Stereology, or the quantitative evaluation of microstructures

The Taupo Volcanic Zone (TVZ) hosts 23 geothermal fields, seven of which are currently utilised for power generation. Ngatamariki geothermal field (NGF) is one of the latest geothermal power generation developments in New Zealand (commissioned in 2013), located approximately 15 km north of Taupo. Samples of reservoir rocks were taken from the Tahorakuri Formation and Ngatamariki Intrusive Complex, from five wells at the NGF at depths ranging from 1354 to 3284 m. The samples were categorised according to whether their microstructure was pore or microfracture dominated. Image analysis of thin sections impregnated with an epoxy fluorescent dye was used to characterise and quantify the porosity structures and their physical properties were measured in the laboratory. Our results show that the physical properties of the samples correspond to the relative dominance of microfractures compared to pores. Microfracture-dominated samples have low connected porosity and permeability, and the permeability decreases sharply in response to increasing confining pressure. The pore-dominated samples have high connected porosity and permeability, and lower permeability decrease in response to increasing confining pressure. Samples with both microfractures and pores have a wide range of porosity and relatively high permeability that is moderately sensitive to confining pressure. A general trend of decreasing connected porosity and permeability associated with increasing dry bulk density and sonic velocity occurs with depth; however, variations in these parameters are more closely related to changes in lithology and processes such as dissolution and secondary veining and re-crystallisation. This study provides the first broad matrix permeability characterisation of rocks from depth at Ngatamariki, providing inputs for modelling of the geothermal system. We conclude that the complex response of permeability to confining pressure is in part due to the intricate dissolution, veining, and recrystallization textures of many of these rocks that lead to a wide variety of pore shapes and sizes. While the laboratory results are relevant only to similar rocks in the Taupo Volcanic Zone, the relationships they highlight are applicable to other geothermal fields, as well as rock mechanic applications to, for example, aspects of volcanology, landslide stabilisation, mining, and tunnelling at depth.

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“…This allows the calculation of microfracture surface area per unit volume using Eq. 5 (Underwood 1969;Wu et al 2000):…”

Section: Thin Section Analysismentioning

confidence: 99%

“…Microfracture surface area was measured using classical stereological techniques outlined by Underwood (1969) and further described by Wu et al (2000) and Heap et al (2014). Using the binary images created in ImageJ the number of microfractures intersecting a grid of parallel and perpendicular lines spaced at 0.1 mm is recorded.…”

Section: Thin Section Analysismentioning

confidence: 99%

Matrix permeability of reservoir rocks, Ngatamariki geothermal field, Taupo Volcanic Zone, New Zealand

et al. 2018

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“…www.the-cryosphere.net/10/2847/2016/ The Cryosphere, 10, 2847-2863, 2016 (Underwood, 1969) to calculate s is to count the number of blue dots per unit length. The estimation for s mf is given by the red contour.…”

Section: Chord Length Distributions and Optical Metricsmentioning

confidence: 99%

Relating optical and microwave grain metrics of snow: the relevance of grain shape

Krol¹,

Löwe²

2016

The Cryosphere

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Abstract. Grain shape is commonly understood as a morphological characteristic of snow that is independent of the optical diameter (or specific surface area) influencing its physical properties. In this study we use tomography images to investigate two objectively defined metrics of grain shape that naturally extend the characterization of snow in terms of the optical diameter. One is the curvature length λ 2 , related to the third-order term in the expansion of the two-point correlation function, and the other is the second moment µ 2 of the chord length distributions. We show that the exponential correlation length, widely used for microwave modeling, can be related to the optical diameter and λ 2 . Likewise, we show that the absorption enhancement parameter B and the asymmetry factor g G , required for optical modeling, can be related to the optical diameter and µ 2 . We establish various statistical relations between all size metrics obtained from the two-point correlation function and the chord length distribution. Overall our results suggest that the characterization of grain shape via λ 2 or µ 2 is virtually equivalent since both capture similar aspects of size dispersity. Our results provide a common ground for the different grain metrics required for optical and microwave modeling of snow.

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“…The surface area of arbuscules in a given length of root was calculated by multiplying the invaginated area by 4\π (Underwood, 1969 ;Cox & Tinker, 1976 ;see Smith et al, 1994). This value was then multiplied by a correction factor of 1.2 to allow for variation in size between immature and mature arbuscules found within the population measured .…”

Section: Calculation Of Area Of Symbiotic Interfacementioning

confidence: 99%

Characterization of two arbuscular mycorrhizal fungi in symbiosis with Allium porrum: inflow and flux of phosphate across the symbiotic interface

Dickson

Smith

1999

New Phytologist

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Individual arbuscular mycorrhizal fungi can differ markedly in their ability to improve the phosphate nutrition and growth of host plants. In particular, Scutellospora calospora is relatively ineffective with some hosts and a growth depression is often observed. We have examined the abilities of S. calospora and Glomus sp. ' City Beach ' strain WUM 16, grown in soils which promote extensive mycorrhizal colonization, to transfer phosphate (P) to Allium porrum. Phosphate uptake from the low-P soils (P ! ) was compared with uptake from soils amended with extra P (P " ). In order to relate P transfer to physiological characteristics of the two fungi, inflow of P via fungus to the plant was combined with the surface areas of intercellular hyphae and arbuscules (symbiotic interfaces) to calculate the amount of P transferred per unit area of interface (P fluxes). ' Hyphal inflows ' and ' hyphal fluxes ' were also calculated on the assumption that P uptake by the plant was the same in mycorrhizal and equivalent nonmycorrhizal (NM) plants (the validity of this assumption is discussed). With both soil P treatments, P was taken up by plants colonized by both mycorrhizal fungi to a greater extent than by the equivalent NM controls. Hyphal inflows to plants in P ! soil that were colonized by Glomus sp. ' City Beach ' were greatest from 14-21 d and decreased at later harvests. With P " soil the inflow via Glomus sp. ' City Beach ' peaked at a very high value at 21-28 d. Hyphal inflows into plants in P ! soil that were colonized by S. calospora increased throughout the experiment, and with P " soil they remained steady at high values. With both fungi, the surface areas of the symbiotic interfaces increased greatly after 14 d, and generally there was little effect of higher soil P. With P ! soil, Glomus sp. ' City Beach ' showed no significant differences in the ratio of surface areas of the two interfaces over the course of the experiment. Scutellospora calospora tended to produce a lower percentage of interfacial area contributed by arbuscules. With the mycorrhizal plants growing in their respective soils, there appeared no consistent differences between the two fungi with respect to fluxes of P across the interfaces. With P ! soil, fluxes via Glomus sp. ' City Beach ' were initially higher than those via S. calospora, but later they were higher with S. calospora. With P " soil the only difference was the 28-42-d period, when Glomus sp. ' City Beach ' produced the higher flux (reflecting the low surface area at the time). The results show that relative inefficiency of S. calospora in its ability to transfer P, as reported by others, may result from different environmental conditions, use of different hosts, or even of different fungal isolates (strains).

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Stereology, or the quantitative evaluation of microstructures

Cited by 127 publications

References 11 publications

Matrix permeability of reservoir rocks, Ngatamariki geothermal field, Taupo Volcanic Zone, New Zealand

Matrix permeability of reservoir rocks, Ngatamariki geothermal field, Taupo Volcanic Zone, New Zealand

Relating optical and microwave grain metrics of snow: the relevance of grain shape

Characterization of two arbuscular mycorrhizal fungi in symbiosis with Allium porrum: inflow and flux of phosphate across the symbiotic interface

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