Sarah Christian | Stanford University (original) (raw)
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Papers by Sarah Christian
Techniques and Methods, 2015
Journal of Geophysical Research, 2000
Catastrophic flank collapses have occurred at many stratovolcanoes worldwide. We present a three-... more Catastrophic flank collapses have occurred at many stratovolcanoes worldwide. We present a three-dimensional (3-D) slope stability analysis for assessing and quantifying both the locations of minimum edifice stability and the expected volumes of potential failure. Our approach can search the materials underlying a topographic surface, represented as a digital elevation model (DEM), and determine the relative stability of all parts of the edifice. Our 3-D extension of Bishop's [1955] simplified limit-equilibrium analysis incorporates spherical failure surfaces, variable material properties, pore fluid pressures, and earthquake shaking. Although a variety of processes can trigger collapse, we focus here on gravitationally induced instability. Even homogeneous rock properties strongly influence the depth and volume of the least stable potential failure. For large failures in complex topography, patterns of potential instability do not mimic local ground surface slope alone. The May 18, 1980, catastrophic failure of the north flank of Mount St. Helens provides the best documented case history to test our method. Using the undeformed edifice topography of Mount St. Helens in an analysis of dry, static slope stability with homogeneous materials, as might be conducted in a precollapse hazard analysis, our method identified the northwest flank as the least stable region, although the north flank stability was within 5% of the minimum. Using estimates of the conditions that existed 2 days prior to collapse, including deformed topography with a north flank bulge and combined pore pressure and earthquake shaking effects, we obtained good estimates of the actual failure location and volume. Our method can provide estimates of initial failure volume and location to aid in assessing downslope or downstream hazards.
Journal of Geophysical Research, 2000
Catastrophic flank collapses have occurred at many stratovolcanoes worldwide. We present a three-... more Catastrophic flank collapses have occurred at many stratovolcanoes worldwide. We present a three-dimensional (3-D) slope stability analysis for assessing and quantifying both the locations of minimum edifice stability and the expected volumes of potential failure. Our approach can search the materials underlying a topographic surface, represented as a digital elevation model (DEM), and determine the relative stability of all parts of the edifice. Our 3-D extension of Bishop's [1955] simplified limit-equilibrium analysis incorporates spherical failure surfaces, variable material properties, pore fluid pressures, and earthquake shaking. Although a variety of processes can trigger collapse, we focus here on gravitationally induced instability. Even homogeneous rock properties strongly influence the depth and volume of the least stable potential failure. For large failures in complex topography, patterns of potential instability do not mimic local ground surface slope alone. The May 18, 1980, catastrophic failure of the north flank of Mount St. Helens provides the best documented case history to test our method. Using the undeformed edifice topography of Mount St. Helens in an analysis of dry, static slope stability with homogeneous materials, as might be conducted in a precollapse hazard analysis, our method identified the northwest flank as the least stable region, although the north flank stability was within 5% of the minimum. Using estimates of the conditions that existed 2 days prior to collapse, including deformed topography with a north flank bulge and combined pore pressure and earthquake shaking effects, we obtained good estimates of the actual failure location and volume. Our method can provide estimates of initial failure volume and location to aid in assessing downslope or downstream hazards.
Water Resources Research, 1995
A method is developed to analyze steady horizontal flow to a well pumped from a confined aquifer ... more A method is developed to analyze steady horizontal flow to a well pumped from a confined aquifer composed of two homogeneous zones with contrasting transmissivities. Zone 1 is laterally unbounded and encloses zone 2, which is elliptical in shape and is several orders of magnitude more transmissive than zone 1. The solution for head is obtained by the boundary integral equation method. Nonlinear least squares regression is used to estimate the model parameters, which include the transmissivity of zone 1, and the location, size, and orientation of zone 2. The method is applied to a hypothetical aquifer where zone 2 is a long and narrow zone of vertical fractures. Synthetic data are generated from three different well patterns, representing different areal coverage and proximity to the fracture zone. When zone 1 of the hypothetical aquifer is homogeneous, the method correctly estimates all model parameters. When zone 1 is a randomly heterogeneous transmissivity field, some parameter estimates, especially the length of zone 2, become highly uncertain. To reduce uncertainty, the pumped well should be close to the fracture zone, and surrounding observation wells should cover an area similar in dimension to the length of the fracture zone. Some prior knowledge of the fracture zone, such as that gained from a surface geophysical survey, would greatly aid in designing the well test.
Water Resources Research, 1995
Steady-state two-phase flow in porous media was studied experimentally, using a model pore networ... more Steady-state two-phase flow in porous media was studied experimentally, using a model pore network of the chamberand-throat type, etched in glass. Four main flow regimes were observed and videorecorded, namely large-ganglion dynamics, small-ganglion dynamics, drop-traffic flow and connected pathway flow. The relative permeabilities are shown to correlate strongly with the flow regimes. The realtive permeability to oil (non-wetting fluid) is minimal in the first regime and increases strongly as the flow mechanism changes. The relative permeability to water (wetting fluid) is minimal in the first domain, increases moderately then increases strongly as the mechanism changes. Qualitative mechanistic explanations for these experimental results are proposed. (from Authors)
Water Resources Research, 1995
Steady-state two-phase flow in porous media was studied experimentally, using a model pore networ... more Steady-state two-phase flow in porous media was studied experimentally, using a model pore network of the chamberand-throat type, etched in glass. Four main flow regimes were observed and videorecorded, namely large-ganglion dynamics, small-ganglion dynamics, drop-traffic flow and connected pathway flow. The relative permeabilities are shown to correlate strongly with the flow regimes. The realtive permeability to oil (non-wetting fluid) is minimal in the first regime and increases strongly as the flow mechanism changes. The relative permeability to water (wetting fluid) is minimal in the first domain, increases moderately then increases strongly as the mechanism changes. Qualitative mechanistic explanations for these experimental results are proposed. (from Authors)
Techniques and Methods, 2015
Journal of Geophysical Research, 2000
Catastrophic flank collapses have occurred at many stratovolcanoes worldwide. We present a three-... more Catastrophic flank collapses have occurred at many stratovolcanoes worldwide. We present a three-dimensional (3-D) slope stability analysis for assessing and quantifying both the locations of minimum edifice stability and the expected volumes of potential failure. Our approach can search the materials underlying a topographic surface, represented as a digital elevation model (DEM), and determine the relative stability of all parts of the edifice. Our 3-D extension of Bishop's [1955] simplified limit-equilibrium analysis incorporates spherical failure surfaces, variable material properties, pore fluid pressures, and earthquake shaking. Although a variety of processes can trigger collapse, we focus here on gravitationally induced instability. Even homogeneous rock properties strongly influence the depth and volume of the least stable potential failure. For large failures in complex topography, patterns of potential instability do not mimic local ground surface slope alone. The May 18, 1980, catastrophic failure of the north flank of Mount St. Helens provides the best documented case history to test our method. Using the undeformed edifice topography of Mount St. Helens in an analysis of dry, static slope stability with homogeneous materials, as might be conducted in a precollapse hazard analysis, our method identified the northwest flank as the least stable region, although the north flank stability was within 5% of the minimum. Using estimates of the conditions that existed 2 days prior to collapse, including deformed topography with a north flank bulge and combined pore pressure and earthquake shaking effects, we obtained good estimates of the actual failure location and volume. Our method can provide estimates of initial failure volume and location to aid in assessing downslope or downstream hazards.
Journal of Geophysical Research, 2000
Catastrophic flank collapses have occurred at many stratovolcanoes worldwide. We present a three-... more Catastrophic flank collapses have occurred at many stratovolcanoes worldwide. We present a three-dimensional (3-D) slope stability analysis for assessing and quantifying both the locations of minimum edifice stability and the expected volumes of potential failure. Our approach can search the materials underlying a topographic surface, represented as a digital elevation model (DEM), and determine the relative stability of all parts of the edifice. Our 3-D extension of Bishop's [1955] simplified limit-equilibrium analysis incorporates spherical failure surfaces, variable material properties, pore fluid pressures, and earthquake shaking. Although a variety of processes can trigger collapse, we focus here on gravitationally induced instability. Even homogeneous rock properties strongly influence the depth and volume of the least stable potential failure. For large failures in complex topography, patterns of potential instability do not mimic local ground surface slope alone. The May 18, 1980, catastrophic failure of the north flank of Mount St. Helens provides the best documented case history to test our method. Using the undeformed edifice topography of Mount St. Helens in an analysis of dry, static slope stability with homogeneous materials, as might be conducted in a precollapse hazard analysis, our method identified the northwest flank as the least stable region, although the north flank stability was within 5% of the minimum. Using estimates of the conditions that existed 2 days prior to collapse, including deformed topography with a north flank bulge and combined pore pressure and earthquake shaking effects, we obtained good estimates of the actual failure location and volume. Our method can provide estimates of initial failure volume and location to aid in assessing downslope or downstream hazards.
Water Resources Research, 1995
A method is developed to analyze steady horizontal flow to a well pumped from a confined aquifer ... more A method is developed to analyze steady horizontal flow to a well pumped from a confined aquifer composed of two homogeneous zones with contrasting transmissivities. Zone 1 is laterally unbounded and encloses zone 2, which is elliptical in shape and is several orders of magnitude more transmissive than zone 1. The solution for head is obtained by the boundary integral equation method. Nonlinear least squares regression is used to estimate the model parameters, which include the transmissivity of zone 1, and the location, size, and orientation of zone 2. The method is applied to a hypothetical aquifer where zone 2 is a long and narrow zone of vertical fractures. Synthetic data are generated from three different well patterns, representing different areal coverage and proximity to the fracture zone. When zone 1 of the hypothetical aquifer is homogeneous, the method correctly estimates all model parameters. When zone 1 is a randomly heterogeneous transmissivity field, some parameter estimates, especially the length of zone 2, become highly uncertain. To reduce uncertainty, the pumped well should be close to the fracture zone, and surrounding observation wells should cover an area similar in dimension to the length of the fracture zone. Some prior knowledge of the fracture zone, such as that gained from a surface geophysical survey, would greatly aid in designing the well test.
Water Resources Research, 1995
Steady-state two-phase flow in porous media was studied experimentally, using a model pore networ... more Steady-state two-phase flow in porous media was studied experimentally, using a model pore network of the chamberand-throat type, etched in glass. Four main flow regimes were observed and videorecorded, namely large-ganglion dynamics, small-ganglion dynamics, drop-traffic flow and connected pathway flow. The relative permeabilities are shown to correlate strongly with the flow regimes. The realtive permeability to oil (non-wetting fluid) is minimal in the first regime and increases strongly as the flow mechanism changes. The relative permeability to water (wetting fluid) is minimal in the first domain, increases moderately then increases strongly as the mechanism changes. Qualitative mechanistic explanations for these experimental results are proposed. (from Authors)
Water Resources Research, 1995
Steady-state two-phase flow in porous media was studied experimentally, using a model pore networ... more Steady-state two-phase flow in porous media was studied experimentally, using a model pore network of the chamberand-throat type, etched in glass. Four main flow regimes were observed and videorecorded, namely large-ganglion dynamics, small-ganglion dynamics, drop-traffic flow and connected pathway flow. The relative permeabilities are shown to correlate strongly with the flow regimes. The realtive permeability to oil (non-wetting fluid) is minimal in the first regime and increases strongly as the flow mechanism changes. The relative permeability to water (wetting fluid) is minimal in the first domain, increases moderately then increases strongly as the mechanism changes. Qualitative mechanistic explanations for these experimental results are proposed. (from Authors)