Modeling of self-potential anomalies near vertical dikes

D.V. Fitterman

doi:10.1190/1.1441456

Home/Archive/USGS

USGS · 70011391

Modeling of self-potential anomalies near vertical dikes

D.V. Fitterman

View original source ↗Export metadata ↓

Abstract

The self-potential (SP) Green's function for an outcropping vertical dike is derived from solutions for the dc resistivity problem for the same geometry. The Green's functions are numerically integrated over rectangular source regions on the contacts between the dike and the surrounding material to obtain the SP anomaly. The analysis is valid for thermoelectrical source mechanisms. Two types of anomalies can be produced by this geometry. When the two source planes are polarized in opposite directions, a monopolar anomaly is produced. This corresponds to the thermoelectrical properties of the dike being in contrast with the surrounding material. When the thermoelectric coefficients change monotonically across the dike, a dipolar anomaly is produced. In either case positive and negative anomalies are possible, and the greatest variation in potential will occur in the most resistive regions. Examples of the effect of changing different model parameters are given for sources that have constant intensity throughout the rectangular source regions. For these patch models the depth to the top of the source region is approximately equal to the distance between the minimum (or maximum) of the anomaly outside of the dike and the edge of the dike. Field data collected over a hot intrusive fissure are presented which have been modeled by the technique described.

Explore related subjects

Geophysics Article

Keep this discovery

Explore connections, maps & timelines

BibTeX RIS

D.V. Fitterman. 1983. Modeling of self-potential anomalies near vertical dikes. https://doi.org/10.1190/1.1441456

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS2023

Cooperative joint inversion of magnetotelluric and microseismic data for imaging the Geysers geothermal field, California, USA

The Geysers geothermal field located in northern California, USA, is the world’s largest electricity-generating geothermal facility. To delineate the spatio-temporal distribution of reservoir steam and recharge water, we have collected microseismic and magnetotelluric (MT) data using a dense array of stations in 2021. The microseismic and MT data have been inverted together using a 3D cooperative joint inversion workflow. The joint inversion exploits a cross-gradient structural constraint because electrical conductivity structures observed in the geothermal field are strongly correlated with

California↗

USGS2022

Inversion of induced polarization-affected towed-transient electromagnetic data in a lateritic regolith geology: A case study from western Tanzania

For several decades, induced polarization (IP) effects on transient electromagnetic (TEM) responses have been observed. These effects can manifest as late-time negative transients or as rapidly decaying curves and are usually associated with highly polarizable bodies. If neglected, IP effects can lead to erroneous resistivity models. Recent work allows IP effects to be incorporated into the inversion of TEM data on a more routine basis. In a recent field survey in western Tanzania, strongly IP-affected TEM signals are observed using a towed-transient electromagnetic (tTEM) system. The survey have been carried out to locate drinking water resources in a weathered regolith setting. In these settings, an inversion of tTEM data using a resistivity-only forward model (i.e., IP neglected) cannot fit the data and severely limits the value of the TEM data for hydrogeologic interpretation. To account for IP effects, we have applied a modified version of the Cole-Cole model called the maximum phase angle (MPA) model to invert IP-affected tTEM data. The MPA model incorporates four inversion model parameters: resistivity ( ⁠ ρ "> ρ ), MPA ( ⁠ ϕ max "> ϕ max ⁠ ), relaxation time ( ⁠ τ "> τ ), and frequency exponent ( ⁠ c "> c ). The MPA model fits the data well and improves the reliability of the resistivity model. In much of the surveyed region, the inverted models using MPA display a three-layer system consisting of an upper resistive laterite layer of varying thickness and an intermediate polarizable conductive unit overlying more resistive weathered basement rocks. The conductive polarizable layer is interpreted as a chemically weathered saprolite separating the surficial and deeper aquifers. Overall, tTEM inversion results provide a local understanding of groundwater systems, especially in such regions with very limited subsurface knowledge.

Kaguruka, Kitagata↗

USGS2022

Evidence-based guidelines for protective actions and earthquake early warning systems

Earthquake early warning systems (EEW) are becoming increasingly available or in development throughout the world. With public alerting in Mexico, Japan, Taiwan, and parts of the United States, it is important to provide evidence-based recommendations for protective action so people can protect themselves when they receive an alert. Best-practice warning communication research suggests that providing a protective action will increase the efficacy of the message. However, given the diversity of earthquakes and building types, as well as social and cultural contexts where these systems exist, the question is: what is the best protective action to recommend? The answer lies in maximizing life-saving protective actions during an earthquake event requires both contextually relevant messaging and widespread public education about appropriate protective actions under a range of conditions. By researching previous earthquake injury literature, examining current best practices and public education campaigns, key protective actions may be determined and used to increase the life-saving potential of earthquake early warning systems.

Geophysics↗