Ryan M. Pollyea

Estimating surface roughness of terrestrial laser scan data using orthogonal distance regression

With the increasing accessibility of terrestrial light detection and ranging scanners (LiDAR), generating tools to elicit meaningful information from high-density point cloud data has become of paramount importance. Surface roughness is one metric that has gained popularity, largely due to the accuracy and density of LiDAR-derived point cloud data. Surface roughness is typically defined as a spread of point distances from a reference datum, the standard deviation of point distances from a model surface being a commonly employed model. Unfortunately, a recent literature review has found that existing surface roughness models are far from standardized and may be prone to error resulting from underlying surface topography. In the research presented here, we develop a surface roughness model that is robust to underlying topographic variability by segmenting the point cloud with a three-dimensional regular grid, establishing local (grid cell) reference planes by orthogonal distance regression, and estimating the surface roughness of each grid cell as the standard deviation of orthogonal point-to-plane distances. This surface roughness model is employed to identify fracture and rubble zone distributions within a terrestrial LiDAR scan from a basalt outcrop in southeast Idaho, and the results are compared to a more common model based on ordinary least-squares plane fitting. Results indicate that the orthogonal regression model is robust to outcrop orientation and that the ordinary least-squares model systematically overestimates surface roughness by contaminating estimates with spatially correlated errors that increase with decreasing grid size.

Experimental evaluation of terrestrial LiDAR-based surface roughness estimates

The rapid proliferation of portable, ground-based light detection and ranging (LiDAR) instruments suggests the need for additional quantitative tools complementary to the commonly invoked digital terrain model (DTM). One such metric is surface roughness, which is a measure of local-scale topographic variability and has been shown to be effective for mapping discrete morphometric features, i.e., fractures in outcrop, landslide scarps, and alluvial fan deposits, to name a few. Several surface roughness models have been proposed, the most common of which is based on the standard deviation of point distances from a reference datum, e.g., DTM panels or best-fit planes. In the present work, we evaluate the accuracy of these types of surface roughness models experimentally by constructing a surface of known roughness, acquiring terrestrial LiDAR scans of the surface at 25 dual-axis rotations, and comparing surface roughness estimates for each rotation calculated by three surface roughness models. Results indicate that a recently proposed surface roughness model based on orthogonal distance regression (ODR) planes and orthogonal point-to-plane distance measurements is generally preferred on the basis of minimum error surface roughness estimates. In addition, the effects of terrestrial LiDAR sampling errors are discussed with respect to this ODR-based surface roughness model, and several practical suggestions are made for minimizing these effects. These include (1) positioning the laser scanner at the largest reasonable distance from the scanned surface, (2) maintaining half-angles for individual scans at less than 22.5°, and (3) minimizing occlusion (shadowing) errors by using multiple, merged scans with the least possible overlap.

Topographically driven fluid flow within orogenic wedges: Effects of taper angle and depth-dependent permeability

The fluid system within a critically tapering orogenic wedge is governed by complex interactions between topographic drive, thermal gradients, prograde dehydration reactions, internal structure, and regional tectonic compaction. Despite this complexity, topography is widely known to be the primary driving potential responsible for basin-scale fluid migration within the upper 7–10 km of an orogenic wedge. In recent years, investigators have revisited the problem of basin-scale fluid flow with an emphasis on depth-decaying permeability, which is a geologic phenomenon that is seldom accounted for in regional flow models. These recent investigations have shown that depth-dependent permeability at the basin scale strongly influences the relationship between local- and regional-scale flow paths. Here we investigate topography driven fluid flow within an orogenic wedge using a numerical modeling experiment designed to assess first-order fluid system behavior when permeability decreases systematically with depth. Critical taper theory is invoked to define two-dimensional basin geometry, and three subaerially exposed orogenic wedge models are presented with critical taper angles of 2°, 4°, and 10°. To assess the combined influence of topographic slope and depth-dependent permeability, a constant rate infiltration is applied at the wedge surface and a transient simulation is performed within each model for 20 m.y. Our results suggest that (1) depth-dependent permeability severely limits the penetration depth of infiltrating water within broadly tapering orogenic wedge systems, (2) fluid system evolution within a narrowly tapering orogenic wedge (i.e., ≤2°) is governed by local-scale topography superimposed on the regional gradient, (3) the influence of subbasin topography on local-scale fluid circulation is suppressed as the regional topographic gradient increases, and (4) the spatial distribution of groundwater residence time is fundamentally different when topographic slope exceeds 3°.

Physical constraints on geologic CO2 sequestration in low-volume basalt formations

Deep basalt formations within large igneous provinces have been proposed as target reservoirs for carbon capture and sequestration on the basis of favorable CO 2 -water-rock reaction kinetics that suggest carbonate mineralization rates on the order of 10 2 –10 3 d. Although these results are encouraging, there exists much uncertainty surrounding the influence of fracture-controlled reservoir heterogeneity on commercial-scale CO 2 injections in basalt formations. This work investigates the physical response of a low-volume basalt reservoir to commercial-scale CO 2 injections using a Monte Carlo numerical modeling experiment such that model variability is solely a function of spatially distributed reservoir heterogeneity. Fifty equally probable reservoirs are simulated using properties inferred from the deep eastern Snake River Plain aquifer in southeast Idaho, and CO 2 injections are modeled within each reservoir for 20 yr at a constant mass rate of 21.6 kg s –1 . Results from this work suggest that (1) formation injectivity is generally favorable, although injection pressures in excess of the fracture gradient were observed in 4% of the simulations; (2) for an extensional stress regime (as exists within the eastern Snake River Plain), shear failure is theoretically possible for optimally oriented fractures if S h ≤ 0.70S V ; and (3) low-volume basalt reservoirs exhibit sufficient CO 2 confinement potential over a 20 yr injection program to accommodate mineral trapping rates suggested in the literature.

Spatial and temporal constraints on regional-scale groundwater flow in the Pampa del Tamarugal Basin, Atacama Desert, Chile

Aquifers within the Pampa del Tamarugal Basin (Atacama Desert, northern Chile) are the sole source of water for the coastal city of Iquique and the economically important mining industry. Despite this, the regional groundwater system remains poorly understood. Although it is widely accepted that aquifer recharge originates as precipitation in the Altiplano and Andean Cordillera to the east, there remains debate on whether recharge is driven primarily by near-surface groundwater flow in response to periodic flood events or by basal groundwater flux through deep-seated basin fractures. In addressing this debate, the present study quantifies spatial and temporal variability in regional-scale groundwater flow paths at 20.5°S latitude by combining a two-dimensional model of groundwater and heat flow with field observations and δ18O isotope values in surface water and groundwater. Results suggest that both previously proposed aquifer recharge mechanisms are likely influencing aquifers within the Pampa del Tamarugal Basin; however, each mechanism is operating on different spatial and temporal scales. Storm-driven flood events in the Altiplano readily transmit groundwater to the eastern Pampa del Tamarugal Basin through near-surface groundwater flow on short time scales, e.g., 100–101 years, but these effects are likely isolated to aquifers in the eastern third of the basin. In addition, this study illustrates a physical mechanism for groundwater originating in the eastern highlands to recharge aquifers and salars in the western Pampa del Tamarugal Basin over timescales of 104–105 years.RésuméLes aquifères de la pampa du bassin du Tamarugal (désert d’Atacama, Nord du Chili) constituent la seule source d’approvisionnement en eau pour la ville côtière d’Iquique et l’industrie minière, importante sur le plan économique. Malgré cela, le système régional des eaux souterraines demeure mal compris. Bien qu’il soit largement accepté que la recharge de l’aquifère trouve son origine dans les précipitations de l’Altiplano et de la Cordillère des Andes à l’est, le débat demeurre pour savoir si la recharge est conduite principalement par des écoulements proches de la surface en réponse aux événements périodiques d’inondation ou bien par un écoulement souterrain empruntant des fractures ancrées en profondeur dans le bassin. En s’attaquant à cette discussion, la présente étude mesure la variabilité spatiale et temporelle des cheminements des eaux souterraines à l’échelle régionale, à la latitude de 20.5°S en combinant un modèle bidimensionnel des transferts d’eaux souterraines et de chaleur avec des observations de terrain et des valeurs de l’isotope δ18O dans les eaux souterraines et les eaux de surface. Les résultats suggèrent que les deux mécanismes de recharge précédemment proposés influencent probablement les aquifères dans la pampa du bassin du Tamarugal; cependant, chaque mécansime fonctionne sur différentes échelles spatiales et temporelles. Des inondations provoquées par des événements orageux dans l’Altiplano se propagent aisément aux eaux souterraines vers l’est de la pampa du bassin du Tamarugal Basin par un écoulement des eaux souterraines proche de la surface sur des échelles de temps courtes, par exemple, 100–101 ans, mais ces effets sont probablement localisés dans les aquifères du tiers oriental du bassin. En complément, cette étude illustre le mécanisme physique pour des eaux souterraines provenant des montagnes orientales pour recharger des aquifères et des salars dans l’ouest de la Pampa du bassin de Tamarugal dans des échelles de temps de 104–105 années.ResumenLos acuíferos de la cuenca de la Pampa del Tamarugal (Desierto de Atacama, norte de Chile) son la única fuente de agua para la ciudad costera de Iquique y para la industria minera de importancia económica. A pesar de esto, el sistema regional de agua subterránea sigue siendo poco conocida. Aunque está ampliamente aceptado que la recarga de los acuíferos se origina en la precipitaciones en el Altiplano y la Cordillera de los Andes al este, sigue habiendo un debate si la recarga es determinada principalmente por el flujo subterráneo cercano a la superficie en respuesta a eventos de inundación periódicos o por el flujo subterráneo basal a través de las fracturas situadas en profundidad en la cuenca. Para el tratamiento de este debate el presente estudio, cuantifica la variabilidad espacial y temporal de las trayectorias de flujo subterráneo a escala regional a una latitud de 20.5 ° S mediante la combinación de un modelo bidimensional del flujo subterráneo y de calor con las observaciones de campo y los valores isotópicos de δ18O en las aguas superficiales y subterráneas. Los resultados sugieren que tanto los mecanismos de recarga de los acuíferos previamente propuestos son probablemente influyentes en los acuíferos de la cuenca de la Pampa del Tamarugal; sin embargo, cada mecanismo está operando en diferentes escalas espaciales y temporales. Las inundaciones por las tormentas en el Altiplano transmiten fácilmente al agua subterránea en la cuenca oriental de la Pampa del Tamarugal través del flujo subterráneo cercano a la superficie en escalas de tiempo corto, por ejemplo, 100–101 años, pero estos efectos probablemente son aislados en los acuíferos del tercio oriental de la cuenca. Además, este estudio ilustra un mecanismo físico para las aguas subterráneas que se originan en las tierras altas del este para recargar los acuíferos y salares en la Cuenca de la Pampa del Tamarugal occidental en escalas de tiempo de 104–105 años.摘要(智利北部阿塔卡马沙漠)Pampa del Tamarugal盆地内的含水层是沿海城市伊基克及经济上重要的开采行业的唯一水源。尽管如此,人们对区域地下水系统仍然知之甚少。虽然普遍认为含水层补给来源于东面的阿尔蒂普拉诺高原和安第斯山脉,但是否主要由周期性洪水事件造成的近地表地下水流或者由穿过深部盆地断裂的基岩地下水通量补给仍存在着争议。为探寻这个争议,目前的研究把地下水与热流二维模型与野外观测数据及地表水和地下水中的同位素δ18O值相结合,量化了纬度20.5°S区域尺度地下水流通道的时空变化。结果显示,上述两种含水层补给机理很可能影响着Pampa del Tamarugal盆地内的含水层;然而,每一种机理处在不同的时空尺度上。阿尔蒂普拉诺高原暴雨引起的洪水事件很容易把地下水通过短时尺度如100–101年近地表地下水流输送到Pampa del Tamarugal盆地,但是这些效果很可能被分散到了盆地东部三分之一区域的含水层。此外,本项研究论述了起源于东部高地的地下水在104–105年时间尺度上补给Pampa del Tamarugal盆地西部含水层和干盐湖的物理机理。ResumoAquíferos dentro da bacia do Pampa do Tamarugal (Deserto do Atacama, norte do Chile) são a única fonte de água para a cidade costeira de Iquique e economicamente importante para a indústria de mineração. Apesar disso, o sistema de águas subterrâneas regional permanece pobremente entendido. Embora seja amplamente aceito que a recarga do aquífero origina-se como precipitação no Altiplano e Cordilheira Andina para o leste, há debates remanescentes se a recarga é oriunda primeiramente pelo fluxo de águas subterrâneas próximo da superfície em resposta a eventos periódicos de inundação ou pelo fluxo basal de águas subterrâneas através de bacias fraturadas profundas. Em resposta a este debate, o presente estudo quantifica a variabilidade espacial e temporal em escala regional dos padrões de fluxo das águas subterrâneas no trajeto em 20.5°S de latitude combinando um modelo bidimensional de águas subterrâneas e fluxo de calor com observações a campo e valores isotópicos de δ18O em águas superficiais e subterrâneas. Os resultados sugerem que ambos mecanismos de recarga do aquífero anteriormente propostos estão provavelmente influenciando dentro da bacia do Pampa do Tamarugal; porém, cada mecanismo está operando em diferentes escalas espaciais e temporais. Eventos de inundação gerados por tempestades no Altiplano prontamente transmitem águas subterrâneas para o leste da bacia do Pampa do Tamarugal através do fluxo próximo à superfície das águas subterrâneas numa curta escala de tempo, p.ex., 100–101 anos, mas estes efeitos são provavelmente isolados para aquíferos do terço leste da bacia. Além disso, este estudo ilustra um mecanismo físico para a origem das águas subterrâneas nas terras altas orientais para recarregar aquíferos e salares no oeste da bacia do Pampa do Tamarugal sob escalas de tempo de 104–105 anos.

TOUGH3: A new efficient version of the TOUGH suite of multiphase flow and transport simulators

The TOUGH suite of nonisothermal multiphase flow and transport simulators has been updated by various developers over many years to address a vast range of challenging subsurface problems. The increasing complexity of the simulated processes as well as the growing size of model domains that need to be handled call for an improvement in the simulators computational robustness and efficiency. Moreover, modifications have been frequently introduced independently, resulting in multiple versions of TOUGH that (1) led to inconsistencies in feature implementation and usage, (2) made code maintenance and development inefficient, and (3) caused confusion to users and developers. TOUGH3—a new base version of TOUGH—addresses these issues. It consolidates both the serial (TOUGH2 V2.1) and parallel (TOUGH2-MP V2.0) implementations, enabling simulations to be performed on desktop computers and supercomputers using a single code. New PETSc parallel linear solvers are added to the existing serial solvers of TOUGH2 and the Aztec solver used in TOUGH2-MP. The PETSc solvers generally perform better than the Aztec solvers in parallel and the internal TOUGH3 linear solver in serial. TOUGH3 also incorporates many new features, addresses bugs, and improves the flexibility of data handling. Due to the improved capabilities and usability, TOUGH3 is more robust and efficient for solving tough and computationally demanding problems in diverse scientific and practical applications related to subsurface flow modeling.

A field sampling strategy for semivariogram inference of fractures in rock outcrops

The stochastic continuum (SC) representation is one common approach for simulating the effects of fracture heterogeneity in groundwater flow and transport models. These SC reservoir models are generally developed using geostatistical methods (e.g., kriging or sequential simulation) that rely on the model semivariogram to describe the spatial variability of each continuum. Although a number of strategies for sampling spatial distributions have been published in the literature, little attention has been paid to the optimization of sampling in resource- or access-limited environments. Here we present a strategy for estimating the minimum sample spacing needed to define the spatial distribution of fractures on a vertical outcrop of basalt, located in the Box Canyon, east Snake River Plain, Idaho. We used fracture maps of similar basalts from the published literature to test experimentally the effects of different sample spacings on the resulting semivariogram model. Our final field sampling strategy was based on the lowest sample density that reproduced the semivariogram of the exhaustively sampled fracture map. Application of the derived sampling strategy to an outcrop in our field area gave excellent results, and illustrates the utility of this type of sample optimization. The method will work for developing a sampling plan for any intensive property, provided prior information for a similar domain is available; for example, fracture maps or ortho-rectified photographs from analogous rock types could be used to plan for sampling of a fractured rock outcrop.

Three‐Phase CO2 Flow in a Basalt Fracture Network

Geologic CO2 sequestration in basalt reservoirs is predicated on permanent CO2 isolation via rapid mineralization reactions. This process is supported by a substantial body of evidence, including laboratory experiments documenting rapid mineralization rates, regional storage estimates indicating large, accessible storage reservoirs, and two successful pilot scale studies. Nevertheless, there remains significant uncertainty in the behavior of CO2 flow within basalt fracture networks, particularly in the context estimating physical trapping potential in early time and as CO2 undergoes phase change. In this study, a Monte Carlo numerical model is designed to simulate a supercritical CO2 plume infiltrating a low permeability flood basalt entablature. The fracture network model is based on outcrop-scale LiDAR mapping of Columbia River Basalt, and CO2 flow is simulated within fifty equally probable realizations of the fracture network. The spatial distribution of fracture permeability for each realization is randomly drawn from a basalt aperture distribution, and ensemble results are analyzed with e-type estimates to compute mean and standard deviation of fluid pressure and CO2 saturation. Results of this model after 10 years of simulation suggests that (1) CO2 flow converges on a single dominant flow path, (2) CO2 accumulates at fracture intersections, and (3) variability in permeability can account for a 1.6 m depth interval within which free CO2 may change phase from supercritical fluid to subcritical liquid or gas. In the context of CO2 sequestration in basalt, these results suggest that physical CO2 trapping may be substantially enhanced as carbonate minerals precipitate within the basalt fracture network.

Advective Heat Transport and the Salt Chimney Effect: A Numerical Analysis

We conducted numerical simulations of coupled fluid and heat transport in an offshore, buried salt diapir environment to determine the effects of advective heat transport and its relation to the so-called “salt chimney effect.” Model sets were designed to investigate (1) salt geometry, (2) depth-dependent permeability, (3) geologic heterogeneity, and (4) the relative influence of each of these factors. Results show that decreasing the dip of the diapir induces advective heat transfer up the side of the diapir, elevating temperatures in the basin. Depth-dependent permeability causes upwelling of warm waters in the basin, which we show to be more sensitive to basal heat flux than brine concentration. In these model scenarios, heat is advected up the side of the diapir in a narrower zone of upward-flowing warm water, while cool waters away from the diapir flank circulate deeper into the basin. The resulting fluid circulation pattern causes increased discharge at the diapir margin and fluid flow downward, above the crest of the diapir. Geologic heterogeneity decreases the overall effects of advective heat transfer. The presence of low permeability sealing horizons reduces the vertical extent of convection cells, and fluid flow is dominantly up the diapir flank. The combined effects of depth-dependent permeability coupled with geologic heterogeneity simulate several geologic phenomena that are reported in the literature. In this model scenario, conductive heat transfer dominates in the basal units, whereas advection of heat begins to affect the middle layers of the model and dominates the upper units. Convection cells split by sealing layers develop within the upper units. From our highly simplified models, we can predict that advective heat transport (i.e., thermal convection) likely dominates in the early phases of diapirism when sediments have not undergone significant compaction and retain high porosity and permeability. As the salt structures mature into more complex geometries, advection will diminish due to the increase in dip of the salt-sediment interface and the increased hydraulic heterogeneity due to complex stratigraphic architecture.