Page 29 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
24 Geotechnical Engineering

The LANDSAT group of satellites gives rapid coverage and spectral diversity
useful for computerized inventorying of resources. This group of satellites is in
north-south polar orbits, covering areas from 120 to 200 m wide on single passes.
Repetitions every few days help penetrate a problem of traditional aerial
photography, cloud cover. Satellite imagery now is capable of fine resolution,
and can be pointed fore and aft to generate stereo pairs for determining ground
elevations relative to known benchmarks. Satellite imagery also offers a highly
sophisticated selection of light wavelengths that are useful for land utilization
studies and military uses.

Satellite photos of the entire world can be viewed with Google after clicking
on Google Earth. This installs a computer program that enables one to move
about and zoom in on a particular city or locality. In selected areas the detail is
such that the view can be tilted to give a three-dimensional effect. This valuable
function is offered at no charge and requires a high-speed internet connection.

The interpretation of aerial imagery should be gist for a computer, but com-
puterized interpretation has not yet caught up with the human photointerpreter.
However, a computer can readily compare images of the same area to detect
changes, such as development of a nuclear facility.

The geotechnical engineer and geomorphologist are most closely attuned to the
relationship between landscape appearance and process. A photographer sees
hills where the geomorphologist sees valleys between the hills and looks for
evidence of fractures, faults, tilted sedimentary rock layers, etc., as they affect
the drainage pattern. Most hills consist of remnants left after erosion, although
some such as volcanoes are constructive.

Airphoto interpretation always should be supplemented with geological maps
and on-site studies to supply ‘‘ground truth.’’ Examples of airphoto interpretation
are shown later in this and in the next chapter.

2.4 THE HUMAN STORY OF PLATE TECTONICS

2.4.1 Continental Drift
Scientists long speculated on reasons why volcanoes tend to line up along arcs
that define continental margins. Examples are the Aleutian Islands, Japan, and a
‘‘ring of fire’’ enclosing the Pacific Ocean basin. An explanation that was
championed by a German meteorologist and explorer, Alfred Wegener, is called
continental drift.

Wegener was not by training a geologist, and his book, The Origins of Continents
and Oceans, was first published in 1915 and viewed with what might conservatively
be called raw skepticism. Nevertheless, unlike geological concepts that derive from

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