AmericanChemicalSociety.com

Results: The first year of this study has focused on collecting gravity and magnetic 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 were hired in January 2009 to assist with the research. One student, Mr. Niti Mankhemthong, has a geophysics background and has been focused on the assembly of existing geological and geophysical information for building our models, as well as the collection of field data. The second student, Mr. Rolando (Ron) Cardenas, is a computational science major who has focused on the development and refinement of software required for the project. We have collected information on topography, well logs, magnetic and gravity data (with assistance from Dr. Richard Saltus of the U.S. Geological Survey), geologic information (with assistance from Dr. Peter Haeussler of the U.S. Geological Survey) and bathymetry. An airborne LiDAR survey was recently flown over a portion of our study area, as well as an airborne gravity survey, and we are awaiting the release of these data, hopefully by the end of the year. Our greatest effort this year was devoted to the collection of new gravity data. The existing data for the region had been collected in the 1960’s and 1970’s at a spacing of 5 to 10 km, not providing the density or quality required for accurate modeling of the BRFS structure. The first data collection trip occurred in late March-early April 2009. Mr. Galen Kaip, staff research specialist, assisted by MS graduate student Brian Eslick, participated in the first trip. The trip focused on establishing good GPS and gravity base sites, locating positions of older survey points, and taking gravity readings in areas that would be either inaccessible or experience heavy traffic noise during the summer. Ground vibration noise from eruptions of Mt. Redoubt in late March hampered our ability to gather data for several days, but we were able to collect over 200 gravity readings on the quieter days. Most data were collected at 500 m spacings, and some were collected at 50 m spacings over select glacial deposits in an effort to better determine the density variations within these deposits. In May 2009 the PI traveled to the field site and was able to locate several of the benchmarks that served as gravity base stations in the 1960’s and 1970’s (based on information from colleagues at the University of Alaska-Fairbanks who had occupied some of the sites for gravity and GPS studies (unpublished) in 2000-2002). She was able to take gravity readings to tie the March-April 2009 survey to the older surveys, as well as to tie to an absolute gravity station located in the central Kenai Peninsula. The majority of the field work took place during three weeks in June 2009 where Mr. Mankhemthong, assisted by Mr. Eslick, collected over 350 additional gravity readings. They also collected hand samples of outcropping rock formations along the survey lines in order to make laboratory determinations of density. Another group from the University of Texas at El Paso was in south-central Alaska in July and August 2009 mapping structural geology along the BRFS as part of a separately funded project. Thus we will be able to use the results of their study to help constrain our geologic models. The remainder of the summer was devoted to reducing GPS and gravity data, tying gravity data from various surveys together, and making preliminary estimates of density variation in the shallow subsurface. Mr. Mankhemthong has submitted an abstract for the fall 2009 meeting of the American Geophysical Union to present these preliminary results. Software for computing the gravity response from a geologic model has been completed. The computation is based on contributions of layers that are represented by gridded elevations, along with gridded densities. These are done over a limited geographic area, and then the contributions of each area are summed. This approach has advantages of allowing rapid updates to model changes in small areas, optimization of computation effort with distance from the gravity stations, and highly parallelized computation structure. Code and data structures to include magnetic and gravity gradiometry measurements have been included, but are not yet tested. Software for the inverse problem (modification of the geologic model based on the potential field measurements) takes 90% of its code from the forward model. The numerical derivative code has been written, and we are testing analytic derivatives now to improve speed and accuracy. 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 are taking her graduate level course in geophysical inverse theory. The PI has recently participated with faculty within the computational science program to write an NSF proposal to fund graduate fellowships for computational science students to conduct interdisciplinary research on multi-physics problems. Impact on Career of Students: Two graduate students in geophysics have learned how to run a successful field campaign to collect gravity data and perform initial processing of the data. Mr. Eslick will be applying the skills he acquired in Alaska to a gravity study of faulting/basin formation in Death Valley, California. These two students will learn how to build geologic models for the analysis of potential field data and will make presentation at upcoming scientific meetings. The computational science student is developing a knowledge base in geophysics, in addition to applying his programming skills to a new set of applications.