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<br />. <br /> <br />-" <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />EM 1110-2-1913 <br />31 Mar 78 <br /> <br /> <br />Table 2-4. Applicable Geophysical Methods of Exploration <br /> <br />Name <br /> <br />1. Seismic methods <br /> <br />a. <br /> <br />Refraction <br /> <br />b. <br /> <br />Continuous <br />vibration <br /> <br />Principle <br /> <br />Based on time required for <br />seismic waves to travel <br />from source to points on <br />ground surface, as mea- <br />sured by geophones spaced <br />at intervals on the <br />surface <br /> <br />Refraction of seismic waves <br />at the interface between <br />different strata gives a <br />pattern of arrival times <br />versus distance at a line <br />of geophones <br /> <br />The travel time of trans- <br />verse or shear waves gen- <br />erated by a mechanical <br />vibrator consisting of a <br />pair of eccentrically <br />weighted disks is re- <br />corded by seismic detec- <br />tors placed at specific <br />distances from the <br />vibrator <br /> <br />( Cont inued) <br /> <br />2-7 <br /> <br />Primary Use <br /> <br />Utilized to deter- <br />mine depth to <br />rock or other <br />lower stratum <br />substantially <br />different in wave <br />velocity than the <br />overlying mate- <br />rial. Generally <br />limited to depths <br />of 5 to 500 ft. <br />Used only where <br />wave velocity in <br />successive layers <br />becomes greater <br />wi th depth <br /> <br />Velocity of wave <br />travel gives in- <br />dication of soil <br />type. Travel <br />time plotted as a <br />function of dis- <br />tance indicates <br />depths or thick- <br />nesses of sub- <br />strata. Useful <br />in determining <br />dynamic modulus <br />of subgrade reac- <br />tion and obtain- <br />ing information <br />