The identification of gas hydrate in sediment cores is of great interest for the study of hydrate distribution in sedimentary sequences, as well as for the detection and sampling of individual gas hydrate occurrences after core recovery. Since hydrate is stable only in high-pressure and low-temperature environments, it dissociates rapidly as cores undergo depressurization and heating during wireline recovery. As this dissociation is an endothermic process, sediment containing hydrate is cooled relative to the surrounding sediment, thus creating a negative temperature anomaly. Previously employed methods for the identification of this thermal anomaly included tactile methods and the use of thermistor arrays (Paull, Matsumoto, Wallace, et al., 1996). However, tactile methods are nonquantitative and thermistor arrays do not provide rapid continuous records of entire cores. Therefore, an infrared (IR) thermal imaging camera was introduced during Leg 201 to scan core liners for thermal anomalies immediately after core recovery.
Infrared radiation (~0.750-350 µm) is emitted by all objects as a function of their temperature. As the temperature of an object decreases, the wavelength of maximum emission increases. For hydrate, longwave infrared (8-12 µm) is the focus. The amount of thermal radiation emitted by an object is dependent upon the emissivity and temperature of the object (Stefan-Boltzmann Law). Emissivity is basically an efficiency factor. An object with an emissivity of 1 is a very efficient energy emitter (or absorber) and is known as a blackbody. In addition to emitting, an object can also reflect or transmit infrared radiation. Kirchhoff's Law states that the total infrared radiation leaving the surface of an object is a combination of emitted radiation (from the object itself), reflected radiation (the object reflects infrared radiation from another source), and transmitted radiation (the amount of infrared radiation coming through the object from another source). For example, a perfect infrared mirror would have an emissivity and transmissivity of 0. In contrast, a perfect infrared window would have an emissivity and reflectivity of 0. For examining relative temperatures, it is important to maintain the same values for these characteristics for each analysis. When examining absolute temperatures, determining the emissivity, transmissivity, and reflectivity of an object is critical. Establishing these parameters was the first step in the development of the infrared camera core scan method. The impact of air temperatures and scanning operators was studied to ensure that time of day or slightly different scanning methods did not impact measured core liner temperatures.
The ultimate goal was to establish a procedure with which hydrate could be identified in cores on the catwalk as quickly and reliably as possible. Toward that end, results from nonhydrate-bearing cores and hydrate bearing cores are presented.