Imaging an Active Normal Fault in Alluvium by High-Resolution Magnetic and Electromagnetic Surveys

Femina, P. C. La; Connor, Charles B.; Stanatakos, John A.; Farrell, D. A.

doi:10.2113/8.3.193

Cited by 9 publications

(7 citation statements)

References 22 publications

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“…[19] A peak-to-peak amplitude of over 1000 nT ( Figure 3b and Table 1) is consistent with vertical separation of basalt that has a strong magnetization [e.g., La Femina et al, 2002]. We consider the magnetic source to be a lava flow unit of high bulk magnetization beneath scoria of low bulk magnetization with some faulting.…”

Section: Magnetic Datamentioning

confidence: 60%

“…Magnetic data are particularly useful for detecting structures in volcanic terrains, due to the high contrast in magnetic properties between basaltic lava, scoria, and alluvium [ Stamatakos et al , 1997]. Magnetic profiles and maps have been used successfully to infer geological features such as faults at a number of sites [e.g., Jones‐Cecil , 1995; Connor et al , 1997; La Femina et al , 2002], even in granitic areas where the magnetic contrast is relatively small [ McPhee et al , 2004]. On basaltic volcanoes, magnetic anomalies associated with faults are often on the order of 100–1000 nT, two to three orders of magnitude larger than the magnetic anomalies induced through electrokinetic effects associated with fluid flow [ Zlotnicki and Le Mouel , 1990; Adler et al , 1999].…”

Section: Methods For Delineating Flow Pathsmentioning

confidence: 99%

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Integrated geophysical and hydrothermal models of flank degassing and fluid flow at Masaya volcano, Nicaragua

Pearson

Kiyosugi

Lehto

et al. 2012

Geochem Geophys Geosyst

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[1] We investigate geologic controls on circulation in the shallow hydrothermal system of Masaya volcano, Nicaragua, and their relationship to surface diffuse degassing. On a local scale ($250 m), relatively impermeable normal faults dipping at $60 control the flowpath of water vapor and other gases in the vadose zone. These shallow normal faults are identified by modeling of a NE-SW trending magnetic anomaly of up to 2300 nT that corresponds to a topographic offset. Elevated SP and CO 2 to the NW of the faults and an absence of CO 2 to the SE suggest that these faults are barriers to flow. TOUGH2 numerical models of fluid circulation show enhanced flow through the footwalls of the faults, and corresponding increased mass flow and temperature at the surface (diffuse degassing zones). On a larger scale, TOUGH2 modeling suggests that groundwater convection may be occurring in a 3-4 km radial fracture zone transecting the entire flank of the volcano. Hot water rising uniformly into the base of the model at 1 Â 10 À5 kg/m 2 s results in convection that focuses heat and fluid and can explain the three distinct diffuse degassing zones distributed along the fracture. Our data and models suggest that the unusually active surface degassing zones at Masaya volcano can result purely from uniform heat and fluid flux at depth that is complicated by groundwater convection and permeability variations in the upper few km. Therefore isolating the effects of subsurface geology is vital when trying to interpret diffuse degassing in light of volcanic activity.

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Section: Magnetic Datamentioning

confidence: 60%

Section: Methods For Delineating Flow Pathsmentioning

confidence: 99%

Integrated geophysical and hydrothermal models of flank degassing and fluid flow at Masaya volcano, Nicaragua

Pearson

Kiyosugi

Lehto

et al. 2012

Geochem Geophys Geosyst

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“…Indeed in numerous cases the ground based surveys will be in response to the need for higher resolution studies of certain anomalies in the aeromagnetic data in order to provide a more constrained interpretation (Blakely et al, 2005). Ground based magnetic profiling has been highly successful in detecting and modelling faults and fractures in sediments (La Femina et al, 2002) and bedrock (Gibson et al, 1996, Dutta et al, 2006, features where secondary hydraulic permeability can often enhance groundwater flow. Geological bodies, such as igneous dykes, can exert controls on groundwater flow directions where they cut across aquifers.…”

Section: Ground Based Magnetic Measurementsmentioning

confidence: 99%

Advancing process‐based watershed hydrological research using near‐surface geophysics: a vision for, and review of, electrical and magnetic geophysical methods

Robinson

Binley

Crook

et al. 2008

Hydrological Processes

243

169

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Abstract:We want to develop a dialogue between geophysicists and hydrologists interested in synergistically advancing process based watershed research. We identify recent advances in geophysical instrumentation, and provide a vision for the use of electrical and magnetic geophysical instrumentation in watershed scale hydrology. The focus of the paper is to identify instrumentation that could significantly advance this vision for geophysics and hydrology during the next 3-5 years. We acknowledge that this is one of a number of possible ways forward and seek only to offer a relatively narrow and achievable vision. The vision focuses on the measurement of geological structure and identification of flow paths using electrical and magnetic methods. The paper identifies instruments, provides examples of their use, and describes how synergy between measurement and modelling could be achieved. Of specific interest are the airborne systems that can cover large areas and are appropriate for watershed studies. Although airborne geophysics has been around for some time, only in the last few years have systems designed exclusively for hydrological applications begun to emerge. These systems, such as airborne electromagnetic (EM) and transient electromagnetic (TEM), could revolutionize hydrogeological interpretations. Our vision centers on developing nested and cross scale electrical and magnetic measurements that can be used to construct a three-dimensional (3D) electrical or magnetic model of the subsurface in watersheds. The methodological framework assumes a 'top down' approach using airborne methods to identify the large scale, dominant architecture of the subsurface. We recognize that the integration of geophysical measurement methods, and data, into watershed process characterization and modelling can only be achieved through dialogue. Especially, through the development of partnerships between geophysicists and hydrologists, partnerships that explore how the application of geophysics can answer critical hydrological science questions, and conversely provide an understanding of the limitations of geophysical measurements and interpretation.

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“…An alternative inversion model was constructed specifying a higher range for the intensity of magnetization, 10 − 17 Amp m −1 , and a shallower range for prism depth. This intensity of remanent magnetization corresponds to the range of measured values in some outcropping YMR basalts, including basalts of SE Crater Flat and the Little Cones (Champion, 1991;Stamatakos et al, 1997;La Femina et al, 2002). Specifically, depth to prism base is allowed to vary 60 − 150 m, while depth to prism top varies 10 − 150 m, again with the requirement that depth-to-top not exceed depth-to-base.…”

Section: Statistics In Volcanologymentioning

confidence: 96%

High-Resolution Ground-Based Magnetic Survey of a Buried Volcano: Anomaly B, Amargosa Desert, NV

George¹,

McIlrath²,

Farrell³

et al. 2014

SIV

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Aeromagnetic surveys over the Amargosa Desert, Nevada, have revealed the presence of several magnetic anomalies that have been interpreted to be caused by buried volcanoes; many of these anomalies have been confirmed by drilling. We present data collected from a high-resolution, ground-based magnetic survey over Anomaly B, the largest of these anomalies, that reveal details about a buried crater and its associated lava flow, not observed in the aeromagnetic surveys. These details provide insight into the nature of the eruption and volume of this buried volcano. Results from non-linear inversion demarcate a crater with a diameter of approximately 700 m and a base approximately 150 m below the ground surface. Coupled with well log data, the inversion results suggest a total volume for the Anomaly B crater area and associated lava flows of approximately 1.0 ± 0.4 km 3 , based on an estimated lava flow field area of 24 km 2 and a lava thickness of 42 ± 15 m. A workflow is presented for processing such large ground-based magnetic data sets with attendant GPS data, filtering these data and constructing maps and models using the provided PERL scripts.

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Imaging an Active Normal Fault in Alluvium by High-Resolution Magnetic and Electromagnetic Surveys

Cited by 9 publications

References 22 publications

Integrated geophysical and hydrothermal models of flank degassing and fluid flow at Masaya volcano, Nicaragua

Integrated geophysical and hydrothermal models of flank degassing and fluid flow at Masaya volcano, Nicaragua

Advancing process‐based watershed hydrological research using near‐surface geophysics: a vision for, and review of, electrical and magnetic geophysical methods

High-Resolution Ground-Based Magnetic Survey of a Buried Volcano: Anomaly B, Amargosa Desert, NV

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