Blog | Advanced Geosciences Inc

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 ...

1D Geophysical Resistivity Survey: Vertical Electrical Sounding

Posted by markus on Dec 16, 2016

Throughout most of the 20th century, Vertical Electrical Sounding (VES) was the dominant geophysical resistivity method. It has been used all over the world for three primary purposes: geotechnical investigation, groundwater exploration, and mineral exploration. VES is performed using either the Wenner electrode configuration described in the ASTM G57 standard or using the Schlumberger electrode configuration. (The Schlumberger method is most commonly used for groundwater and mineral exploration, because it is less labor intensive than the Wenner method.)

Below, we’ll describe VES’s...

In-Depth Descriptions Of 3 Soil Resistivity Testing Techniques

Posted by markus on Nov 22, 2016

Soil resistivity testing is performed for a variety of important applications, including:

  • Testing grounding grids: Grounding grids are primarily used in industrial settings—like electrical substations—where expensive machinery needs to be protected. The U.S. National Electric Code sets forth that every substation must have at least one grounding rod to protect equipment from lightning damage and to ensure that a stray current from the neutral doesn’t electrocute a person nearby.
  • Cathodic protection: Cathodic protection is a way to protect underground or...

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