geophysical prospection and survey dives on the fish‐weir

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:

- [email protected]

- [email protected]

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)

mailto:[email protected]

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


v


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


11:02:05 Time 11:02:05

Survey File Event 0


X 574050.9




Heading 286.7

Fish Altitude 3.90



Length 5

Width 0

Notes Length: 5.0

ix


11:08:46 Time 11:08:46

Survey File Event 0


X 574042.4




Heading 115.8

Fish Altitude 3.80



Length 4.5

Width 0

Notes Length: 4.5

x


11:08:52 Time 11:08:52

Survey File Event 0


X 574037.5




Heading 116.7

Fish Altitude 3.40

Range to Target 9.9


Length 5

Width 0

Notes Length: 5.0

xi


11:08:40 Time 11:08:40

Survey File Event 0


X 574020.8




Heading 118.5

Fish Altitude 4.10



Length 0

Width 0

Notes Fishpot

xii


11:13:50 Time 11:13:50

Survey File Event 0


X 574038.2




Heading 294.1

Fish Altitude 3.10



Length 0

Width 7.1

Notes Width: 7.1 with fishpot included

xiii


11:13:57 Time 11:13:57

Survey File Event 0


X 574019.9




Heading 291.4

Fish Altitude 3.30

Range to Target 8.6


Length 0

Width 0

Notes Fishpot and embankment included

xiv


12:12:53 Time 12:12:53

Survey File Event 0


X 574049.2




Heading 203.3

Fish Altitude 5.50



Length 42

Width 4.9

Notes Length: 42.0, Width at middle: 4.9 . Embankment lengthways

Image on following page

xvi


12:20:03 Time 12:20:03

Survey File Event 0


X 574045.4




Heading 197.6

Fish Altitude 3.10



Length 52.9

Width 4.9

Notes Length: 52.9 Width: 4.9 at middle. Embankment lengthways,

Image on following page

xviii

Figure3:Locationofthetargetsonthefeature(thePetitDavierisatthetopoftheimage).

xix

Contactinventoryoffig.3.


12:20:09 Time 12:20:09

Survey File Event 0

StarfishLog_20130226_121839_ss1.XTF

X 574043.1




Heading 189.9

Fish Altitude 10.60


Height Above Bottom 0.3

Length 0

Width 0

Notes Height: 0.3

xx


12:19:57 Time 12:19:57

Survey File Event 0


X 574050.9




Heading 204.6

Fish Altitude 11.10



Length 0

Width 0

Notes Height: 0.1

xxi


12:19:52 Time 12:19:52

Survey File Event 0


X 574052.2




Heading 207.7

Fish Altitude 15.10



Length 0

Width 0

Notes Height: 0.2

xxii


12:19:48 Time 12:19:48

Survey File Event 0


X 574056.8




Heading 209.4

Fish Altitude 13.80



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

geophysical prospection and survey dives on the fish‐weir

Documents