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Results

     The second year of this study has focused on collecting additional gravity data for the region of the Border Ranges Fault System (BRFS) to constrain the structure of the BRFS and determine its control on the formation of the Cook Inlet Basin, an important oil and gas producing region of south-central Alaska.  Two doctoral students and one MS student were involved in the research project during 2009-2010.  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.  An MS level student, Mr. Slade Jones, assisted Mr. Mankhemthong in the collection of gravity data during the 2010 field season.

     The two main efforts this year were the collection of new gravity data and continued software development.  A data collection trip occurred in June and July 2010 where Mr. Mankhemthong, assisted by Mr. Jones, collected ~350 new gravity and GPS readings in the region extending from Anchorage to Palmer, Alaska.  They also collected hand samples of outcropping rock formations along the survey lines in order to make laboratory determinations of density. 

     The remainder of the summer was devoted to reducing GPS and gravity data, tying gravity data from the 2009 and 20010 surveys together, and making preliminary estimates of density variation in the shallow subsurface.  Mr. Mankhemthong has been working on 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.  He presented preliminary results of this technique at the fall 2009 meeting of the American Geophysical Union.  He has also begun developing 2-D models to match select gravity profiles and will present this work at the fall 2010 meeting of the American Geophysical Union.

     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.  Initial tests on synthetic data sets have begun.  Code and data structures to include magnetic and gravity gradiometry measurements have been included, but are not yet tested.

     We are finding the need to develop additional grid manipulation tools to assist in building and modifying the complex geologic models. An example of this sort of complexity is where gravity stations located on a mountain road that parallels a deep glacial lake, cross two reverse faults separating three major terrains.

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.  A total of 3 students from this program took her graduate level course in geophysical inverse theory.  The PI has been asked to participate in a gravity survey related to exploration for geothermal resources on a portion of the Ft. Bliss military reservation in southern New Mexico.  This planned survey would help support students and lead to more opportunities to apply software developed with PRF funding.

Impact on Career of Students

       Three 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. Brian Eslick, an MS student who participated in the 2009 field work, applied the skills he acquired in Alaska to a gravity study of faulting/basin formation in Death Valley, California.  He successfully completed his thesis in the summer of 2010 and now is employed by a major oil company.  Mr. Mankhemthong has participated in several local gravity surveys and now assists in the training of other graduate students who are planning gravity surveys as part of their research.  He also attended the 2009 meeting of the American Geophysical Union to present his results.  Mr. Jones applied his field skills in a study of sand dune morphology at White Sands National Monument in July and August 2010.  Mr. Cardenas is developing a knowledge base in geophysics, in addition to applying his programming skills to a new set of applications.