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<br />EM 1110-2-1913 <br />31 Mar 78 <br /> <br />Table 2-4. <br /> <br />(Continued) <br /> <br />e <br /> <br />Name <br /> <br />Primary Use <br /> <br />b. Continuous <br />vibration <br />(Continued) <br /> <br />2. Electrical methods <br /> <br />a. <br /> <br />Resistivity <br /> <br />b. Equipotential <br />mapping <br /> <br />Principle <br /> <br />Based on the difference in <br />electrical conductivity <br />or resistivity of strata. <br />Resistivity of subsoils <br />at various depths is <br />determined by passing a <br />known current between two <br />electrodes and measuring <br />the potential difference <br />between two intermediate <br />electrodes. Resistivity <br />is correlated to material <br />type <br /> <br />Location of lines of equal <br />potential around a <br />current electrode <br /> <br />. <br /> <br />for the natural <br />period of vibra- <br />tion for founda- <br />tions of vibrat- <br />ing structures <br /> <br />~ <br /> <br />Used to determine <br />depths up to <br />100 ft of hori- <br />zontal subsurface <br />strata. Princi- <br />pal applications <br />for investigating <br />foundations of <br />dams, levees, and <br />other large <br />structures, par- <br />ticularly in ex- <br />ploring granular <br />river channel <br />deposits or bed- <br />rock surfaces. <br />Also used to mea- <br />sure depth to <br />saturated zones <br />or aquifers <br /> <br />Delineation of <br />vertical bound- <br />aries and zones <br />of limited hori- <br />zontal extent. <br />Can trace lines <br />of water flow or <br />locate bodies <br />such as clay <br />plugs <br /> <br />. <br /> <br />. <br /> <br />.. <br /> <br />2-8 <br /> <br />e <br />