The United States Geological Survey defines gravel deposits as alluvial accumulations of small unconsolidated rock fragments, such as pebbles and cobbles, used in construction as fill, ground cover or aggregate for concrete. Gravel is appearing more and more in landscaping design as a water and maintenance saving measure and is an integral part of road building and construction as well. Finding lucrative deposits of gravel is big business and becoming more and more important to the development of new roads and construction sites. Airborne electromagnetic (EM) data is being used to find buried gravel deposits that aerial photographs miss.
Complete a detailed aerial survey of the area in which you search for gravel deposits. A helicopter equipped with aerial photography and surveying equipment expedites the process.
Outfit the helicopter with electromagnetic mapping hardware and fly over the surveyed area. Take electromagnetic readings as the helicopter flies above the quadrants outlined in the aerial survey.
Measure the resistivity of the terrain to locate underground deposits of gravel. Use the resistivity differencing technique to interpret the EM readings.Take readings at 3,600 Hz and at 7,200 Hz and record them using resistivity software.
Subtract the natural log of the resistivity values generated from the 3,600 Hz readings from the natural log of the 7,200 Hz resistivity values. The result indicates positive contours over areas where the upper material is more resistive than the material beneath. The positive contours are deposits of sand and gravel. Negative contours indicate a conductive cover over resistant bedrock.