Thermophysical properties of the Earth's crust: In situ measurements from continental and ocean drilling

Abstract

Knowledge of the variation of thermal conductivity and heat production with depth is critical to understanding the thermal state of the crust. Continental and oceanic drill holes provide unique opportunities for the calibration of empirical and theoretical models relating crustal thermal properties to physical and chemical properties. These calibrations are based on more than 30,000 well log measurements of acoustic velocity (both compressional and shear), density, porosity, radioelement concentrations (K, U, Th), and major element chemistry (Al, Si, Ca, Fe, S, Mg + Na), in formations ranging in age from 5 to 620 Ma and in composition from granitic to gabbroic. The mineralogical data available from induced gamma ray spectroscopy logs, when combined with known mineral thermal conductivities in a model for the thermal conductivity of a crystalline aggregate, enable the in situ application of a laboratory estimation technique and yield continuous conductivity profiles. However, mineralogy‐based conductivity models cannot account for the anisotropic conductivity observed in foliated metamorphic rocks. A phonon conduction model, which utilizes acoustic velocity and bulk density measurements, is accurate within ±15% in both isotropic and anisotropic formations of the upper continental crust and in oceanic crustal layers 2C and 3. In oceanic crustal layers 2A and 2B, however, the effects of fracturing on compressional and shear velocities lead to inaccurate results. The heat flow‐heat production relationship, which provides one method of determining subsurface heat production, has well‐documented limitations. Allis (1979) and Rybach and Buntebarth (1982, 1984), on the basis of laboratory measurements, have proposed quantitative relationships between heat production and rock compositional and physical properties. The well log K, U, and Th data indicate that heat production can be predicted from compositional data within a factor of 2 or 3. Predictions of heat production from acoustic velocity and bulk density measurements are only accurate to within an order of magnitude. Three factors account for these results: (1) the natural variability of velocity and density for a given rock composition, (2) the tendency of U and Th to associate with secondary rather than primary minerals, and (3) U and Th redistribution through fluid flow and metamorphism. Thus seismic velocity based heat production profiles can, at best, be taken as first‐order estimates which may misrepresent the complexity of the continental crust. However, the correlation of formations of known heat production with subsurface structures, when combined with a suite of existing techniques for estimating heat production, may lead to more accurate results.