Soil Gas Sampling

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Exploration Technique: Soil Gas Sampling

Exploration Technique Information
Exploration Group: Field Techniques
Exploration Sub Group: Field Sampling
Parent Exploration Technique: Gas Sampling
Information Provided by Technique
Stratigraphic/Structural: Identify concealed faults that act as conduits for hydrothermal fluids.
Hydrological: Identify hydrothermal gases of magmatic origin.
Thermal: Differentiate between amagmatic or magmatic sources heat.
Soil Gas Sampling:
Soil gas sampling is sometimes used in exploration for blind geothermal resources to detect anomalously high concentrations of hydrothermal gases in the near-surface environment. Identification of high concentrations of hydrothermal gas species may indicates the presence of enhanced permeability (faults) and high temperature hydrothermal activity at depth. Soil gas data may also be used to study other important aspects of the geothermal system, such as distinguishing between magmatic and amagmatic sources of heat. The technique may also be used for ongoing monitoring of the geothermal system during resource development and production.
Other definitions:Wikipedia Reegle

Soil gas sampling may be used to analyze the volatile gas contents emanating from a hydrothermal system. Typical geothermal resources expel anomalously high concentrations of gases indicating the presence of enhanced permeability and of high temperature hydrothermal activity that may be present at depth. Soil gas sampling is particularly useful in areas where surface manifestations that traditionally evidence the presence of an active geothermal system are scarce. The technique may also be used for ongoing monitoring of the geothermal system during resource development and production.

USGS scientist collecting a soil gas sample for volatile-organic compound analysis. Photo featured on the U.S. Geological Survey website.[1]

Use in Geothermal Exploration
Soil gas sampling is used to identify anomalous concentrations of hydrothermal gases (CO2, CH4, H2S, SO2, SO3, N2, O2, H2, Ar, Xe, Hg, He, Rn, and/or Th) associated with magmatic activity that have seeped into soils in the near-surface environment. High concentrations of these gas species in soils relative to background concentrations can assist in delineating main upflow regions and areas of increased subsurface permeability (e.g. concealed, such as shear fault zones).[2]

Soil gas sampling techniques are also used in ongoing measurement of the gas contents of soils in volcanic hazard monitoring. For example, gas flux and soil gas sampling techniques have been used to monitor potentially deadly CO2 emissions at the Long Valley Caldera, CA since the early 90s, when a period of intense seismic activity associated with dike intrusion and/or magmatic fluid migration caused an increase in CO2 flux to the surface, resulting in localized tree kills around Horseshoe Lake at the base of Mammoth Mountain.[3][4]

Field Procedures
A pilot hole is created at each site to the depth from which gas will be sampled using a hammer drill or slide hammer. A sampling depth between 0.5 and 2 m is generally considered to be below the depth influenced by meteorological processes.(Hinkle, 1994; Segovia et al., 1987) A probe consisting of rod with a vapor recovery point and attached tubing is lowered into the pilot hole. The 2 m temperature probes used for shallow temperature surveys in the Great Basin region have recently been modified to allow for simultaneous soil gas sampling using a 3 mm Teflon tube.[5] A low vacuum is pulled in the tubing and soil probe prior to sampling in order to purge atmospheric air and avoid contamination of the gas sample. A standard sampling bottle or copper tube (under vacuum) is then attached to the purged tubing system, resealed, and finally removed to collect the gas sample.[6]

Exact field procedures may vary depending on the vapor sampling apparatus used. For detailed information on sampling methods and analysis of soil gas samples and their use in evaluating geothermal resources in different environments, consult Lechler et al. (2009),[5] Voltattorni et al. (2010),[2] Klingel et al. (2011).[6]

Best Practices
Soil gas sampling is best carried out by a qualified hydrologist, geologist, geochemist, or field technician familiar with current sampling standards. A practical understanding of how soil gas contents relate to biogenic processes and hydrothermal activity within the geothermal system is also ideal for the purposes of data interpretation, application of various chemical and isotopic geothermometers, and geochemical modeling of the reservoir.

Analysis and comparison of hydrothermal gas concentrations in soil gas samples can be used to:[6]

  • Presence or absence of hydrothermal gases of magmatic origin.
  • Differentiate between amagmatic or magmatic sources of soil gases.
  • Temperature estimates of the geothermal source reservoir based on multiple gas geothermometers.
  • Determine geometry and/or location of the geothermal source.
  • Estimate relative depth to an inferred magmatic source.
  • Identify concealed faults that act as conduits for hydrothermal fluids.
  • Estimate relative fracture effective porosity versus porous medium effective porosity.
  • Assist in siting locations for exploratory slim holes and surface geophysical surveys.

Potential Pitfalls
Failure to properly purge the sampling tube prior to collecting the soil gas sample may lead to atmospheric contamination, resulting in dilution of hydrothermal gases in the sampled volume. It is also important to note that anomalous concentrations of gases in soils are defined on a site-by-site basis based on local soil-gas background values. As such, an absolute quantitative value of what constitutes a soil-gas anomaly is difficult to define. A normal probability plot is ideal for distinguishing anomalous values from background and outlier measurements, but requires a large set of samples to reliably determine the “normal” background soil gas contents of a particular geothermal area.

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