Compact high-resolution temperature loggers for measuring the thermal gradients of marine sediments

Abstract

Currently, two major types of ship-operated underwater instruments are used for heat flow measurements including a Lister-type heat probe (LTHP) and small temperature loggers (STLs) that are attached to a solid-steel lance or a core barrel. In both operations, penetration friction heat introduces a transient disturbance to the temperature of the surrounding sediments. A pragmatic approach is to extrapolate a cylindrical temperature decay function to estimate the equilibrium ambient temperature (EAT) and equilibrium ambient temperature gradient (EATG) of the sediments from short temperature recordings. The extrapolated EAT and EATG will greatly affect the estimate of the base of the gas hydrate stability zone (BGHS). In order to achieve a better extrapolation of EAT, EATG, and 1-s temperature sampling requirements, compact (22.2 cm × 2.2 cm) high-resolution temperature loggers (CHTLs) have been designed to work with a sediment core barrel. The mechanical and electronic design of the CHTL is detailed in the text. With a 24-bit, low noise A/D converter embedded in the mix-signal microprocessor, including a highly stable reference resistor based ratiometric scheme, the CHTL is capable of resolving 0.1 m°C in the range of −1 to 25°C. It has a memory capacity of 4 Mbyte which can work continually up to 16 days with a 1-s sampling interval. From a data processing efficiency consideration, field experiments indicate that adopting short support fins to attach the CHTLs to a relatively small size core barrel is better than using high support fins. A similar approach such as extrapolating the cylindrical temperature decay function to estimate EAT, EATG can be obtained from regressing a direct calculated temperature gradient of short recorded data. The resulting EATG accuracy may be significantly improved through the application of the proposed correction formula and therefore is much better than that which is directly calculated from the extrapolated EATs.