefeature(thePetitDavierisatthetopoftheimage).
vii
Contactinventoryoffig.2.
Name Date 26/02/2013
10:56:36 Time 10:56:36
Survey File Event 0
StarfishLog_20130226_105528_ss1.hs2
X 574046.9
Capture File Y 5391985.3
10-56-36.JPG WGS84 Latitude 48 40.591248 N
WGS84 Longitude 001 59.647597 W
Heading 107.7
Fish Altitude 3.80
Range to Target 12.6
Height Above Bottom 0
Length 0
Width 4.9
Notes Width: 4.9
viii
Name Date 26/02/2013
11:02:05 Time 11:02:05
Survey File Event 0
StarfishLog_20130226_110151_ss1.hs2
X 574050.9
Capture File Y 5391988.1
11-02-05.JPG WGS84 Latitude 48 40.592731 N
WGS84 Longitude 001 59.644307 W
Heading 286.7
Fish Altitude 3.90
Range to Target 21.4
Height Above Bottom 0
Length 5
Width 0
Notes Length: 5.0
ix
Name Date 26/02/2013
11:08:46 Time 11:08:46
Survey File Event 0
StarfishLog_20130226_110743_ss1.hs2
X 574042.4
Capture File Y 5391980.4
11-08-46.JPG WGS84 Latitude 48 40.588636 N
WGS84 Longitude 001 59.651317 W
Heading 115.8
Fish Altitude 3.80
Range to Target 17.2
Height Above Bottom 0
Length 4.5
Width 0
Notes Length: 4.5
x
Name Date 26/02/2013
11:08:52 Time 11:08:52
Survey File Event 0
StarfishLog_20130226_110743_ss1.hs2
X 574037.5
Capture File Y 5391953.2
11-08-52.JPG WGS84 Latitude 48 40.573991 N
WGS84 Longitude 001 59.655602 W
Heading 116.7
Fish Altitude 3.40
Range to Target 9.9
Height Above Bottom 0
Length 5
Width 0
Notes Length: 5.0
xi
Name Date 26/02/2013
11:08:40 Time 11:08:40
Survey File Event 0
StarfishLog_20130226_110743_ss1.hs2
X 574020.8
Capture File Y 5391956.9
11-08-40.JPG WGS84 Latitude 48 40.576107 N
WGS84 Longitude 001 59.669172 W
Heading 118.5
Fish Altitude 4.10
Range to Target 14.6
Height Above Bottom 0
Length 0
Width 0
Notes Fishpot
xii
Name Date 26/02/2013
11:13:50 Time 11:13:50
Survey File Event 0
StarfishLog_20130226_111253_ss1.hs2
X 574038.2
Capture File Y 5391965.1
11-13-50.JPG WGS84 Latitude 48 40.580409 N
WGS84 Longitude 001 59.654904 W
Heading 294.1
Fish Altitude 3.10
Range to Target 21.5
Height Above Bottom 0
Length 0
Width 7.1
Notes Width: 7.1 with fishpot included
xiii
Name Date 26/02/2013
11:13:57 Time 11:13:57
Survey File Event 0
StarfishLog_20130226_111253_ss1.hs2
X 574019.9
Capture File Y 5391958.8
11-13-57.JPG WGS84 Latitude 48 40.577139 N
WGS84 Longitude 001 59.669885 W
Heading 291.4
Fish Altitude 3.30
Range to Target 8.6
Height Above Bottom 0
Length 0
Width 0
Notes Fishpot and embankment included
xiv
Name Date 26/02/2013
12:12:53 Time 12:12:53
Survey File Event 0
StarfishLog_20130226_121126_ss1.hs2
X 574049.2
Capture File Y 5391987.8
12-12-53.JPG WGS84 Latitude 48 40.592581 N
WGS84 Longitude 001 59.645696 W
Heading 203.3
Fish Altitude 5.50
Range to Target 11.2
Height Above Bottom 0
Length 42
Width 4.9
Notes Length: 42.0, Width at middle: 4.9 . Embankment lengthways
Image on following page
xv
xvi
Name Date 26/02/2013
12:20:03 Time 12:20:03
Survey File Event 0
StarfishLog_20130226_121839_ss1.hs2
X 574045.4
Capture File Y 5391979.9
12-20-03.JPG WGS84 Latitude 48 40.588345 N
WGS84 Longitude 001 59.648877 W
Heading 197.6
Fish Altitude 3.10
Range to Target 12.4
Height Above Bottom 0
Length 52.9
Width 4.9
Notes Length: 52.9 Width: 4.9 at middle. Embankment lengthways,
Image on following page
xvii
xviii
Figure3:Locationofthetargetsonthefeature(thePetitDavierisatthetopoftheimage).
xix
Contactinventoryoffig.3.
Name Date 02/26/2013
12:20:09 Time 12:20:09
Survey File Event 0
StarfishLog_20130226_121839_ss1.XTF
X 574043.1
Capture File Y 5391968.5
12-20-09.JPG WGS84 Latitude 48 40.582209 N
WGS84 Longitude 001 59.650874 W
Heading 189.9
Fish Altitude 10.60
Range to Target 10.6
Height Above Bottom 0.3
Length 0
Width 0
Notes Height: 0.3
xx
Name Date 02/26/2013
12:19:57 Time 12:19:57
Survey File Event 0
StarfishLog_20130226_121839_ss1.XTF
X 574050.9
Capture File Y 5391989.0
12-19-57.JPG WGS84 Latitude 48 40.593217 N
WGS84 Longitude 001 59.644298 W
Heading 204.6
Fish Altitude 11.10
Range to Target 10.8
Height Above Bottom 0.3
Length 0
Width 0
Notes Height: 0.1
xxi
Name Date 02/26/2013
12:19:52 Time 12:19:52
Survey File Event 0
StarfishLog_20130226_121839_ss1.XTF
X 574052.2
Capture File Y 5391998.9
12-19-52.JPG WGS84 Latitude 48 40.59855 N
WGS84 Longitude 001 59.643132 W
Heading 207.7
Fish Altitude 15.10
Range to Target 14.5
Height Above Bottom 0.4
Length 0
Width 0
Notes Height: 0.2
xxii
Name Date 02/26/2013
12:19:48 Time 12:19:48
Survey File Event 0
StarfishLog_20130226_121839_ss1.XTF
X 574056.8
Capture File Y 5392003.5
12-19-48.JPG WGS84 Latitude 48 40.601 N
WGS84 Longitude 001 59.639334 W
Heading 209.4
Fish Altitude 13.80
Range to Target 13.8
Height Above Bottom 0.4
Length 0
Width 0
Notes Height: 0.3
xxiii
Figure4:Strataboxprofile(27_SBP10_21_11.seg)illustratingthestartoftheweiratitssouthernendinSBP15.
xxiv
Figure5:Strataboxtargets,includingSBP15,onsonarmosaic.
xxv
Starfish450datasheet
xxvi
Starfish450datasheet
xxvii
Strataboxdatasheet
xxviii
Strataboxdatasheet
Appendix3