Dale Werkema - Academia.edu (original) (raw)

Papers by Dale Werkema

Symposium on the Application of Geophysics to Engineering and Environmental Problems 2011, 2011

The leading edge, Feb 1, 2015

Ground Water, Jan 6, 2016

Journal of Applied Geophysics, Feb 1, 2017

$Research paper thumbnail of An overview of geophysical technologies appropriate for characterization and monitoring at fractured-rock sites$

Journal of Environmental Management, Dec 1, 2017

AGU Fall Meeting Abstracts, Dec 1, 2011

ABSTRACT M. Karaoulis (1), D.D. Werkema (3), A. Revil (1,2), A., B. Minsley (4), (1) Colorado Sch... more ABSTRACT M. Karaoulis (1), D.D. Werkema (3), A. Revil (1,2), A., B. Minsley (4), (1) Colorado School of Mines, Dept. of Geophysics, Golden, CO, USA. (2) ISTerre, CNRS, UMR 5559, Université de Savoie, Equipe Volcan, Le Bourget du Lac, France. (3) U.S. EPA, ORD, NERL, ESD, CMB, Las Vegas, Nevada, USA . (4) USGS, Federal Center, Lakewood, 10, 80225-0046, CO. Abstract We propose 2D and 3D forward modeling and inversion package for DC resistivity, time domain induced polarization (IP), frequency-domain IP, and seismic refraction data. For the resistivity and IP case, discretization is based on rectangular cells, where each cell has as unknown resistivity in the case of DC modelling, resistivity and chargeability in the time domain IP modelling, and complex resistivity in the spectral IP modelling. The governing partial-differential equations are solved with the finite element method, which can be applied to both real and complex variables that are solved for. For the seismic case, forward modeling is based on solving the eikonal equation using a second-order fast marching method. The wavepaths are materialized by Fresnel volumes rather than by conventional rays. This approach accounts for complicated velocity models and is advantageous because it considers frequency effects on the velocity resolution. The inversion can accommodate data at a single time step, or as a time-lapse dataset if the geophysical data are gathered for monitoring purposes. The aim of time-lapse inversion is to find the change in the velocities or resistivities of each model cell as a function of time. Different time-lapse algorithms can be applied such as independent inversion, difference inversion, 4D inversion, and 4D active time constraint inversion. The forward algorithms are benchmarked against analytical solutions and inversion results are compared with existing ones. The algorithms are packaged as Matlab codes with a simple Graphical User Interface. Although the code is parallelized for multi-core cpus, it is not as fast as machine code. In the case of large datasets, someone should consider transferring parts of the code to C or Fortran through mex files. This code is available through EPA's website on the following link http://www.epa.gov/esd/cmb/GeophysicsWebsite/index.html Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy.

Symposium on the Application of Geophysics to Engineering and Environmental Problems 2015, Mar 26, 2015

Ground Water, May 23, 2017

The time-lapse joint inversion of geophysical data is required to image the evolution of oil rese... more The time-lapse joint inversion of geophysical data is required to image the evolution of oil reservoirs during production and enhanced oil recovery, CO2 sequestration, geothermal fields during production, active volcanoes, and the evolution of contaminant plumes. Joint inversion schemes reduce the uncertainty of the model in each monitoring stage, while time lapse inversion algorithms reduce time-related artifacts. There are several approaches that are possible to perform the time-lapse joint inverse problem. In this work, we select the structural joint inversion approach of Meju and Guallardo (the inversion looks for models with strcutural similarities) to perform the joint inversion of DC resistivity and seismic data. Timelapse inversion is performed with an actively timeconstrained (ATC) approach. In this approach, the subsurface is defined as a space-time model. All the snapshots are inverting together assuming a regularization over time of the sequence of snapshots. Still, this approach is flexible enough to allow for abrupt time-related changes in the areas where there are significant indications that these changes are effectively occurring. At the same time, the ATC approach removes inversion artefacts corresponding to spatial artefacts that are randomly distributed over time. We show the advantage of combining strcutural joint inversion and time-lapse inversion using a synthetic case corresponding to cross-hole seismic and DCresistivity data. We show that this approach reduces strongly artefact both with respect to individual inversion of the resistivity and seismic datasets and also with respect to the joint inversion of both data sets at each time step.

Journal of Applied Geophysics, 2018

Geophysical Journal International, Aug 17, 2011

2015 AGU Fall Meeting, Dec 17, 2015

Symposium on the Application of Geophysics to Engineering and Environmental Problems 2021, Jun 11, 2021

Hydrological Processes, 2017

Groundwater flow advects heat, and thus, the deviation of subsurface temperatures from an expecte... more Groundwater flow advects heat, and thus, the deviation of subsurface temperatures from an expected conduction‐dominated regime can be analysed to estimate vertical water fluxes. A number of analytical approaches have been proposed for using heat as a groundwater tracer, and these have typically assumed a homogeneous medium. However, heterogeneous thermal properties are ubiquitous in subsurface environments, both at the scale of geologic strata and at finer scales in streambeds. Herein, we apply the analytical solution of Shan and Bodvarsson (), developed for estimating vertical water fluxes in layered systems, in 2 new environments distinct from previous vadose zone applications. The utility of the solution for studying groundwater‐surface water exchange is demonstrated using temperature data collected from an upwelling streambed with sediment layers, and a simple sensitivity analysis using these data indicates the solution is relatively robust. Also, a deeper temperature profile re...