Electromagnetic methods
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Electromagnetic methods
من طرف اللؤلؤ المكنون الجمعة فبراير 19, 2010 8:47 pm
ELECTROMAGNETIC METHODS
INTRODUCTION
Electromagnetic geophysical surveys measure ground conductivity by the process of electromagnetic induction. An EM system consists of a transmitter and a receiver coil spaced at a standard configuration. The main systems used for site investigation are the Geonics EM 31, EM 34-3 and EM 38. All of the systems work on similar principles but use different operating frequencies, providing a range of depth penetrations and resolutions for different applications.
Low frequency systems are utilised for investigating subsurface ground conditions, or finding large underground cavities such as caves and mine workings. Intermediate frequency ones are more useful for locating discrete features such as sinkholes or abandoned mineshafts. High frequency systems work best on small targets such as buried tanks, drums and other metal artifacts.
Electromagnetic EM 31 survey of an overgrown brownfield site
SYSTEMS & OPERATION
During survey operations the EM system is suspended above ground, avoiding direct contact. This operational mode makes EM surveys rapid and cost effective compared to conventional resistivity surveys. The transmitting coil outputs a primary electromagnetic field, which 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, responsive to bulk changes in lithology, groundwater and ground contamination. Metal produces strong secondary fields, so the in-phase component is a useful indicator of buried metal targets.
Data is collected as point readings taken at regular intervals along a survey grid set out over the site area. The spacing between grid-lines and reading stations is dependant upon the target size. Generally smaller targets require closer survey lines and denser spaced readings .
Locating air raid shelters below a former airfield
PROCESSING & INTERPRETATION
The data readings acquired during sitework operations are recorded on a digital data logger, for later downloading to a PC for post-survey processing and interpretation. The most commonly used interpretation procedure is contouring, carried out with specialist interactive software to produce contour plans.
The contoured data is analysed in detail by our experts to identify anomalous features relative to the general background. Once identified, the anomalies are correlated with local ground conditions. Survey results are presented as plans tied in to site co-ordinates, in a readily understandable engineering compatible format.
APPLICATIONS
Finds voids & solution features in soil and rock
Locates former mineshafts, crown holes & subsidence features
Detects bedrock discontinuities & mineralised veining
Identifies former landfill site & associated leachate plumes
Locates buried UST's and chemical waste drums
Detects saline groundwater incursions & heavy metal concentrations
INTRODUCTION
Electromagnetic geophysical surveys measure ground conductivity by the process of electromagnetic induction. An EM system consists of a transmitter and a receiver coil spaced at a standard configuration. The main systems used for site investigation are the Geonics EM 31, EM 34-3 and EM 38. All of the systems work on similar principles but use different operating frequencies, providing a range of depth penetrations and resolutions for different applications.
Low frequency systems are utilised for investigating subsurface ground conditions, or finding large underground cavities such as caves and mine workings. Intermediate frequency ones are more useful for locating discrete features such as sinkholes or abandoned mineshafts. High frequency systems work best on small targets such as buried tanks, drums and other metal artifacts.
Electromagnetic EM 31 survey of an overgrown brownfield site
SYSTEMS & OPERATION
During survey operations the EM system is suspended above ground, avoiding direct contact. This operational mode makes EM surveys rapid and cost effective compared to conventional resistivity surveys. The transmitting coil outputs a primary electromagnetic field, which 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, responsive to bulk changes in lithology, groundwater and ground contamination. Metal produces strong secondary fields, so the in-phase component is a useful indicator of buried metal targets.
Data is collected as point readings taken at regular intervals along a survey grid set out over the site area. The spacing between grid-lines and reading stations is dependant upon the target size. Generally smaller targets require closer survey lines and denser spaced readings .
Locating air raid shelters below a former airfield
PROCESSING & INTERPRETATION
The data readings acquired during sitework operations are recorded on a digital data logger, for later downloading to a PC for post-survey processing and interpretation. The most commonly used interpretation procedure is contouring, carried out with specialist interactive software to produce contour plans.
The contoured data is analysed in detail by our experts to identify anomalous features relative to the general background. Once identified, the anomalies are correlated with local ground conditions. Survey results are presented as plans tied in to site co-ordinates, in a readily understandable engineering compatible format.
APPLICATIONS
Finds voids & solution features in soil and rock
Locates former mineshafts, crown holes & subsidence features
Detects bedrock discontinuities & mineralised veining
Identifies former landfill site & associated leachate plumes
Locates buried UST's and chemical waste drums
Detects saline groundwater incursions & heavy metal concentrations
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عدد المساهمات : 490
تاريخ التسجيل : 19/11/2009
العمر : 34
Geoman- Admin
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عدد المساهمات : 830
تاريخ التسجيل : 04/11/2009
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الموقع : الزرقاء - الرصيفة
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