In Situ Measurement of Temperature
Temperature is a major parameter controlling dynamic Earth processes. Borehole temperature measurements are important for understanding heat transfer from Earth’s interior, lithospheric evolution, hotspot volcanism, gas hydrate stability, and fluid flow in marine sediments. Consequently, temperature was one of the initial downhole properties measured during DSDP (Von Herzen and Maxwell, 1964).Throughout DSDR ODP, and IODR new tools and analysis approaches have continuously been developed and improved (e.g., Uyeda and Horai, 1982; Horai, 1985; Fisher and Becker, 1993; Davis et al., 1997; Heeseman et al., 2006). A 2004 IODP workshop on downhole tools confirmed that precise downhole temperature measurements were critical to fulfillment of programmatic objectives in all primary research themes (Flemings et al., 2004).
The most efficient tool for measuring temperature in boreholes is the advanced piston corer temperature tool (APCT), which measures sediment temperatures as the core is being taken (Horai and Von Herzen, 1985). The APCT allows for the measurement of in situ temperatures in the undisturbed sediments that have not yet been reached by the drill bit. The APCT has undergone two major upgrades to improve sensor and data sampling accuracy and stability while retaining the same efficient physical format (Heeseman et al., 2006). For deeper sediments that are too stiff to be sampled with the APCT, the Davis-Villinger temperature tool (DVTP) was developed (Davis et al., 1997). The DVTP also measures in situ pore pressure, although obtaining reliable pressure measurements has been challenging because of the long time constant of the pressure response and fractures induced in the sediment when the probe is inserted (Villinger et al., 2010).
compiled into a regional summary (Figure 3.1), which illustrates that shallow basement permeabilities are consistently three to seven orders of magnitude higher than the overlying sediment column and supports the early observations of a stratified permeability structure controlled by depth within the basement (Fisher, 2005; Becker and Fisher, 2008). The widespread nature of large-scale basement fluid circulation has profound implications for the formation and continuation of subseafloor microbial communities, the creation of ore deposits and gas hydrates, and the overall chemical and heat budget of the oceans.
In some cases, borehole temperature measurements indicated down- or uphole fluid exchange between the ocean and basement formations that could be used to estimate formation permeability (Figure 3.1). However, such open flow also represented a perturbation to in situ conditions and revealed the need for tools with long-term in situ monitoring capabilities. The development of the CORK (Circulation Obviation Retrofit Kit) has led to widespread use for long-term measurement of temperatures, pressures, and fluid fluxes (Box 3.2). The first long-term observatories were established on the Juan de Fuca Ridge (ODP Leg 139). Pressure records from these observatories after 14 months showed high lateral fluid fluxes and short residence times in very permeable upper basement (Davis and Becker, 2002; Fisher, 2005). The first cross-hole experiment (ODP Leg 168), and the first three-dimensional CORK array (IDOP Leg 301), also along the Juan de Fuca Ridge, continued to add to the picture of large lateral fluid fluxes and high permeabilities, and recorded transient flow events associated with seismic activity and tides (e.g., Fisher et al., 2008).
FIGURE 3.1 Summary of borehole permeability determinations in oceanic basement rocks, based on packer and temperature (flow-meter) experiments. Vertical axis is depth into basement, accounting for differences in sediment thickness. Most seafloor measurements have been made in basaltic crust, but two sets of data (ODP Holes 857D and 735B) are from sediment/sill and gabbroic lithologies, respectively. Note range of values and relatively consistent depth trends. SOURCE: Fisher, 2005.