the selected area. Unfortunately 100 per cent is difficult

to define in this context due to the varying degrees of

efficiency for different techniques. For instance an exten-

sive visual search may cover every square centimetre of

the sea-bed but miss an object a few millimetres long, or

a larger object camouflaged by a temporary dusting of light

silt. Similarly, with remote-sensing techniques, magne-

tometer search-corridors separated by 50 m (163 ft)

might be sufficient to detect a large wooden wreck con-

taining iron cannon, but might not detect other smaller

targets of archaeological significance, such as a single

cannon.

It is important to realize that the fact that nothing has

been detected during a search does not necessarily mean

that nothing is there. The development of increasingly

sophisticated remote-sensing equipment (see chapter 13)

has made investigating the sea-bed a more reliable science,

but there is still some way to go before it is possible to

be certain of discovering all the available evidence.

Safety

Whichever search method is chosen, it is essential that team

members are trained to undertake the technique safely.

(Refer to chapter 6 for further information concerning

safety on archaeological sites.) Many of the inherent

problems associated with the following techniques, how-

ever, can be alleviated with planning and practice. This

S

earches can be divided into two types: those deploy-

ing a diver or a submersible and relying on the

human eye or hand-held equipment, and remote-

sensing surveys usually employing acoustic or magnetic

equipment and remotely operated vehicles (ROVs)

deployed from a boat, or other craft, on or above the

surface. This chapter covers the first type of search. For

detailed information about remote-sensing techniques, refer

to chapter 13.

Positioning

Whatever search method is used, it is vital to know where

you are and where you have been (see chapter 11 for posi-

tion-fixing techniques). This will save time in search

operations and will significantly enhance the information

recorded. A search only has value if the position of the

area covered and the identified targets, together with

other pertinent observations, are accurately reported and

recorded. Divers should complete detailed dive-logs,

recording all information of potential interest for subse-

quent analysis.

Coverage

Unless a well thought-out sampling strategy has been

devised, a common objective is 100 per cent coverage of

Contents

u Underwater search methods

u Positioning

u Coverage

u Safety

u Diver search methods

12 Underwater Search Methods

Underwater Archaeology: The NAS Guide to Principles and Practice Second Edition Edited by Amanda Bowens

© 2009 Nautical Archaeological Society ISBN: 978-1-405-17592-0