Summary statistics from training images as prior information in probabilistic inversion

A strategy is presented to incorporate prior information from conceptual geological models in probabilistic inversion of geophysical data. The conceptual geological models are represented by multiple-point statistics training images (TIs) featuring the expected lithological units and structural patterns. Information from an ensemble of TI realizations is used in two different ways. First, dominant modes are identified by analysis of the frequency content in the realizations, which drastically reduces the model parameter space in the frequency-amplitude domain. Second, the distributions of global, summary metrics (e.g. model roughness) are used to formulate a prior probability density function. The inverse problem is formulated in a Bayesian framework and the posterior pdf is sampled using Markov chain Monte Carlo simulation. The usefulness and applicability of this method is demonstrated on two case studies in which synthetic crosshole ground-penetrating radar traveltime data are inverted to recover 2-D porosity fields. The use of prior information from TIs significantly enhances the reliability of the posterior models by removing inversion artefacts and improving individual parameter estimates. The proposed methodology reduces the ambiguity inherent in the inversion of high-dimensional parameter spaces, accommodates a wide range of summary statistics and geophysical forward problems

Effect of different quantities of a sugared clear fluid on gastric emptying and residual volume in children: a crossover study using magnetic resonance imaging

Background Gastric emptying in the first 2 h after 7 ml kg−1 of sugared clear fluid has recently been investigated in healthy children using magnetic resonance imaging (MRI). This study aims to compare gastric volume and emptying half-life during 1 h after 3 or 7 ml kg−1 sugared clear fluid intake. Methods Fourteen healthy volunteer children aged 11.1 (8.2-12.5) yr were investigated prospectively after administration of 3 and 7 ml kg−1 diluted raspberry syrup in a randomized order, after overnight fasting (baseline). Gastric content volume (GCVw) was assessed with a 1.5 Tesla MRI scanner in a blinded fashion. Data are presented as median (range) and compared using the Wilcoxon test. Results Baseline GCVw was 0.39 (0.04-1.00) and 0.34 (0.07-0.75) before intake of 3 and 7 ml kg−1 syrup, respectively (P=0.93). GCVw was 0.45 (0.04-1.55)/1.33 (0.30-2.60) ml kg−1 60 min after ingestion of 3/7 ml kg−1 syrup (P=0.002). Thus GCVw had declined to baseline after 3 ml kg−1 (P=0.39) but not after 7 ml kg−1 (P=0.001) within 60 min. T1/2 was 20 (10-62)/27 (13-43) min (P=0.73) after 3/7 ml kg−1. Conclusion In healthy volunteer children, residual GCVw 1 h after intake of 3 ml kg−1 syrup is significantly smaller than that after 7 ml kg−1 and within the range of baselin

Probabilistic electrical resistivity tomography of a CO2 sequestration analog

Electrical resistivity tomography (ERT) is a well-established method for geophysical characterization and has shown potential for monitoring geologic CO2 sequestration, due to its sensitivity to electrical resistivity contrasts generated by liquid/gas saturation variability. In contrast to deterministic inversion approaches, probabilistic inversion provides the full posterior probability density function of the saturation field and accounts for the uncertainties inherent in the petrophysical parameters relating the resistivity to saturation. In this study, the data are from benchtop ERT experiments conducted during gas injection into a quasi-2D brine-saturated sand chamber with a packing that mimics a simple anticlinal geological reservoir. The saturation fields are estimated by Markov chain Monte Carlo inversion of the measured data and compared to independent saturation measurements from light transmission through the chamber. Different model parameterizations are evaluated in terms of the recovered saturation and petrophysical parameter values. The saturation field is parameterized (1) in Cartesian coordinates, (2) by means of its discrete cosine transform coefficients, and (3) by fixed saturation values in structural elements whose shape and location is assumed known or represented by an arbitrary Gaussian Bell structure. Results show that the estimated saturation fields are in overall agreement with saturations measured by light transmission, but differ strongly in terms of parameter estimates, parameter uncertainties and computational intensity. Discretization in the frequency domain (as in the discrete cosine transform parameterization) provides more accurate models at a lower computational cost compared to spatially discretized (Cartesian) models. A priori knowledge about the expected geologic structures allows for non-discretized model descriptions with markedly reduced degrees of freedom. Constraining the solutions to the known injected gas volume improved estimates of saturation and parameter values of the petrophysical relationship. (C) 2014 Elsevier B.V. All rights reserved

Effect of different quantities of a sugared clear fluid on gastric emptying and residual volume in children: a crossover study using magnetic resonance imaging

BACKGROUND: Gastric emptying in the first 2 h after 7 ml kg(-1) of sugared clear fluid has recently been investigated in healthy children using magnetic resonance imaging (MRI). This study aims to compare gastric volume and emptying half-life during 1 h after 3 or 7 ml kg(-1) sugared clear fluid intake. METHODS: Fourteen healthy volunteer children aged 11.1 (8.2-12.5) yr were investigated prospectively after administration of 3 and 7 ml kg(-1) diluted raspberry syrup in a randomized order, after overnight fasting (baseline). Gastric content volume (GCV(w)) was assessed with a 1.5 Tesla MRI scanner in a blinded fashion. Data are presented as median (range) and compared using the Wilcoxon test. RESULTS: Baseline GCV(w) was 0.39 (0.04-1.00) and 0.34 (0.07-0.75) before intake of 3 and 7 ml kg(-1) syrup, respectively (P=0.93). GCV(w) was 0.45 (0.04-1.55)/1.33 (0.30-2.60) ml kg(-1) 60 min after ingestion of 3/7 ml kg(-1) syrup (P=0.002). Thus GCV(w) had declined to baseline after 3 ml kg(-1) (P=0.39) but not after 7 ml kg(-1) (P=0.001) within 60 min. T(1/2) was 20 (10-62)/27 (13-43) min (P=0.73) after 3/7 ml kg(-1). CONCLUSION: In healthy volunteer children, residual GCV(w) 1 h after intake of 3 ml kg(-1) syrup is significantly smaller than that after 7 ml kg(-1) and within the range of baseline