Improvements in storm surge surrogate modeling for synthetic storm parameterization, node condition classification and implementation to small size databases

Surrogate models are becoming increasingly popular for storm surge predictions. Using existing databases of storm simulations, developed typically during regional flood studies, these models provide fast-to-compute, data-driven approximations quantifying the expected storm surge for any new storm (not included in the training database). This paper considers the development of such a surrogate model for Delaware Bay, using a database of 156 simulations driven by synthetic tropical cyclones and offering predictions for a grid that includes close to 300,000 computational nodes within the geographical domain of interest. Kriging (Gaussian Process regression) is adopted as the surrogate modeling technique, and various relevant advancements are established. The appropriate parameterization of the synthetic storm database is examined. For this, instead of the storm features at landfall, the features when the storm is at closest distance to some representative point of the domain of interest are investigated as an alternative parametrization, and are found to produce a better surrogate. For nodes that remained dry for some of the database storms, imputation of the surge using a weighted k nearest neighbor (kNN) interpolation is considered to fill in the missing data. The use of a secondary, classification surrogate model, combining logistic principal component analysis and Kriging, is examined to address instances for which the imputed surge leads to misclassification of the node condition. Finally, concerns related to overfitting for the surrogate model are discussed, stemming from the small size of the available database. These concerns extend to both the calibration of the surrogate model hyper-parameters, as well as to the validation approaches adopted. During this process, the benefits from the use of principal component analysis as a dimensionality reduction technique, and the appropriate transformation and scaling of the surge output are examined in detail

Pulsar-wind nebulae and magnetar outflows: observations at radio, X-ray, and gamma-ray wavelengths

We review observations of several classes of neutron-star-powered outflows: pulsar-wind nebulae (PWNe) inside shell supernova remnants (SNRs), PWNe interacting directly with interstellar medium (ISM), and magnetar-powered outflows. We describe radio, X-ray, and gamma-ray observations of PWNe, focusing first on integrated spectral-energy distributions (SEDs) and global spectral properties. High-resolution X-ray imaging of PWNe shows a bewildering array of morphologies, with jets, trails, and other structures. Several of the 23 so far identified magnetars show evidence for continuous or sporadic emission of material, sometimes associated with giant flares, and a few possible "magnetar-wind nebulae" have been recently identified.Comment: 61 pages, 44 figures (reduced in quality for size reasons). Published in Space Science Reviews, "Jets and Winds in Pulsar Wind Nebulae, Gamma-ray Bursts and Blazars: Physics of Extreme Energy Release

Stimulation rationale for shale gas wells: a state-of-the-art report

Despite the large quantities of gas contained in the Devonian Shales, only a small percentage can be produced commercially by current production methods. This limited production derives both from the unique reservoir properties of the Devonian Shales and the lack of stimulation technologies specifically designed for a shale reservoir. Since October 1978 Science Applications, Inc. has been conducting a review and evaluation of various shale well stimulation techniques with the objective of defining a rationale for selecting certain treatments given certain reservoir conditions. Although this review and evaluation is ongoing and much more data will be required before a definitive rationale can be presented, the studies to date do allow for many preliminary observations and recommendations. For the hydraulic type treatments the use of low-residual-fluid treatments is highly recommended. The excellent shale well production which is frequently observed with only moderate wellbore enlargement treatments indicates that attempts to extend fractures to greater distances with massive hydraulic treatments are not warranted. Immediate research efforts should be concentrated upon limiting production damage by fracturing fluids retained in the formation, and upon improving proppant transport and placement so as to maximize fracture conductivity. Recent laboratory, numerical modeling and field studies all indicate that the gas fracturing effects of explosive/propellant type treatments are the predominate production enhancement mechanism and that these effects can be controlled and optimized with properly designed charges. Future research efforts should be focused upon the understanding, prediction and control of wellbore fracturing with tailored-pulse-loading charges. 36 references, 7 figures, 2 tables