Reports: AC848312-AC8: Constraints on the Structure of the Border Ranges Fault System, South-Central Alaska from Integrated 3-D Inversion of Gravity/Magnetic Data

Diane I. Doser , University of Texas (El Paso)


The no-cost extension year of this study focused on collecting additional gravity data for the region of the northwestern Border Ranges Fault System (BRFS) to constrain the structure of the BRFS and determine its relation to the Cook Inlet Basin and Susitna basins, important oil and gas producing region of south-central Alaska, as well as the Castle Mountain fault, a seismically active structure located just north of the BRFS.  Three doctoral students and one undergraduate student were involved in the research project during 2010-2011.  One doctoral student, Mr. Niti Mankhemthong, has a geophysics background and has focused on the assembly of existing geological and geophysical information for building our models, as well as the collection of field data, and developing preliminary 2-D geologic models.  The second doctoral student, Mr. Rolando (Ron) Cardenas, is a computational science major who has focused on the development and refinement of software required for the project.  A PhD level student, Mr. Pawan Budhathoki, assisted Mr. Mankhemthong in the collection of gravity data during the 2011 field season.  Mr. Shane Schinagel, an undergraduate student in geophysics, assisted with making maps and compiling geologic information.

Efforts for the final year were the collection of new gravity data, analysis of two dimensional profiles, and continued software development.  A data collection trip occurred in August 2011 where Mr. Mankhemthong, assisted by Mr. Budhathoki, collected ~450 new gravity and GPS readings in the region extending from Palmer to Houson, Alaska.  They also collected hand samples of outcropping rock formations along the survey lines in order to make laboratory determinations of density. 

Mr. Mankhemthong has completed a manuscript related to an inversion method we have developed to estimate near surface bulk density in regions of thick glacial alluvium where hand samples are not available for analysis and will be submitting it to the Journal of Environmental and Engineering Geology in September 2011.  He is also finalizing 2-D geologic models of the region and presented preliminary results of this work at the fall 2010meeting of the American Geophysical Union.  He successfully defended his dissertation proposal in August 2011 and hopes to finish his work by summer 2012.  He is currently building 3-D models for software developed by doctoral student Ron Cardenas.

The software inverts the free-air gravity anomaly using an a priori 3-D geologic model built from layers that are represented by gridded elevations, along with gridded densities. The free-air gravity is computed using vertical line element approximations. These are done over a limited geographic area, and then the contributions of each area are summed. This approach allows rapid updates to model changes in small areas, optimizes of computation effort with distance from the gravity stations, and gives a highly parallelized computation structure. Uncertainties on gravity anomalies and geologic models are also included in the inversion process to increase stability and provide meaningful estimates of covariance and information density matrices.  Mr. Cardenas has tested his model on an area near Skilak Lake on the Kenai Peninsula where there are extreme topographic and density variations.  Code and data structures to include magnetic measurements have been included, but are not yet tested.   Mr Cardenas also successfully defended his dissertation proposal (based on the software development) in July 2011.

Impact on Career of PI

            The funded research has led to the PI establishing stronger ties with the graduate program in computational science.  One PhD student in computational science is participating in the research.  The PI has several graduate students conducting gravity studies in the El Paso-Juarez region to study fault control of local aquifers and to better model how the shape of local basins may influence strong ground motion during large earthquakes.  One Mexican student working on this project has received funding from CONACYT (the Mexican equivalent of the US’s NSF) for tuition support for doctoral studies that will involve use of the 3-D modeling software.

Impact on Career of Students

            Graduate students in geophysics/geology have learned how to run a successful field campaign to collect gravity data and perform initial processing of the data.   Mr. Pawan Budhathoki will apply skills he learned in the Alaska study to a planned gravity survey of northeastern El Paso.  Mr. Slade Jones, who participated as a field assistant last summer (2010) was employed by a major oil company in the summer of 2011.  Mr. Mankhemthong continues to assist in the training of other graduate students who are planning gravity surveys as part of their research.  He also attended the 2010 meeting of the American Geophysical Union to present his results.  Mr. Cardenas is developing a knowledge base in geophysics, in addition to applying his programming skills to a new set of applications.  He will be presenting results of his work at the 2011 meeting of the American Geophysical Union.

Converging on Alaska
Dr. Ridgway
Polyene Synthesis
Dr. O'Neil
Dr. Bali
Faults and Fluid Flow
Dr. Huntington