Interdisciplinary Methods and Applications in Geophysics (IMAGe) Active

Science Issue and Relevance

Mineral resource studies are fundamentally subsurface studies — from the need to characterize the geological structures that host important deposits to better understanding the landscape and environmental impacts of resource development, sophisticated geophysical methods and instruments are needed. Constantly evolving modern geophysical techniques, in particular airborne geophysical methods, present new opportunities for large-scale subsurface characterization, but also require new computational tools and software algorithms that translate geophysical datasets into meaningful interpretations that can support management decisions. The Mineral Resources Program, and the USGS as a whole, has a continuing need for the development of state-of-the-art geophysical methods and instruments that have application to mineral resource and mineral environmental studies.

Sources/Usage: Public Domain.

Methods to Address Issue

The IMAGe project focuses on the development of novel geophysical techniques that improve our ability to understand Earth's subsurface, with broad relevance to the Mineral Resources Program and the USGS Science Strategy. Our goal is to develop and maintain state-of-the-art geophysical capabilities that support the diverse science needs of USGS projects that aim to meet the challenges of the 21st century by helping to improve the economic and environmental health and prosperity of people and communities across the Nation and around the world.

The main objectives of our research include:

Frontier Geophysical Methods: Develop novel data analysis and interpretation algorithms that help maintain USGS at the forefront of airborne geophysics. Activities will include:

• develop expertise in the use of fixed-wing airborne electromagnetic (AEM) systems which are effective for regional-scale mapping activities;
• evaluating tools for the analysis of airborne induced polarization effects, a cutting-edge approach to identifying mineralized geologic targets;
• evaluating nodal seismometers for passive seismic studies and developing open source workflows to facilitate use and analysis of these data;
• evaluation of compensation and calibration methods for UAS-based aeromagnetic surveys.

Computational Geophysics: Support maintenance of existing software codes developed over many years by Geology, Geophysics, and Geochemistry Science Center (GGGSC) researchers that are used for the processing and analysis of airborne potential field and electromagnetic datasets. Develop new advanced computational capabilities including cooperation with the USGS Advanced Research Computing (ARC) group to facilitate the implementation of geophysical codes on the USGS high-performance computing cluster (Yeti). Current focus includes:

• developing new algorithms for high performance and high throughput parallel computing needs in cooperation with GGGSC researchers,
• facilitating publication of new and existing codes through the USGS Github group, 
• facilitate access and use of the Yeti high-performance computing system,
• developing hdf5 and netCDF data standards for geophysical datasets, along with open source workflows to facilitate processing, visualization, and archival of datasets.

Quantitative geologic inferences from integrated geophysical methods: Many geologic mapping targets are best resolved through the use of integrated geophysical methods that are sensitive to different physical characteristics. We will continue research into methods for combined geologic interpretations using multiple geophysical methods. Examples include: models of hydrothermally altered regions based on magnetic and resistivity data collected at Mt. Iliamna, AK, and models of the Stillwater layered mafic intrusion based on gravity, magnetic, and seismic reflection data.

Uncertainty quantification: There are two fundamental sources of uncertainty in developing geological interpretations from geophysical data. First is the underlying uncertainty in geophysical properties (e.g. electrical resistivity, magnetic susceptibility, density, or seismic velocity) given a specific type of geophysical survey along with measurement errors. The second is the uncertainty in the relationship between geophysical and geological properties. We will address multiple aspects of uncertainty quantification for different types of geophysical data. Work will include:

• Continued development of GeoBIPy open source software for uncertainty quantification of airborne electromagnetic data, and extension to other data types
• Probabilistic geologic modeling to quantify the uncertainty in geological model structure given prior expert knowledge, geophysical data, and other existing observations. One application will be to investigate uncertainty in the shape and location of carbonatite deposits in the Mountain Pass project based on gravity and magnetic data. We will also build tools to quantify geological model uncertainty based on airborne electromagnetic datasets along with geostatistical simulation tools.
• An open-source toolbox for physical property relationships and uncertainty. We will develop an source software application using Python that will allow users, in a stochastic framework, to estimate physical properties such as temperature, water content, clay content, partial melt, or porosity based on geophysical parameters such as resistivity, density, or magnetic susceptibility.

Facilities

Geophysical Test Site - In 2015, we began to establish a geophysical test site at the USGS Boulder Geomagnetic Observatory. We are continuing to collect new baseline datasets for a broad suite of geophysical instruments. The test site will be used to calibrate and assess the accuracy of in-house and commercial geophysical tools, to develop and test new instrumentation, and for training Geology, Geophysics, and Geochemistry Science Center scientists and partners.

Previous Project Activities

Geophysical Instrument Development - The USGS and its Mineral Resources Program required the development of new geophysical instrumentation, and the maintenance of existing geophysical instrumentation.

Alaska Geophysical Data Processing - Alaska is a geologic frontier with substantial mineral resource potential. Complex geology, limited outcrop, and high logistical costs make airborne geophysics essential for efficient reconnaissance. We performed an advanced analysis of electromagnetic data to map geologic trends, structural geologic and tectonic patterns, and identify key lithologies for direct integration with geologic framework and mineral potential studies.

Monthly Webinar Series - We coordinated an internal, monthly webinar series to highlight cutting-edge geophysical research and applications related to our project's research, as well as other innovative work related to remote sensing and geochemistry by Geology, Geophysics, and Geochemistry Science Center scientists and other USGS colleagues.

Petrophysics Laboratory - The Petrophysics Laboratory is a multi-user facility that provides physical property data of earth materials for geophysical research. The Laboratory is equipped to make physical property measurements on rocks and sediment including density, magnetic properties, electrical properties, and radiometric properties. Knowledge of a site's physical properties is an important asset in planning geophysical surveys; particularly in selection of an appropriate method and in selecting optimum survey parameters such as array size and frequency. Knowing the range of physical properties for a given area help refine geophysical interpretations and provide justification for constraining model input parameters. The integration of physical property data with mineralogical and geochemical data provides an important link between the rocks and ores to their observed geophysical signatures.

Geophysical Instrumentation Laboratory - The Geophysical Instrumentation Laboratory provides electronic, software, and mechanical design and fabrication services for projects within the Geology, Geophysics, and Geochemistry Science Center.

• Machine shop with tools for wood, metal, plastic, and fiberglass fabrication
• Expertise with LabVIEW software for data acquisition and signal processing
• Electronic design, analysis and fabrication capability includes signal filtering, digital logic and circuit design, and more

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