Remote sensing and airborne geophysics in the assessment of natural aggregate resources

Natural aggregate made from crushed stone and deposits of sand and gravel is a vital
element of the construction industry in the United States. Although natural aggregate is a high
volume/low value commodity that is relatively abundant, new sources of aggregate are becoming
increasingly difficult to find and develop because of rigid industry specifications, political
considerations, development and transporation costs, and environmental concerns, especially in
urban growth centers where much of the aggregate is used. As the demand for natural aggregate
increases in response to urban growth and the repair and expansion of the national infrastructure,
new sources of natural aggregate will be required. The USGS has recognized the necessity of
developing the capability to assess the potential for natural aggregate sources on Federal lands; at
present, no methodology exists for systematically describing and evaluating potential sources of
natural aggregate. Because remote sensing and airborne geophysics can detect surface and nearsurface
phenomena, these tools may useful for detecting and mapping potential sources of natural
aggregate; however, before a methodology for applying these tools can be developed, it is
necessary to understand the type, distribution, physical properties, and characteristics of natural
aggregate deposits, as well as the problems that will be encountered in assessing their potential
value.
There are two primary sources of natural aggregate: (1) exposed or near-surface igneous,
metamorphic, and sedimentary bedrock that can be crushed, and (2) deposits of sand and gravel
that may be used directly or crushed and sized to meet specifications. In any particular area, the
availability of bedrock suitable for crushing is a function of the geologic history of the area - the
processes that formed, deformed, eroded and exposed the bedrock. Deposits of sand and gravel
are primarily surficial deposits formed by the erosion, transportation by water and ice, and
deposition of bedrock fragments. Consequently, most sand and gravel deposits are Tertiary or
Quaternary in age and are most common in glaciated areas, alluvial basins, and along rivers and
streams.
The distribution of potential sources of natural aggregate in the United States is closely
tied to physiography and the type of bedrock that occurs in an area. Using these criteria, the
United States can be divided into 12 regions: western mountain ranges, alluvial basins, Columbia
Plateau, Colorado Plateau and Wyoming basin, High Plains, nonglaciated central region, glaciated
central region, Piedmont Blue Ridge region, glaciated northeastern and Superior uplands, Atlantic
and Gulf coastal plain, Hawaiian Islands, and Alaska. Each region has similar types of natural
aggregate sources within its boundary, although there may be wide variations in specific physical
and chemical characteristics of the aggregates within a region.
Conventional exploration for natural aggregate deposits has been largely a ground-based
operation (field mapping, sampling, trenching and augering, resistivity), although aerial photos
and topographic maps have been extensively used to target possible deposits for sampling and
testing. Today, the exploration process also considers other factors such as the availability of the
land, space and water supply for processing purposes, political and environmental factors, and distance from the market; exploration and planning cannot be separated.
There are many physical properties and characteristics by which aggregate material is
judged to be acceptable or unacceptable for specific applications; most of these properties and
characteristics pertain only to individual aggregate particles and not to the bulk deposit. For
example, properties of crushed stone aggregate particles such as thermal volume change,
solubility, oxidation and hydration reactivity, and particle strength, among many others, are
important consi