Page 185 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

164 Maritime Archaeology: A Technical Handbook, Second Edition
When working in muddy or sandy situations on a wreck site where the archaeological remains are likely to be deeply buried, some form of probe survey can be advantageous. Provided the probe is used sensibly, the extent of the buried site can be determined, and this can be very important in plan- ning an excavation. The visible site may only be 10% of that buried, for example, which will have major implications on the planning of the exca- vation. Additionally, with careful probing it may be possible to determine the location of the bottom of the hull of a site, thus giving an idea of the depth of material on the site. Used sensibly, probing is unlikely to damage anything except the most fragile material and potential harm can be kept to the absolute minimum by not probing too violently.
IV. GROUND-PENETRATING RADAR
Ground-penetrating radar (GPR) is generally applied on land for detect- ing subsurface anomalies. This system uses pulses of ultra-high frequency radio waves (microwaves) that are projected into the ground through an antenna. The transmitted energy is reflected from various objects depend- ing on their conductivity and received by the antenna. Recently an ultra- wideband (UWB) GPR has been developed. This is basically an advanced form of GPR that operates by producing billions of short-duration, low- powered radio frequency pulses. The GPR consists mainly of a PC for storing and viewing the present measurement and a dipole antenna unit with send and receive antenna. The system sends short pulses in the range of nanoseconds and uses a synthetic sampling on receive. In a post- processing step it is possible to view any slice in the three-dimensional data volume. As the signal analysis is done in a near sensor fashion, it includes the physical properties of electromagnetic waves, especially polarization effects and resonance effects.
GPR has been used in a wide range of archaeological situations, however, because the high-frequency radiation cannot penetrate seawater, it is not applicable under water or where material lies inundated in a sea- water environment.