<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML> <HEAD> <TITLE>Electromagnetic methods, electromagnetic surveys - Subsurface Geotechnical</TITLE> <link rel="canonical" href="https://www.geophysical.biz/electromagnetic-method.htm" /> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; CHARSET=utf-8"> <META HTTP-EQUIV="Content-Language" CONTENT="en-GB"> <META NAME="description" CONTENT="Electromagnetic methods, electromagnetic surveys and ground conductivity"> <META NAME="keywords" CONTENT="electromagnetic methods, EM survey, ground conductivity, geophysical surveys, electromagnetic geophysics, electromagnetic surveying"> <link rel="icon" type="image/ico" href="https://www.geophysical.biz/favicon.ico"/> <script type="text/javascript"> var _gaq = _gaq || []; _gaq.push(['_setAccount', 'UA-19923460-1']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); </script> <STYLE TYPE="text/css"> h6.c10 {font-family: arial, verdana} div.c9 {margin-left: 2em} h5.c1 {font-family: arial} span.c8 {color: #400080} span.c8 {color: #400080} div.c7 {margin-left: 4em} p.c6 {font-family: arial, verdana} span.c5 {color: #FF0000} h2.c4 {font-family: arial, verdana} span.c3 {color: #0000FF} h3.c2 {font-family: arial, verdana} span.c1 {font-family: arial, verdana} span.c2 {font-family: arial} span.c3 {color: #0000FF} {font-family: arial, verdana} h4.c4 {font-family: arial, verdana} </STYLE> </HEAD> <BODY> <img src="https://d5nxst8fruw4z.cloudfront.net/atrk.gif?account=PzLgk1a0CM00WY" style="display:none" height="1" width="1" alt=""> <SPAN CLASS="c1"><IMG SRC="logoweb5.png" WIDTH="250" HEIGHT= "215" ALIGN="right" ALT="company logo"></SPAN> <H3 CLASS="c2"> GEOPHYSICAL SURVEY FACT FILE </H3><SPAN CLASS="c1"><BR> <BR></SPAN> <H2 CLASS="c4"> <SPAN CLASS="c3">ELECTROMAGNETIC METHODS</SPAN> </H2><SPAN CLASS="c1"><BR> <BR> <BR></SPAN> <H4 CLASS="c4"> Tel (44) (0)1905 619531 </H4> <HR> <DIV ID="navbar"> <h5 class="c1"> <A HREF="index.htm">Home</A> | <A HREF="gpr1.htm">Ground Penetrating Radar</A> | <A HREF="magnetic.htm">Magnetic</A> | <A HREF="res1.htm">Resistivity Imaging</A> | <A HREF= "soil-resistivity.htm">Earthing &amp; Soil Corrosivity</A> | <A HREF="self-potential.htm">Self Potential</A> | <A HREF="seisrf1.htm">Seismic</A> | <A HREF= "water1.htm">Water Geophysics</A> | <A HREF= "void1.htm">Void Detection</A> | <A HREF= "tank1.htm">Finding USTs</A> | <A HREF= "links.htm">Links</A> | <A HREF= "geophysical-services.htm">Capability Profile</A> | <A HREF="contact.htm">Contact</A> </H5> <HR> </DIV><SPAN CLASS="c1"><BR> <BR></SPAN> <H3 CLASS="c2"> <SPAN CLASS="c5">INTRODUCTION</SPAN> </H3> <TABLE CLASS="image" ALIGN="right"> <CAPTION ALIGN="bottom"> <EM>Electromagnetic EM 31 survey of an overgrown brownfield site</EM> </CAPTION> <TR> <TD> <IMG SRC="webfoto29.jpg" WIDTH="430" HEIGHT="287" HSPACE= "30" BORDER="2" ALIGN="right" ALT= "EM 31 geophysical survey of brownfield site"> </TD> </TR> </TABLE> <P CLASS="c6"> Electromagnetic surveys have been one of our specialist geophysical methods for many years. EM conductivity surveys measure ground conductivity by the process of electromagnetic induction. The principal electromagnetic systems used for site investigation are the Geonics EM 31, EM 34-3 and EM 38 ground conductivity meters. 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. </P> <P CLASS="c6"> Low frequency EM 34-3 systems can be effective for finding large underground cavities such as caves and mine workings but are rarely for smaller targets. The EM 31 operates at an intermediate frequency and is useful for locating discrete features such as sinkholes, abandoned mineshafts and underground storage tanks UST's. The high frequency EM 38 system is best for detecting small targets buried at shallow depth, such as chemical waste drums and metal artifacts. </P><SPAN CLASS="c1"><BR> <BR> <BR> <BR> <BR></SPAN> <TABLE CLASS="image" ALIGN="left"> <CAPTION ALIGN="bottom"> <EM>Locating air raid shelters below a former airfield</EM> </CAPTION> <TR> <TD> <IMG SRC="webfoto30.jpg" WIDTH="430" HEIGHT="288" HSPACE= "25" BORDER="2" ALIGN="left" ALT= "geophysical investigation of an airfield by EM 31"> </TD> </TR> </TABLE><SPAN CLASS="c1"><BR> <BR></SPAN> <H3 CLASS="c2"> <SPAN CLASS="c5">SYSTEMS &amp; OPERATION</SPAN> </H3> <P CLASS="c6"> During normal survey operations the EM system is suspended above ground to avoid direct contact. This operational mode makes EM surveys rapid and cost effective in comparison to conventional resistivity surveys. 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 </P> <P CLASS="c6"> 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 dependant upon the target size. Generally smaller targets require closer survey lines and denser spaced readings. </P><SPAN CLASS="c1"><BR></SPAN> <TABLE CLASS="image" ALIGN="right"> <CAPTION ALIGN="bottom"> <EM>Searching for buried metal artefacts using the EM 38</EM> </CAPTION> <TR> <TD> <IMG SRC="webfoto40.jpg" WIDTH="450" HEIGHT="327" HSPACE= "10" BORDER="2" ALIGN="right" ALT="EM 38 Survey"> </TD> </TR> </TABLE><SPAN CLASS="c1"><BR> <BR> <BR></SPAN> <H3 CLASS="c2"> <SPAN CLASS="c5">PROCESSING &amp; INTERPRETATION</SPAN> </H3> <P CLASS="c6"> The site data is 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 CAD format. </P><SPAN CLASS="c1"><BR><BR><BR><BR> <IMG SRC="webplot1.jpg" WIDTH="500" HEIGHT="361" HSPACE="60" VSPACE="30" BORDER="1" ALIGN="left" ALT= "EM 31 geophysical contour plan"><BR> <BR><BR></SPAN> <DIV CLASS="c7"> <H3 CLASS="c2"> <SPAN CLASS="c5">APPLICATIONS</SPAN> </H3> </DIV><SPAN CLASS="c1"><SPAN CLASS= "c8"><STRONG><BR></STRONG></SPAN></SPAN> <DIV CLASS="c9"> <UL> <LI> <SPAN CLASS="c1"><SPAN CLASS="c8"><STRONG>Finding voids &amp; solution features in soil and rock</STRONG></SPAN></SPAN> </LI> </UL> </DIV> <DIV CLASS="c9"> <UL> <LI> <SPAN CLASS="c1"><SPAN CLASS="c8"><STRONG>Locating abandoned mineshafts, crown holes &amp; subsidence features</STRONG></SPAN></SPAN> </LI> </UL> </DIV> <DIV CLASS="c9"> <UL> <LI> <SPAN CLASS="c1"><SPAN CLASS="c8"><STRONG>Identifying bedrock discontinuities &amp; mineralised veining</STRONG></SPAN></SPAN> </LI> </UL> </DIV> <DIV CLASS="c9"> <UL> <LI> <SPAN CLASS="c1"><SPAN CLASS="c8"><STRONG>Defining former landfill sites &amp; associated leachate plumes</STRONG></SPAN></SPAN> </LI> </UL> </DIV> <DIV CLASS="c9"> <UL> <LI> <SPAN CLASS="c1"><SPAN CLASS="c8"><STRONG>Detecting buried UST's &amp; dumped chemical waste drums</STRONG></SPAN></SPAN> </LI> </UL> </DIV> <DIV CLASS="c9"> <UL> <LI> <SPAN CLASS="c1"><SPAN CLASS="c8"><STRONG>Mapping soil types and land drainage systems</STRONG></SPAN></SPAN> </LI> </UL> </DIV><SPAN CLASS="c1"><BR> </SPAN> <SPAN CLASS="c2"><STRONG> Phone us at: <SPAN CLASS="c3">01905 619531</SPAN> or e-mail us at: <A HREF= "mailto:info@geophysical.biz">info@geophysical.biz</A></STRONG> <BR><BR><BR><BR><BR></SPAN> <h5 class="c1"> <A HREF="sitemap.htm">site map</A> </H5> <TABLE ALIGN="right"></TABLE><SPAN CLASS="c1"><BR> </SPAN> <hr> <DIV ID="navbar2"> <h5 class="c1"> <A HREF="index.htm">Home</A> | <A HREF="gpr1.htm">Ground Penetrating Radar</A> | <A HREF="magnetic.htm">Magnetic</A> | <A HREF="res1.htm">Resistivity Imaging</A> | <A HREF= "soil-resistivity.htm">Earthing &amp; Soil Corrosivity</A> | <A HREF="self-potential.htm">Self Potential</A> | <A HREF="seisrf1.htm">Seismic</A> | <A HREF= "water1.htm">Water Geophysics</A> | <A HREF= "void1.htm">Void Detection</A> | <A HREF= "tank1.htm">Finding USTs</A> | <A HREF= "links.htm">Links</A> | <A HREF= "geophysical-services.htm">Capability Profile</A> | <A HREF="contact.htm">Contact</A> </H5> <HR> </DIV> <H6 CLASS="c10"> <EM>(C) 2014 SUBSURFACE GEOTECHNICAL</EM> </H6> </BODY> </HTML>