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. EM 1110-2-1913 31 Mar 78 . Exploration from the ground surface will generally be adequate for levee design purposes. Geophysical ground surface exploration can involve: (1) seismic refraction and (2) electrical resistivity, Information ob- tained from seismic refraction surveys includes material velocities, delineation of interfaces between zones of differing velocities, and the depths to these interfaces, The electrical resistivity survey is used to locate and define zones of different electrical properties such as pervious and impervious zones or zones of higher ionic activity such as clayey strata, Both methods require distinct differences in prop- erties of foundation strata materials in order to be effective, The resistivity method requires a high resistivity contrast between mate- rials being located, while the seismic method requires high contrast in wave transmission velocities. Furthermore, the seismic method requires that any underlying stratum transmit waves at a higher velocity than the overlying stratum. Difficulties arise in the use of the seismic method if the surface terrain and/or layer interfaces are steeply sloping or irregular instead of relatively horizontal and smooth. Therefore, in order to use these methods, one must be fully aware of what they can and cannot do. EM 1110-2-1802 (ref. A-3a(3)) describes the use of both seismic refraction and electrical resistivity. Dobrin (ref. A-5c) is a valuable, general text on geophysical exploration, Applicable geophys- ical exploration methods based on operation from the ground surface are summarized in table 2-4. c, Recent developments in the use of downhole logging devices have shown that these tools can be used with success in correlating subsur- face soil and rock stratification and in providing quantitative engineer- ing parameters such as porosity, density, water content, and moduli, They also provide valuable data for interpreting surface geophysical data, The pUL~ose in using these methods is to allow cost sa\rings to be made in the exploration program without lessening the quality of the information obtained. This can be done by reducing the number of borings required to determine subsurface stratification and by allowing sampling to be done only in those zones where samples are necessary for laboratory testing, thus reducing the number of undisturbed samples. Section III, Field Testing 2-11. Preliminary Strength Estimates, It is often desirable to estimate foundation strengths during Phase 1 of the exploration program, Various methods of preliminary appraisal are listed in table 2-5. 2-12. Vane Shear Tests. Where undisturbed samples are not being ob- tained or where samples of acceptable quality are difficult to obtain, in situ vane shear tests may be utilized as a means of obtaining 2-9 . ~. ~ ~: