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From: Ole Gron
Subject: Survey Techniques
Date: 02/16
Time: 05:59 PM
Date: Fri, 16 Feb 1996 21:23:39 PST
From: Ole Gron [ccc7466@vi*.cy*.dk*]
Sender: Underwater Archaeology Discussion List
[Underwater Archaeology Discussion List]
Reply-To: ccc7466@vi*.cy*.dk* [ccc7466@vi*.cy*.dk*]
Subject: Survey Techniques
Modern Archaeological Survey Techniques
Summary of a meeting held at the
Centre for Maritime Archaeology
National Museum of Denmark
March 20, 1995
Organizer: Ole Gron
Three subjects were highlighted:
A) Acoustic systems
B) The use of ROVs
C) Radar techniques.
1. Introduction by Ole Gron.
The basic intention of the meeting was to create a forum for debate
and
exchange of experiences between archaeological users, and the
scientists and
engineers. It was proposed that meetings should last no longer than
one day
because of the problem of finding time to attend longer conferences.
It was
also suggested that meetings should be held every second year, and
that it is
important to have a good meal in the evening since this is where
contacts are
made. The initial meeting was restricted to participants from North
Western
Europe but future meetings may be extended to cover a wider
geographic area.
It was suggested that a group of 4 - 5 technicians and
archaeologists should
collect information about archaeological survey equipment and methods.
The
group would study availability, cost, and performance, and then
communicate
the results to all the other participants. This arrangement could be
exploited to improve the chances of access to new equipment so that
it can be
tested and evaluated for archaeological purposes on behalf of the
supporting
institutions.
2. An evaluation of different types of acoustic penetration equipment
for
archaeological survey by Ole Gron.
My job is at the moment to develop archaeological survey strategies
for the
prehistoric and present-day coastal zone of Denmark. Due to the fact
that
most of the interesting archaeological structures in this area are
located in
extremely shallow water, it is logical to base such a strategy on
small
boats. Ideally these should be easily transported on a trailer and
have an
enclosed cabin to help protect electronic equipment. Precise
navigation is
necessary to be able to relocate anomalies recorded during survey.
This is
obtained by using DGPS satellite-navigation which allows us to
operate with a
precision of about 11m. A basic problem we have is how to sail slowly
enough
to obtain good horizontal resolution. The optimum speed is a few
metres/minute but, at such slow speeds, most small boats are very
difficult
to control.
A basic rule of acoustic penetration is that the higher the frequency,
the
better the resolution and the worse the penetration, whereas the
lower the
frequency, the worse the resolution but the better the penetration.
For
archaeological survey with single frequency equipment a normal
compromise is
about 30 kHz.
The strategy chosen has been to start the seismic survey on known
and
well-recorded artificial structures and cultural layers, and to
observe how
they are represented on the various types of equipment. In Haderslev
Fjord
there are two well known defensive blockades, 'F Lei' embedded in a
soft
bottom and 'Magrethes Bro' in a more sandy bottom.
A CHIRP II system, sweeping from 2-23 kHz to produce an optimal
relation
between penetration and resolution, (price 60-70.000$) gave a very
clear
picture of 'F Lei', whereas 'Magrethes Bro' could not be seen clearly.
A
problem was that the boat's speed was too high due to a large motor
being
used. A similar CHIRP system was used to follow what seems to be the
continuation of a known culture layer 1.5m below the top of the
bottom at the
partly excavated old stone age site Blak in Roskilde Fjord.
Experiments have
shown that this system can be operated in only 30-40 cm of water with
good
penetration of sediments.
A 30 kHz Knudsen sediment-echosounder (price 20.000$) produced a
reasonably
good picture of 'F Lei' but was not able to show 'Magrethes Bro'.
A 30 kHz Elac sediment-echosounder (price 30.000$), produced a
reasonably
good picture of 'F Lei' whereas 'Magrethes Bro' could not be seen
clearly. It
produced a lot of multiple images which in some places made the
results
difficult to interpret. However the Elac echosounder was used
successfully to
distinguish structures looking like the single posts of a known
Viking Age
bridge in Ravinge Enge. These were located in the bottom of an
artificial
lake at an operating speed of 2m/minute. An unknown structure
parallel to the
bridge was also observed. Unfortunately it has not yet been possible
to test
other systems here for comparison. At the Blak locality in Roskilde
Fjord,
where the CHIRP system achieved good penetration, it proved
impossible to
penetrate the sediments with the Elac system. This was probably due
to the
rather sandy sediments.
A standard 28 kHz Furuno echosounder (price 7.000$), was used to test
the
difference between this and the specialized penetration-systems
available. 'F
Lei' could be distinguished but not with a resolution sufficient for
professional work. The surveys of 'F Lei' showed that 2 people
working on a
small boat were able to record the outline of the structure in 4
hours and
reveal details not observable by divers.
A lightweight 370 kHz Imaginex sidescan (price 20.000$) was used in
Haderslev
Fjord to study wooden constructions visible on the bottom. This was
especially successful with 'F Lei' where a series of horizontal logs
were
exposed. The equipment was also successfully used to record the
trunks and
stumps of trees from submerged Stone Age forests.
3. Archaeological survey in the Eckernfvrde by Svnke Hartz, Harald
L|bke and
Gerd Hoffmann.
The project was started in 1994 as a collaboration between the
Institute for
Marine Geology (GEOMAR), Kiel, the Archaeological Museum
(Landesmuseum) in
Kiel, and the Institute for Electronics, Rostock. The aim is to
distinguish
anthropogenic sediments from the natural ones in the Holocene
deposits from
the last 7000 years. The research area is the western part of the
Kieler Bay
and the project mainly deals with the Stone Age Cultures in this area.
These
studies were supplemented by limited investigations in the hinterland
to
record the pattern of presence and absence of a human population
during the
same period.
In the marine areas the following techniques were used:
1) Sidescan
2) Sediment-echosounder (or a boomer in areas of deeper
sediment)
3) Analysis of cores combined with X-ray analysis
4) Pollen analysis
5) Dinoflagellats (as an indicator of salinity from North Sea
influences
in the bay environment)
The project also involved survey in the coastal areas of Schleswig-
Holstein
for submerged settlements from the Ertebolle Culture. Systematic
excavation
has until now only been carried out on one site, but several others
have been
located in this area by dredgers.
In the future it is planned to combine excavations of Ertebolle sites
that
contain preserved culture layers with detailed geological analysis of
cores
and peat layers. In this way it should be possible to reconstructthe
prehistoric landscape in the rese arch area.
4. The development of an acoustic penetration device by Wolf Dieter
Heinitz.
The University of Rostok has been working with acoustics in sediments
under
water since 1970. In connection with a climate research programme
investigations have been carried out in the North Sea, the Baltic and
in a
number of deep sea basins of relevance for the research programme.
In collaboration with Dr. Willi Kramer from Landesamt f|r Vor- und
Fruhgeschichte, Schleswig, the University of Rostock has been
involved in
archaeological surveys with the echosounder-system SEL-93 developed
in
Rostock. Its frequencies are 5, 10 and 20 kHz. Originally it was
constructed
for work in deeper waters but at the moment it is being
modified for shallow water operations . Experience has shown that in
hot
summer weather the development of gases in the sediments can
seriously reduce
the effectiveness of the instrument. However with this equipment it
proved
possible to locate a wreck covered by sediments and not detectable
using
sidescan. In the Schlei has it has also been possible to
locate a submerged prehistoric island covered by sediments.
5. Postprocessing of sidescan data by Henning Christoffersen.
This is a Windows-based system for digital postprocessing of sidescan
data.
Using this data in combination with precise navigation (DGPS) it is
possible
to produce high resolution maps of the seabed. It can also be used to
make
photographic images of wrecks and other structures or to produce
detailed
information about the character of the seabed including particle size,
vegetation etc.
The program has an automatic facility for distinguishing between
objects in
the area. This application has been developed for the military to
help
distinguish between mines and stones, but the facility may also be
of
interest to archaeologists.
To obtain good sonar images, especially from digital systems, it is
an
advantage to use heavy sidescan fish because they are more stable in
the
water and do not pitch.
6. The use of sidescan and ROV's for archaeological survey by Bert
Westenberg.
In recent years archaeological survey has been carried out in Sweden
by a
group organized by The Maritime Museum in Stockholm. In the past the
focus
was on coastal areas, whereas this year the investigations will
concentrate
on the big lake, Vdneren. In the period 1970-1980 we used a Klein
sidescan
with 100 and 500 kHz, but now we use an EG&G system combined with
DGPS-positioning.
A number of organisations collaborated to buy an ROV. The Technical
highschools in Gothenburg and Stockholm, the Board of West Coast
Fisheries,
and the Maritime Museum in Stockholm are now jointly involved in the
'Sea Owl
Project'. The ROV can be positioned at all angles and attitudes and
therefore
is a good camera platform for making photo mosaics, but the vehicle
can be
equipped in numerous other ways. With the CAT-option, a sonar
tracking
device, the exact position of the vehicle can be controlled so that
its grid
co-ordinate within an area can be defined
7. The use of ROV's by Dag Nfvestad.
Archaeological survey had to be undertaken in connection with the
construction of a bridge over Oslo Fjord, where the water is up to 70
m deep
and currents can reach 4-5 knots. In co-operation with the Swedish
consortium, a survey was undertaken using their 'Sea Owl' equipped
with a
Mesotech sonar and the CAT system for positioning. Compared to
traditional surveys based on visual inspection by divers this system
was
cheaper and faster.
8) Survey by ROV's by Paal Nymoen, Svein Kristiansen and Frederik
Lide
On two sites have underwater-archaeological surveys been carried out
as a
collaboration between The Departments of Marine Technology and
Archaeology at
the University of Trondheim.
The one site is a Medieval harbour at the mouth of Trondheim Fjord,
the other
a russian wreck located partly on shallow water and partly 300 m deep.
In 1994 the ROV was equipped with a Mesotech sonar for investigation
of
anomalies that might be parts of the russian wreck. The result
indicates that
this is a quite successful approach.
A HYBALL ROV has been bought for future investigations and has been
equipped
with cameras and sonars. A frame built ROV is being developed
specially for
marine archaeological purposes.
9 Marius, an autonomous underwater vehicle by Anders Bjerrum.
This vehicle was developed in 1991-1994 as part of the MAST I
programme of
the European Community. The vehicle is equipped with its own
batteries and
on-line video. It can operate as an autonomous unit navigating and
avoiding
obstacles by a RESON multibeam sonar, and other navigation systems
can be
applied. It can also be operated by cable from a ship or
by acoustic communication from a ship or a seabed station. Almost any
kind of
application can be mounted on the vehicle.
10. The technology employed on the 'Kronan' project by Lars Einarsson.
Sidescan and ROV's have been used in the investigation of the wreck
of the
'Kronan' located near Oland, Sweden. Future plans involve survey
using this
equipment in other locations along the Swedish coast. The Klein
Sidescanner
operates at 100 and 500 kHz and cost 75.000$, in future it is planned
to
connect it to DGPS and a Track-point II system for precise
location of the data recovered.
The ROV is a 'PHANTOM 300' costing 75.000$ with video, camera and
monitors
included. The advantage of this model is the low weight of 25 kg and
the low
price of spare parts. The disadvantage is that it is as sensitive to
currents
as most other ROV's.
The experience has shown that the use of sidescan and ROV has reduced
the
diving time necessary. By improving our knowledge of an area before
excavation starts, sampling and diving can be better planned.
11. Experiences with DGPS positioning by Martin Dean.
The Archaeological Diving Unit's 'ROV' is a diver supplied with air
from the
surface and wearing a KM 27b helmet equipped with a Hi-8 video camera.
The
clear voice communication and broadcast quality video allow for the
collection of visual data, together with the ability for the diver to
discuss
specific details with other archaeologists on the surface. The
ADU is soon to experiment with a variety of software to try to
capture video
images and then take survey measurements from them.
One of the ADU's tasks is to check the position of archaeological
sites for
legal protection. Until recently horizontal sextant angles were used
to plot
positions but over the last few years we have been using Navstar GPS
system
(2800$) which displays the position error calculated to ARMS.
As a stand-alone unit errors of up to 100m can be regularly recorded,
but
with our own base station and radio modem link (another 2800$ +
4900$) we
have been able to reduced the error to about 5m. Unfortunately due
to
restrictions on the power of the radio link, the base station has to
be set
up within 3 km of the working area.
Since 1993 the ADU has used a commercially available differential
GPS
correction signal based on selected lighthouses in British waters.
The
Scorpio DGPS system requires a decoding receiver (800$) and a chip
to
unscramble the signal (1500$ for three year s). This gives an
accuracy down
to 4m although when working off areas like the west coast of
Scotland, both satellite and the differential correction signals can
be
masked by the mountainous topography.
12. Radar detection of sub-surface structures from planes and
satellites by
Preben Gudmandsen.
The title of this paper has been changed into 'What do we believe we
can do
with radar observations'.
Until now no archaeological structures have been located by this
technology.
A picture of the Gudme area made by airborne 5.3 gHz radar shows that
it is
difficult to penetrate the surface. This is because of vegetation and
the
humidity of the soil which gives a high loss within the upper few
centimetres
penetrated. Some years ago surveys were made on the Greenland ice-cap
with
60-300 mHz equipment. Because the signal loss is limited in ice, it
was
possible to measure its thickness in this way.
A summary of the technology behind the penetration radar systems and
the
problems related to their applications was given. Since radar waves
cannot
penetrate water even with a very restricted salinity (except under
very
special circumstances) radar will not be suited for marine surveys.
13. The use of radar survey in archaeology by Finn Ole Nielsen.
The title of this presentation has been changed to 'What might a
radar
discover'.
High quality conventional air-photos are available from many areas,
but where
there is a high clay content in the soil the chances of detecting
prehistoric
structures are minimal, whereas on more sandy soils prehistoric sites
can
often be observed because of differces in the crop growth between
the
disturbed and undisturbed areas.
Since penetration of clay is also a problem to radar, we cannot
expect this
technology to broaden the area where observations can be made.
Possibly radar
technology will make a more detailed observation of differences in
the crops
possible.
14. The use of georadar (Ground Penetration Radar) in archaeology by
Frank
Andreasen.
Georadars can be pulled by car or carried by hand depending on how
much power
they use and consequently how heavy they are.
A number of profiles made by georadar were shown, with the deepest
penetration being 30m. Sand is the best material for penetration
whereas clay
is the worst. Due to the potential for georadar-work a sandy area in
north-western Jutland was chosen to reconstruct the development of a
landscape through the last 5000 years. The formation of the present
landscape
involves tectonics as well as aeolian and fluvial erosion and
sedimentation
of material.
Scientists have for some time discussed whether the 'Sloj'-channel in
the
Viking Age could have connected the Aggersborg fortification directly
with
the North Sea. Several profiles proved the existence of such a
structure
filled by aeolean sand. A date for the time that the channel was
closed can
hopefully be obtained through the planned boring programme
in the area.
In archaeology georadars can be used: for mapping geological
phenomena in
areas dominated by sand, and thus in reconstruction of the
palaeogeographical
conditions and the living conditions (problems with sand dunes
covering
fields, locating possible sailing routes and farming areas etc.) for
making
non-destructive sections through linear features such as defence
systems,
dykes, channels, moats, roads and plough tracks below the dunes. to
estimate
how far it is reasonable to extrapolate data from point sources (e.g.
borings).
15. New survey methods - new fields of archaeology by Willi Kramer.
Survey with a low frequency sidescan (100 kHz) and a short range DGPS
system
has been carried out over the last three years. An eighth-century
defensive
blockade of log built wooden boxes was used for the testing of new
technical
methods such as the SEL- 93 sediment echosounder from Rostock
University (see
contribution 4). Investigations by divers were also carried out.
Sidescan is regarded as promising for locating wrecks in deeper
waters, but
penetrating echosounders will be more important for locating
settlements
related to 'fossil' coastlines.
16. Discussion
The discussion demonstrated that there was great interest in having
access to
modern survey equipment. Part of the discussion focused on prices and
how to
reduce costs. It was suggested that a group should be formed to
organise
testing of equipment, and this was positively supported by the
meeting. It
was also proposed that there should be exchange of equipment
between the institutions involved in underwater archaeology, together
with
communication of experiences and results. The idea of further one-
day
meetings was also positively supported.
Ole Gron, Research Associate
Centre for Maritime Archaeology
National Museum of Denmark
Havnevej 7, 4000 Roskilde
Denmark
phone: +45 4632 1600
fax: +45 4632 2477
e-mail: nmf-og@sk*.na*.mi*.dk*
home phone/fax: +45 6221 4046
home e-mail: ccc7644@vi*.cy*.dk*
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From: Ole Gron <ccc7466@vi*.cy*.dk*>
Subject: Survey Techniques
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