ability to create and maintain tunnels and underground openings becomes more and more important. Underground openings are essential. They serve as sewers and accommodate storm drainage. They carry water from distant sources to cities and provide efficient access for many transportation systems, including highways and subways. And large underground caverns, often left by mining enterprises, are increasingly being used to store petroleum and natural gas as well as radioactive waste. Greater Kansas City is now a world leader in conversion of mined underground space to low-energy, secure commodity storage. Underground excavations also house electricity-generating stations and defense command posts. Underground car parking is vital to many communities. The use of underground space will grow further as older population centers are redeveloped. Already the city of Austin, Texas, has a subterranean utility and freight supply trafficway, and Toronto, Canada, has an underground mall network and pedestrian trafficway.
Use of underground space requires geological understanding; the nature of the surrounding earth material—its composition, fractures, and engineering properties—is the primary consideration for space utilization and design. The stability of an excavation, as well as that of adjacent structures, depends on the earth material characteristics and groundwater control. Transportation and infrastructure routes follow excavations, embankments, and tunnels; all require geological analysis for structural integrity and to alleviate effects on adjacent facilities.
Engineering geophysics was born in the late 1950s, the child of technologies developed for petroleum and mining exploration. It was the advent of nuclear power plant siting, which required detailed subsurface structural and positional accuracy, that led to the use of improved oil field geophysical techniques to detect near-surface stratigraphic anomalies, the presence of groundwater, and faults. An amazing array of engineering geophysical technology is now available—based on the refraction and reflection of sound and electromagnetic waves, electromagnetic properties of earth materials, electromagnetic fields, natural or induced radioactivity, and gravitational attraction of earth materials—as are borehole seismic and television imaging devices. These innovations contribute to the abilities of the engineering geophysicist, but, as with so much in the earth sciences, they need to be interpreted in light of visually observed geology. Boreholes and outcrop observations are still necessary to confirm remotely sensed data.
In the continuing search for new sources of industrial minerals, the possible health hazards from occupational exposure to mineral dusts have become an important factor in deciding what minerals can safely be exploited. More recently, concern has grown that exposure to various mineral dusts even in nonoccupational environments may cause disease or death. This concern has focused on asbestos dusts (or dusts perceived to be asbestos-like), radon, and zinc mining. Each of these is given below as examples, and each illustrates how inadequate comprehension about the nature and use of geological materials can lead to profound and quite unnecessary regulatory problems.
Asbestos is a nonscientific commercial term for certain silicate minerals that separate into flexible, heat-resistant, chemically inert fibers. Their particular mechanical, chemical, and especially thermal properties have been valuable in yarns, cloth, paper, paint, brake linings, tiles, insulation, fillers, filters, putty, and cement—applications requiring incombustible, nonconducting, or chemically resistant material. The world consumption of asbestos is about 4 million metric tonnes per year; that of the United States is disproportionately small, just over 80,000 metric tonnes. Of the six types of asbestos that have been mined commercially, only three are quantitatively important—a serpentine mineral called chrysotile and two amphibole minerals called amosite and crocidolite. The three other minerals, which are rarely used as asbestos, are the amphiboles—anthophyllite, actinolite, and tremolite.
Since 1972 the Environmental Protection Agency (EPA) has taken steps to limit human exposure to asbestos on the grounds that the fibers are carcinogens. This conclusion is based on epidemiological observations that former asbestos miners and fabricators experience an increased incidence of lung cancer and mesothelioma, a tumor of the chest cavity lining. Several mineralogists have questioned the EPA's classification of all these different minerals together and consideration of them all as equally hazardous. Recent medical research suggests that chrysotile asbestos is not hazardous outside the workplace—and perhaps not even there.
The serpentine mineral chrysotile is a layer sili-