Geophysics of Rio Grande Basins
Geophysical Methods and Techniques
Different rock units and the fluids contained in them can be characterized by their physical properties, such as density, magnetization, and electrical resistivity. Geophysicists use indirect methods to measure the differences in underground physical properties. This allows them to probe the subsurface without having to drill a hole. For example, measuring the variations in the Earth's gravity or magnetic field at different places gives information on the density or magnetization of the rocks underground. Measuring the effects of an electric current transmitted through the ground gives clues to the electrical resistivity of the subsurface materials, a measure of how well or how poorly the materials and their fluids conduct electricity.
Geophysical interpretations require integration with geologic mapping and topical studies, seismic data, hydrologic models, and drillhole information. Our studies use geophysical data and techniques that include:
- High-resolution aeromagnetic
- Airborne time-domain electromagnetic
- Ground-based magnetotelluric and audio-magnetotelluric
Why Gravity Data?
Gravity data are particularly effective in determining the subsurface configuration of basins within the Rio Grande rift owing to the generally large contrast in density between the low-density basin fill (Santa Fe Group sediments) and the moderate- to high-density bedrock composing the basin floor and sides (pre-Miocene sedimentary rocks and Precambrian basement). In addition, gravity data can be used to determine the location of basement uplifts and other major structures.
Why Aeromagnetic Data?
Aeromagnetic data are geophysical data acquired from aircraft that measure the subtle variations in the Earth's magnetic field due to differences in the magnetic properties of the underlying rocks. Although aeromagnetic data are insensitive to the presence of water, differing magnetic properties of certain rock types can be detected and used to infer many aspects of the subsurface geology that control the presence, quality, and flow of ground water. Knowledge of the magnetic properties as well as volume and depth of rock bodies are important for understanding the geologic sources of aeromagnetic anomalies. Generally, sedimentary rocks and sediments are much less magnetic than igneous and metamorphic rock types, but all rock types can produce anomalies when they are juxtaposed, such as at faults.
Why time-domain electromagnetic data?
Electromagnetic (EM) methods are used to map the electrical resistivity (or its inverse, electrical conductivity) of the subsurface. Resistivities of sediments are determined by rock porosity, the electrical resistivity of the pore substance, and the presence of certain electrically conductive minerals, such as clays. Mapping resistivity variations can provide a framework for predicting hydrologic conditions in areas less explored by drilling. Resistivity maps can provide direct input to ground-water flow models that are critical to water management agencies.
When correlated with lithologic and geophysical borehole logs, airborne time-domain electromagnetic (TEM) methods can determine changes in the electrical resistivity with depth that are related to variations in grain size and hydraulic properties. These geophysical results can be used to place spatial limits on coarse-grained aquifers, especially the axial sand and gravel deposits of the ancestral Rio Grande, and can help delineate facies changes within the basin fill that may control local and regional ground water flow.
Why magnetotelluric data?
The magnetotelluric (MT) method is a passive-surface electromagnetic geophysical technique that measures variations in the Earth's natural electromagnetic field to investigate the electrical resistivity structure of the subsurface from depths of tens of meters to tens of kilometer. The primary purpose of the MT surveys was to map changes in electrical resistivity with depth that are related to differences in various rock types that help control the properties of aquifers in the region. The MT method is well suited for studying complicated geological environments because the electric and magnetic fields are sensitive to vertical and horizontal variations in resistivity. The method is capable of establishing whether the electromagnetic fields are responding to subsurface rock bodies of effectively 1-, 2-, or 3-dimensions.
Brief description of the geophysical methods used and their hydrogeologic targets.
|Geophysical Method||Geophysical Measurement||Associated Physical Property||Type of Geophysical Map, Units||Hydrogeologic Target|
|Gravity Method||Ground measurements of variations in the Earth's gravity field||bulk rock density||Gravity anomaly map, milligals (mGal)||thickness of the Santa Fe Group|
|Aeromagnetic Surveys||Airborne measurements of variations in the Earth's magnetic field||total rock magnetization||Aeromagnetic anomaly map, nanoTeslas (nT)||faults within the Santa Fe Group buried igneous rocks|
|Airborne time-domain electromagnetic (TEM) Surveys||Airborne monitoring of the time-varying effects of shutting off an electrical current induced in the earth||electrical resistivity, or its inverse, electrical conductivity||Electrical resistivity maps for different depth slices, ohm-meters (ohm-m)||permeability of sediments related to grain-size variations|
|Ground-based magnetotelluric and audio-magnetotelluric Method||Measures variations in the Earth's natural electromagnetic fields using natural sources such as lightning strikes||electrical resistivity, or its inverse, electrical conductivity||Two-dimensional finite-element resistivity models (distance v. depth)||faults within the Santa Fe Group lithologic or structural boundaries|