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In recent years there have been enormous advances in nautical archaeology through developments
in SONAR technologies as well as in manned and robotic submersible vehicles. The number of
sunken vessel discoveries has escalated in many of the seas of the world in response to the
widespread application of these and other new tools. Customarily, surviving artifacts within the
debris field of a wreck are collected and then moved to laboratories, centers, or institutions for
analyses and possible conservation. Frequently, the conservation phase involves chemical
treatments to stabilize an artefact to standard temperature, pressure, and humidity instead of an
undersea environment. Many of the artefacts encountered at an underwater site are now
characterized and restored in-situ in accordance with modern trends in art conservation. Two
examples of this trend are exemplified by the resting place of the wreck of the Titanic in the
Atlantic and the Cancun Underwater Park in the Caribbean Sea. These two debris fields have been
turned into museums for diving visitors.
Several research groups have investigated the possibility of adapting the well-established analytical
tool Laser Induced Breakdown Spectroscopy (LIBS) to in-situ elemental analyses of underwater
cultural, historic, and archaeological artefacts where discovered, rather than as a phase of a salvage
operation. As the underwater laser ablation associated with LIBS generates a “snowplough”
shockwave within the aqueous matrix, the atomic emission spectrum is usually severely attenuated
in escaping from the target. Consequently, probative experiments to date generally invoke a
submerged air chamber or air jet to isolate water from the interaction zone as well as employ more
complex double-pulse lasers. These measures impose severe logistical constraints on the
examination of widely dispersed underwater artefacts. In order to overcome this constraint we
report on water-immersion LIBS experiments performed with oblique laser irradiation and spectral
detection at the complementary angle so as to view emission from behind the shockwave. Targets
of silver, gold, and copper have been studied. It is found that this approach enables LIBS detection
in water both in emission and in absorption. It appears that underwater inverse LIBS may be
especially useful in underwater archaeology.
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