Heat flow fundamentals/Basic physics This session introduces the idea that almost all geological phenomena is driven by thermal gradients and discusses the three mechanisms of heat transport: diffusion, convection, and radiation. The timescales of different modes of cooling of the primary Earth systems will be discussed. Coupling between these systems will be briefly analyzed with particular reference to superficial thermal perturbations caused by climatic phenomena.
Thermo-physical properties In this session we will investigate the intrinsic thermo-physical properties of rocks, minerals and fluids that influence the steady-state and transient flow of heat in the Earth. We will look at the definitions, impacts, and inter-relationships between different properties. The dominant properties controlling heat conduction are thermal conductivity, thermal diffusivity, density, and specific heat capacity. Hydraulic conductivity, fluid viscosity and the coefficient of thermal expansion influence advective heat transport. Internal heat generation, latent heat of fusion, latent heat of vaporization, exothermic and endothermic reactions can also affect the overall balance and movement of heat through the Earth’s crust over geological time. While many of these properties are scalar, some are better defined as tensors with inherent possibility of anisotropy.
Temperature measurements This session introduces the different temperature sensors used for geothermal measurements (such as mercury and metallic expansion thermometers, Pt resistances, thermistors, thermocouples, and fiber-optic cables), their accuracy and the calibration procedures. The different survey techniques are presented with particular reference to determining the temperature gradient in continental settings (stop-start precision logging, continuous logging, fiber optic monitoring). Moreover, we will briefly review some indirect temperature indicators (groundwater geothermometers, xenolith data, seismic constraints on temperatures like the Curie point depth).
Thermal conductivity measurements In this session we will examine the challenges, strategies, and instruments for measuring the thermal conductivity of rocks—the dominant physical property value required for crustal heat flow determinations. Thermal conductivity controls the rate of conductive heat flow under the influence of an imposed temperature gradient. Challenges and strategies related to obtaining and preparing samples for measurement; ensuring that samples represent the true in situ properties of the rock; and identifying and quantifying anisotropy. The main recommended instruments for measuring the thermal conductivity of rocks can be divided into those that employ the ‘optical scanning technique’, the ‘divided bar technique’, and the ‘line-source technique.’
Error and uncertainty estimation In this session, we will address the problem of the error in thermal measurements and how they affect the uncertainty in heat flow determinations. If the measured temperature is that of the virgin rock and the accuracy of depth determination is warranted, the terrestrial heat flow value depends not only on errors in the thermal gradient and the rock’s thermal properties but also on the specific method chosen to combine the different physical parameters. The interval method and the Bullard plot are classical approaches to determine the heat flow. Uncertainty in temperature measurements can derive from inherent biases or random errors. Thermal conductivity values can be biased by poor porosity estimates. In deep boreholes, radiogenic heat generation can produce observable effects on the inferred heat flow.
Marine heat flow measurements In this session we will explore how the study of the Earth's temperature field has evolved to include marine heat flow measurements. Even though temperature-depth data have been acquired around the world since the 1930's, the distribution of observations remained uneven with continental data mainly reflecting the locations of boreholes associated with exploitation-related activities. In contrast marine heat flow probes constructed in the 60's were meant for rapid observations at sea allowing heat flow data to be collected in major offshore surveys. This session includes a review on the different instruments for marine heat flow data, data processing and the key corrections used when dealing with marine heat flow data.
Transient disturbances to steady state heat flow In this session we will examine the differences between transient and steady state heat transfer. In steady-state heat transfer, the temperature is constant throughout time. In transient heat transfer, the temperature changes with time. But if there is an abrupt change in temperature or disturbance, an equilibrium temperature or steady-state will be attained after some period of time. This phenomenon is known as unsteady or transient heat transfer. This session will include a revision of the usual transient disturbances that arise from the actual direct heat flow measurements depending on the method of acquisition and other environmental factors that can play a role.
Reporting and publishing heat flow data In this session, we will show examples of how heat-flow data are reported in the literature and analyze the history of development in data compilation. The first compilation of the global heat flow database, designed to be accessible on a computer, was available only in the 1970s. A few pieces of information could be included in that database due to the state of information technology at that time. Although progress in thermal studies over the last decades has resulted in a fast accumulation of new data, the structure of that database has remained unchanged for about fifty years. In 2020 the International Heat Flow Commission initiated a process to create a new authenticated database to fulfil the requirements of modern research by including detailed metadata descriptions and database interoperability. We will examine the new GHFD structure and the individual fields that hold information related to the heat-flow determinations and allow the inference of the type and quality of heat-flow data.
Determination of thermal conductivity and physical properties in the Laboratory Estimation and interpretation of thermal conductivity and thermal diffusivity of dry and saturated rocks measured under ambient laboratory conditions using four different methods.
- The optical scanning technology (TCS)
- Needle Probe (TLS)
- Transient Plane Source (TPS)
- Laser Flash Analytics (LFA)
Downhole temperature logging Temperature and natural gamma-ray will be logged at a nearby borehole. Estimation of thermal gradient and uncertainties. Heat flow calculation and correction of environmental effects to determine surface heat flow.
Thermal modeling This session presents the fundamental equations of balance of energy of geological processes in steady and transient state.The finite element method for the numerical modeling thermal processes in the Earth. 1D models for isotherms of the continental crust, oceanic lithosphere, and hydrothermal circulation around a fault. 1D models of isotherms in sedimentary basins. 1D thermal perturbations caused by climate temperature changes. 2D models of the cooling of igneous bodies and heating by a strike-slip fault. 3D thermal effects.
Heat flow for geothermal energy In this session we will discover how a comprehensive understanding of crustal heat flow can greatly enhance the efficiency of exploration and characterization of geothermal energy resources. This includes ensuring that the maximum information is obtained from conventional ‘temperature gradient wells’, and that realistic values of rock and fluid properties are entered into sophisticated geothermal exploration and production modelling software. But the principles of crustal heat flow can also be applied at the regional scale to predict the temperature at any particular depth, and especially where high temperatures might coincide with productive reservoir formations. Knowing how and why heat moves in the subsurface can also help predict the redistribution of heat by convection, and aid in the interpretation of observed surface geothermal manifestations.
Heat flow for geodynamic modeling This session discusses the rheological response of Earth materials to variations in temperature and stress. Empirical laws that describe the rheology of the different shells of the Earth are also examined: brittle and ductile deformation on the crust and lithosphere and modes of flow in the mantle. Finally, this section introduces frictional constitutive laws that describe slip-weakening in friction, stick-slip instabilities, the dynamics of earthquakes, and the temperature dependence of these of these phenomena.