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Wenner Array: Electrical Resistivity Methods, Part 1
Posted by hasan on Oct 06, 2017
Electrode arrays are different arrangements of electrodes used to perform geophysical resistivity measurements. Electrode arrays were developed in order to make field measurements more efficient and data interpretation easier. In this series, we’ll explore 11 electrode arrays. The first array we’ll discuss is the Wenner array.
What is the Wenner array?
The Wenner electrode array is the simplest of arrays; in it, the four electrodes—A, M, N, and B—are placed in line and spaced equidistant from each other. The two outer electrodes, A and B, are current electrodes,...
The Streaming-Potential Method: Its 2 Main Uses
Posted by markus on Feb 28, 2017
Streaming-potential (SP) is an electrical surveying method used when fluids or electrolytes—typically water—move through porous areas in the ground. This creates a change in voltage that can be read using a voltmeter and two electrodes. Streaming-potential is synonymous with “self-potential” and “spontaneous-potential,” though the latter two designations are used when the cause of the spontaneous voltage is chemical processes associated...
An Overview Of The Self-Potential (Spontaneous-Potential) Method
Posted by markus on Feb 15, 2017
Before we dive into details, let’s start with a primer on three different terms that are abbreviated “SP.” In this article, we’ll be discussing self-potential, which is synonymous with spontaneous-potential. The self-potential method in geophysics refers to an electrical surveying method used for looking at electrical anomalies in the ground. It is primarily used in mineral exploration. Streaming-potential, on the other hand, is caused by a fluid —typically water with dissolved minerals (an electrolyte)—moving in the ground, which causes a change in voltage. This change is detected when a...
A Closer Look At 3D Resistivity Surveys
Posted by markus on Feb 14, 2017
One-dimensional (1D) resistivity surveys are not ideal because they assume that the geological layers are horizontal and homogeneous—which is rarely the case. Two-dimensional (2D) resistivity surveys are an improvement but still an approximation, as they assume the geology continues infinitely in both directions perpendicular to the survey line. While this method is more accurate than the 1D Vertical Electrical...
3D Time-Lapse Surveys: Watching Resistivity Change Over Time
Posted by markus on Feb 13, 2017
The three-dimensional (3D) time-lapse resistivity survey is the holy grail of resistivity surveys. It is an extremely accurate and effective method that allows you to not only see the surveyed area in three dimensions—but also how the resistivity in the surveyed area changes over time.
Method, Technology, & Applications
To perform a 3D time-lapse resistivity survey, you place your electrodes in a grid and record multiple surveys over an extended period of time. Using AGI’s ...
An Introduction To Induced Polarization (IP) Surveying
Posted by markus on Jan 20, 2017
Induced polarization (IP) is the Earth’s capacity to hold an electric charge over time. IP measures the voltage decay curve after the injected current is shut off. The higher the IP, the longer over time the charge is held—IP decays over time, typically a few seconds but sometimes up to minutes, and will eventually disappear. IP is especially useful for mineral exploration applications.
How does induced polarization surveying work?
IP surveying is an exploration method involving the measurement of the slow decay of voltage in the ground following the cessation of an...
Inversion Modeling In Geophysics: The Why & How
Posted by markus on Jan 19, 2017
Inversion is the mathematical process of calculating cause from a set of observations. In resistivity work, it is used to calculate the resistivity of different formations in the ground from a set of readings taken at the surface or between boreholes.
In geophysics, an electrical resistivity survey is conducted to map the subsurface of the earth. The measurements are performed using four electrodes placed in contact with the earth. Two are for injecting a current, and the...
7 Types Of Resistivity Instruments & Equipment You Need (& Why)
Posted by markus on Jan 18, 2017
Have you ever prepared for a vacation, only to arrive at your destination and realize you’d neglected to pack important attire and equipment for your holiday? Recently, in my haste to prepare for a trip, I didn’t make a list and found myself in this very situation—I ended up wearing wool sweaters and jeans in an 80-degree climate.
While this may seem like a silly example—it was just a few days—not knowing what you need to bring to a survey site has much bigger ramifications. If you bring the wrong geophysical instruments and equipment for a survey, you may not get the best data, or...
Detecting Karst At Friesenhahn Cave Using 3D Electrical Resistivity
Posted by markus on Jan 12, 2017
Dr. Keith N. Muhlestein, PG, REM, from the University of Texas at San Antonio’s Environmental Science and Engineering department, used AGI equipment for an electrical resistivity survey to image Karst regions within Friesenhahn Cave in Texas.
Background
Friesenhahn Cave is located near the Balcones Escarpment along the Edwards Plateau of central Texas in northern Bexar County. It is well known as a Pleistocene epoch predator cave. It contains an exceptional variety of megafauna fossils including ...
2D Resistivity Surveys: The Benefits, Limitations, & Technology
Posted by markus on Jan 04, 2017
In the 1980s and early 1990s, geophysicists became largely disenchanted with electrical resistivity because of the inadequacies of Vertical Electrical Sounding (VES). Specifically, the VES method assumes that the imaged geology is horizontally layered and that each layer is homogeneous. Of course, this often isn’t the case, causing some project managers (and their clients) to become fed up with the unusable results.
Around this time, we created the Sting R1—the industry’s first 2D electrical resistivity imaging instrument—and tested the instrument on what is now called ...
