16 research outputs found
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A crust and upper mantle model of Eurasia and North Africa for Pn travel time calculation
We develop a Regional Seismic Travel Time (RSTT) model and methods to account for the first-order effect of the three-dimensional crust and upper mantle on travel times. The model parameterization is a global tessellation of nodes with a velocity profile at each node. Interpolation of the velocity profiles generates a 3-dimensional crust and laterally variable upper mantle velocity. The upper mantle velocity profile at each node is represented as a linear velocity gradient, which enables travel time computation in approximately 1 millisecond. This computational speed allows the model to be used in routine analyses in operational monitoring systems. We refine the model using a tomographic formulation that adjusts the average crustal velocity, mantle velocity at the Moho, and the mantle velocity gradient at each node. While the RSTT model is inherently global and our ultimate goal is to produce a model that provides accurate travel time predictions over the globe, our first RSTT tomography effort covers Eurasia and North Africa, where we have compiled a data set of approximately 600,000 Pn arrivals that provide path coverage over this vast area. Ten percent of the tomography data are randomly selected and set aside for testing purposes. Travel time residual variance for the validation data is reduced by 32%. Based on a geographically distributed set of validation events with epicenter accuracy of 5 km or better, epicenter error using 16 Pn arrivals is reduced by 46% from 17.3 km (ak135 model) to 9.3 km after tomography. Relative to the ak135 model, the median uncertainty ellipse area is reduced by 68% from 3070 km{sup 2} to 994 km{sup 2}, and the number of ellipses with area less than 1000 km{sup 2}, which is the area allowed for onsite inspection under the Comprehensive Nuclear Test Ban Treaty, is increased from 0% to 51%
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Tomography and Methods of Travel-Time Calculation for Regional Seismic Location
We are developing a laterally variable velocity model of the crust and upper mantle across Eurasia and North Africa to reduce event location error by improving regional travel-time prediction accuracy. The model includes both P and S velocities and we describe methods to compute travel-times for Pn, Sn, Pg, and Lg phases. For crustal phases Pg and Lg we assume that the waves travel laterally at mid-crustal depths, with added ray segments from the event and station to the mid crustal layer. Our work on Pn and Sn travel-times extends the methods described by Zhao and Xie (1993). With consideration for a continent scale model and application to seismic location, we extend the model parameterization of Zhao and Xie (1993) by allowing the upper-mantle velocity gradient to vary laterally. This extension is needed to accommodate the large variation in gradient that is known to exist across Eurasia and North African. Further, we extend the linear travel-time calculation method to mantle-depth events, which is needed for seismic locators that test many epicenters and depths. Using these methods, regional travel times are computed on-the-fly from the velocity model in milliseconds, forming the basis of a flexible travel time facility that may be implemented in an interactive locator. We use a tomographic technique to improve upon a laterally variable starting velocity model that is based on Lawrence Livermore and Los Alamos National Laboratory model compilation efforts. Our tomographic data set consists of approximately 50 million regional arrivals from events that meet the ground truth (GT) criteria of Bondar et al. (2004) and other non-seismic constraints. Each datum is tested to meet strict quality control standards that include comparison with established distance-dependent travel-time residual populations relative to the IASPIE91 model. In addition to bulletin measurements, nearly 50 thousand arrival measurements were made at the national laboratories. The tomographic method adjusts Pn velocity, mantle gradient, and a node-specific crustal slowness correction for optimized travel-time prediction
Hydrogenation of Naturally-Derived Nepetalactone as a Topical Insect Repellent
Dihydronepetalactone
(DHN) is a safe and effective topical insect
repellent,,, comparable
in efficacy to that of <i>N</i>,<i>N</i>-diethyl-<i>m</i>-toluamide (DEET). The latter is the most commonly used
active ingredient, found in many commercial insect repellents for
a broad range of biting insects. DHN can be produced by hydrogenating
nepetalactone (NL), which is the primary ingredient of the essential
oil obtained from the renewably sourced catmint plant, <i>Nepeta
cataria</i>. Optimizing the hydrogenation reaction to produce
DHN from catmint oil is a key economic driver for the process. Prior
to the study described here, Six Sigma methodologies were used to
select palladium on carbon (5% Pd/C) as the catalyst of choice. The
hydrogenation step was studied as a function of critical process variables
and the composition of the oil. As described in this article, a robust,
two-step hydrogenation process was developed to maximize the yield
of the desired DHNs from treated catmint oil. It was observed that
the composition of the catmint oil, vis-à-vis, the relative
amounts of <i>trans–cis</i> and <i>cis–trans</i>-nepetalactone isomers, had a major impact on the activity and selectivity
of the catalyst. This study also focused on minimizing the formation
of a less desirable byproduct, puleganic acid. On the basis of the
process variables tested in this study, temperature was found to have
a strong effect on the activity and selectivity of the catalyst. Higher
pressure enhanced the activity of the catalyst but it did not significantly
impact the formation of undesired byproducts, such as puleganic and
nepetalic acids. Spiking experiments with suspected catalyst poisons,
such as dimethyl sulfide, dimethyl sulfoxide, nepetalic acid, and
puleganic acid were also performed to study catalyst deactivation.
Sulfur was identified as the main factor for the catalyst deactivation.
Possible reaction mechanisms for the formation of less desirable puleganic
and nepetalic acids have been suggested