Field Techniques

Jump to: navigation, search

Exploration Technique: Field Techniques

Exploration Technique Information
Exploration Group: Field Techniques
Exploration Sub Group: None
Parent Exploration Technique: Exploration Techniques
Information Provided by Technique
Lithology: Map surface geology and hydrothermal alteration. Rock samples are used to define lithology. Field and lab analyses can be used to measure the chemical and isotopic constituents of rock samples. Bulk and trace element analysis of rocks, minerals, and sediments. Identify and document surface geology and mineralogy. Rapid and unambiguous identification of unknown minerals.[1]
Stratigraphic/Structural: Locates active faults in the area of interest. Map fault and fracture patterns, kinematic information. Can reveal relatively high permeability zones. Provides information about the time and environment which formed a particular geologic unit. Microscopic rock textures can be used to estimate the history of stress and strain, and/or faulting.
Hydrological: Can reveal whether faults are circulating hydrothermal fluids. Map surface manifestations of geothermal systems. Water composition and source of fluids. Isotope geochemistry can reveal fluid circulation of a geothermal system.
Thermal: Identify and delineate shallow thermal anomalies. Map surface temperature. Water temperature. Used to locate active hydrothermal systems. Thermal conductivity of a rock sample can provide information to calculate heat flow. Hydrothermal alteration of a rock sample can indicate certain temperature or fluid compositions.
Field Techniques:
Field techniques include a broad array of methods used to characterize a geothermal system based on data and analysis of samples collected in the field. Surface mapping, shallow temperature probe surveys, and portable XRF/XRD analysis represent a valuable set of basic, cost effective tools for evaluating geothermal prospects. These methods can provide a geothermal exploration program with considerable information about a hydrothermal system prior to the application of expensive laboratory analysis or exploration drilling techniques. Systematic field sampling is also critical for reliable characterize a geothermal resource. Some of the physical and chemical properties of rock samples can be estimated by visual inspection, but accurate determination of these properties requires detailed laboratory analysis. Surface or subsurface fluid sampling is also routinely performed to characterize the chemical, thermal, or hydrological properties of a hydrothermal system. Combinations of these sampling techniques have traditionally been used to obtain important information used to determine whether or not a viable power generation or heat utilization facility can be developed at a prospect.
Other definitions:Wikipedia Reegle

Field investigations are a critical component of any successful geothermal exploration program, and provide useful reconnaissance during the early stages of prospect evaluation through to production of a geothermal resource. There are a number of techniques that can be applied in geothermal exploration to obtain data directly in the field, ranging from traditional geologic mapping to recently developed/refined analytical methods that can be applied outside of the laboratory setting. Basic mapping of surface lithology, hydrothermal alteration, and faults provides important information regarding the geologic setting, history of hydrothermal discharge, and the location of structures that may function as high permeability pathways for fluid circulation within the geothermal system. Shallow temperature measurements have also been used to detect deeper geothermal aquifers since the early 1980s. Recent refinements to shallow temperature probe survey methods have been used successfully to detect blind systems in the Great Basin that lack readily discernable surface manifestations. The relatively low cost of applying these techniques makes them excellent tools in the early stages of geothermal exploration, owing to the wealth of information they can provide prior to the selection of drilling targets.

Recent miniaturization of components of traditional laboratory-based analytical instruments has enabled the development of portable XRD and XRF devices that can be used in the field to enhance traditional mapping techniques. These tools can be used to analyze the bulk composition of rock, mineral, and sediment samples and to identify unknown minerals in the field, thereby assisting mappers in identifying hydrothermal alteration products that visually difficult to distinguish.[1]

Field sampling is typically performed in conjunction with surface investigations to characterize important properties of the geothermal system under study. Reservoir properties determined through analysis of rock samples can assist in defining a geothermal resource by providing information regarding the temperature history, volume, heat source, degree of alteration/water-rock interaction, and rate of fluid recharge that occur in the hydrothermal system. There are dozens of techniques that can be applied to measure physical and chemical properties of a single rock sample, as well as the hydrological and thermal environment that the rock came from. Reservoir rock properties strongly influence the unique chemical signature of geothermal fluids, which evolve through specific processes within the dynamic hydrothermal system. Water and gas samples can be collected at the surface from springs, geysers, fumaroles, or drilled wells, or from a specific depth interval in geothermal wells. For geothermal resource areas that lack surface manifestations, soil sampling can be a useful tool for identifying a blind geothermal system at depth. Soils act similar to a sponge and absorb certain elements or mineral alterations that are associated with hydrothermal activity.

Geologist taking a soil sample in the field. Photo from the Kansas State University website[2].

Use in Geothermal Exploration
Mapping of geologic units and faults are important exploration techniques that assist in understanding the setting of the geothermal system, identifying hydrothermal alteration, and pinpointing high permeability conduits for hydrothermal fluids. Curewitz & Karson (1997) document the common relationship between active faulting and hydrothermal systems.[3] In addition to the collection surface data, field mapping may be used to "ground truth" data collected by other methods, such as remote sensing.

Shallow temperature probe surveys are an extremely useful and cost-effective tool for approximating the temperatures of deeper geothermal systems and mapping shallow thermal anomalies in order to find blind geothermal systems. These surveys are only effective if the geothermal system is not overlain by a cold groundwater aquifer, which will mask the temperature anomaly that could be seen in a shallow survey. Shallow temperature surveys are often conducted prior to drilling a temperature gradient borehole.

Reports of portable XRF and XRD analyses in geothermal exploration are scant, as these techniques have only been available for the past decade. Potential uses include detailed characterization of lithologic units, identification of unrecognized hydrothermal minerals, and differentiation of optically similar clays and mixed layer clays that form from distinct different weathering and hydrothermal alteration processes.

Water, gas, soil and rock samples are also typically collected for laboratory analysis during field investigations. A single rock sample is used to characterize a section of a well or formation, while a collection of rock samples can be used to determine if whether reservoir properties are sufficient to sustain a power generation or heat utilization facility. Drilling wells and collecting drill cores is a common practice for determining the location of a potential geothermal reservoir. Comprehensive characterization of a reservoir requires detailed knowledge and analysis of the rock samples collected from a well. Typically the most critical information gathered are physical properties such as porosity, permeability, and thermal conductivity. However, there are other more detailed techniques that can reveal chemical composition, alteration mineralogy, the peak temperature experienced by the hydrothermal system, etc.

Water and gas sampling are routinely used in geothermal exploration and monitoring to characterize the chemical composition of the fluid, measure the temperature, or conduct isotope studies to derive the provenance of thermal fluids. Fluid sampling is a critical aspect of characterizing a geothermal system because the water chemistry, temperature and source can reveal the quality of the resource. Water chemistry is largely controlled by temperature, water-rock interactions, volume of water vs rock, residence time, and contributions from other fluids (mixing), such as cold groundwater, seawater, magmatic fluids, etc.[4] Waters that discharge at the surface are commonly over saturated with silica or carbonate at surface conditions and precipitate sinter or travertine, respectively.[5][4] Some geothermal fluids that reach the surface form acid-sulfate springs, generated from rising steam and volatile compounds that condense and mix with an overlying freshwater aquifer, whereupon the H2S in the steam oxidizes to form sulfuric acid.[5][4][6]

Soil sampling is typically performed in a methodical and structured way, so that the results of the geochemical analysis can be plotted spatially on a map. An active hydrothermal system releases fluids with chemical signatures that are anomalous in typical surface environments. The most volatile gases are able to escape the heat source and permeate through overlying formations and structures, casting an imprint either on the soil or into the atmosphere that is used to locate a potential geothermal resource. Analysis of soil samples is particularly useful for locating blind or covered structures in geothermal areas, which act as conduits for the ascending fluids. The successful identification of these soils can reveal relatively high permeability zones associated with otherwise unknown geothermal resources.

No exploration activities found.

      Print PDF