Geophysical Measurements

Currently, I take Environmental Measuring and Monitoring course in KTH. This course is so foreign and I thought, I have a high chance to not working in this area in the future. Even so, it is always good to learn something new and challenging. Furthermore, I really want to understand this course!! Thus, here is what I understand so far.

There are several geophysical measurements namely electrical resistivity method, electromagnetic method, gravity method, etc.

Let’s discuss each of them one  by one 🙂

 

Gravity Method

Main principle: This method measures the differences in gravity field due to difference in rock density. The basis theory of gravity method is Newton’s law of attraction between two point masses m1 and m2. Gravitation Netwon Law said: “The gravitational force between two bodies is a force of attraction whose magnitude is directly proportional to the mass of each object and inversely proportional to the square of the distance between the two.”

Units: Gal (=1cm/s2), mGal, µGal. 1 mGal = 10-3 Gal = 10-3 cm/s2

Density of rock types:

Untitled

 Source: Presentation of Bosse Olofsson in EMM course in KTH

Example of measurement:

Untitled Source: Presentation of Bosse Olofsson in EMM course in KTH

Equipment: Gravimeters

fihjidsi.jpg

Source: http://agungbarus-mahasiswaunsyiah.blogspot.se/

Interpretation: Several corrections have to be done: latitude correction (subtract the normal gravity), free-air correction (increase in height from Earth’s centre, therefore a positive correction is added), bouguer correction (as terrain is assume to be flat and a plate is added), and terrain correction (depends on terrain difficulty)

Untitled

 Source: Presentation of Bosse Olofsson in EMM course in KTH

Advantage: The gravity method is a relatively cheap, non-invasive, non-destructive remote sensing method

Disadvantages: 

Functions: bedrock type and stratigraphy, soil thickness, fractures exploration, hydrocarbon prospecting.

 

Electromagnetic Method

Main principle: EM conductivity surveys measure ground conductivity by the process of electromagnetic induction. The systems work on similar principles consisting of a transmitter and receiver coil spaced at a fixed configuration, but use different operating frequencies to provide a range of depth penetration and resolution for different applications. Low frequency can be effective for finding large underground cavities such as caves and mine workings but are rarely applicable for smaller targets. The high frequency is best for detecting small targets buried at shallow depth, such as chemical waste drums and metal artifacts (http://www.geophysical.biz, 2017).

How it works: A primary electromagnetic field output by the transmitting coil induces a secondary field in the ground. The receiving coil measures the magnitude of the secondary field (quadrature component) and the ratio between primary and secondary fields (in-phase component). Quadrature fields are proportional to ground conductivity, being responsive to bulk changes in lithology, groundwater and ground contamination. The presence of metal produces strong secondary fields, making the in-phase component a useful indicator of the presence of buried metal objects. EM data is typically collected as point readings of ground conductivity or in-phase taken at regular intervals along a survey grid that has been set out over the site area. The spacing of the grid-lines and reading stations is dependent upon the target size. Generally smaller targets require closer survey lines and denser spaced readings (http://www.geophysical.biz, 2017).

Generalized picture of electromagnetic induction prospecting. (Klein and Lajoie 1980; copyright permission granted by Northwest Mining Association and Klein)

Source: https://archive.epa.gov/, 2017

Equipment:

geophysical investigation searching for abandoned mineshafts by EM 31

Source: http://www.geophysical.biz, 2017

Advantages:  rapid and cost effective in comparison to conventional resistivity surveys and no direct contact with ground (http://www.geophysical.biz, 2017).

Weaknesses: not good for a place with a lot of noises and urban places

Functions: Finding voids & solution features in soil and rock, locating abandoned mineshafts, crown holes & subsidence features, identifying bedrock discontinuities & mineralised veining, defining former landfill sites & associated leachate plumes, detecting buried UST’s & dumped chemical waste drums, and mapping soil types and land drainage systems

Ground Penetrating Radar

Main principle: Ground-penetrating radar (GPR) uses in the microwave band (UHF/VHF frequencies (10 MHz to 2.6 GHz)) of the radio spectrum, and detects the reflected signals from subsurface structures. Higher frequencies may provide improved resolution and vice versa.

High frequency (>500 MHz): •Gives detailed information •High accuracy •Small penetration depth (often 0-3 m).

Low frequency (50-500 MHz): •Low accuracy •Long wavelength •High penetration depth (often 10-50 m)

How it works: A GPR transmitter emits electromagnetic energy into the ground. When the energy encounters a buried object or a boundary between materials having different permittivities, it may be reflected or refracted or scattered back to the surface.

Image result for ground penetrating radar

Source: www.worksmartinc.net, 2017

Advantages: Good for dry sandy soils or massive dry materials such as granite, limestone, and concrete because these materials tend to be resistive rather than conductive, and the depth of penetration could be up to 15-metre (49 ft).

Weaknesses: The principal disadvantage of GPR is that it is severely limited by less-than-ideal environmental conditions. Bad for  moist and/or clay-laden soils and materials with high electrical conductivity that cause causes loss of signal strength in which penetration may be as little as a few centimetres. Other than that, rocky or heterogeneous sediments scatter the GPR signal, weakening the useful signal while increasing extraneous noise.

Functions: Geological (stratigraphy, fracture mapping, cavities), exploration (peat, gravel, groundwater levels), environmental (plume mapping, landfills, burried objects), glaciological (ice thickness, snow stratigraphy, ice structures), engineering (road pavement, void detection, location of pipes etc), and archaeology (burried structures, graves).

 

Vertical Electrical Sounding

Main principle: Vertical electrical sounding (VES) is a geophysical method for investigation of a geological medium which based on the estimation of the electrical conductivity or resistivity of the medium. The estimation is performed based on the measurement of voltage of electrical field induced by the distant grounded electrodes (current electrodes) (Wikipedia, 2017).

How it works: The electrodes A and B are current electrodes which are connected to a current source; N and M are potential electrodes which are used for the voltage measurements (Wikipedia, 2017).

Wenner configuration (Source: Wikipedia, 2017)

Schlumberger configuration (Source: Wikipedia, 2017)

Image result for wenner vertical electrical sounding the fieldThe interpretation of the measurements can be performed based on the apparent resistivity values. The depth of investigation depends on the distance between the current electrodes. In order to obtain the apparent resistivity as the function of depth, the measurements for each position are performed with several different distances between current electrodes (Wikipedia, 2017).

 

 

VES measurement with multi-electrode arrays:

Image result for multi electrode arrays for geophysics

Source: http://www.landviser.net

Resistivity of materials:

Untitled

Source: Peltoniemi,1988

Advantages: the most used method for geoelectric surveying, because it is one of the cheapest geophysical method and it gives very good results in many area of interest.

Applicable on: Soil, Rock and Groundwater

Function: – Aquifer description – Bedrock depth – Mapping geological boundaries – Identification of fractures in the bedrock – Mapping of sand and gravel deposits – Sol / Strate unconsolidated – Mineral Exploration – Geological mapping – Mapping groundwater depth – Mapping spots brine – Groundwater Contamination – Inorganic pollutants leak detection – Leaking landfill – Maps of saline soils – Identify wells – Old settlements – Depth mapping rocks – salt water intrusion

 

 

 

 

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