a. Flight strips. Most aerial photographs are taken as flight strips
with 60 percent or more overlap between pictures along the flight line and
20 to 30 percent side overlap between parallel flight lines.
b. Interpretation. When overlapping pictures are viewed
stereoscopically, ground relief appears. From the appearance of land forms
or erosional or depositional features, the character of soil or rock may be
interpreted (see Reference 3, Terrain Analysis, A Guide to Site Selection
Using Aerial Photographic Interpretation, by Way, for guidance on
interpretation and terrain analysis with respect to issues in site
development).
3. LIMITATIONS. Interpretation of aerial photographs and other remote
sensed data requires considerable experience and skill, and results obtained
depend on the interpreter's proficiency. Spot checking in the field is an
essential element in photo-geologic interpretation.
a. Accuracy. Accuracy is limited where dense vegetation obscures
ground features (unless SLAR imagery is used) and is dependent upon the
scale, sensors, film products and enlargements. Recently, computer
enhancements of multi-spectral imagery has made LANDSAT data compatible
with conventional aerial photography.
b. Utility. For intensive investigations within developed areas,
aerial photographs are not essential to exploration. Although valuable,
the technique does not provide quantitative information for site specific
foundation conditions. However, photo-interpretation greatly aids
qualitative correlation between areas of known and unknown subsurface
conditions.
Section 4.
GEOPHYSICAL METHODS
1. UTILIZATION. See Table 3 for onshore and Table 4 for offshore
geophysical methods and application.
a. Advantages. In contrast to borings, geophysical surveys explore
large areas rapidly and economically. They indicate average conditions
along an alignment or in an area rather than along the restricted vertical
line at a single location as in a boring. This helps detect irregularities
of bedrock surface and interface between strata.
b. Applications. Geophysical methods are best suited to prospecting
sites for dams, reservoirs, tunnels, highways, and large groups of
structures, either on or offshore. They also have been used to locate
gravel deposits and sources of other construction materials where properties
differ substantially from adjacent soils. Downhole, uphole and cross-hole
seismic surveys are used extensively for determining dynamic properties of
soil and rock at small strains.
(1) Rippability-velocity relationships for various rock types are
given in DM-7.2 Chapter 1.
7.1-59
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