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Introduction
Most applications of airborne geophysics for engineering projects involve soil mapping or near-surface geological mapping with electromagnetic systems. Whether locating aggregates, or mapping overburden to measure depth, stability or water content, airborne geophysics helps cover large areas quickly.
Aggregates
 More tonnes of aggregate are mined each year than any other mineral resource. Large urban areas are becoming starved for aggregates. Whether derived from sand and gravel in overburden or sedimentary rock, aggregate deposits generally are more resistive than the surrounding sediments.
Airborne electromagnetic surveys have been successfully used to locate and delineate aggregate deposits. An EM system capable of getting very near surface information is generally most useful, although deep deposits are being mined.
Construction Planning
Depth and type of overburden, type of bedrock, and groundwater conditions are important facts to know when planning construction. Airborne electromagnetic surveys can provide initial estimates of this information at low cost, particularly when the project covers large or remote areas, as is the case for pipeline or power line construction. Shallow detection EM surveys have been used to map the overburden thickness to delineate areas where trenching or blasting will be necessary. Magnetic data collected simultaneously with the EM aids in mapping the bedrock types.
Soil types have been mapped to delineate areas of high porosity or unstable ground. This has been used to identify areas where flood control levees may be prone to underseepage. High clay content (conductive) soils are often sought for industrial locations, where the low porosity forms a natural barrier to contaminant seepage.
When planning construction in bedrock, the rock type, competence, and fracturing can also be very important. Faults and water-bearing shear zones can be significant factors in designing tunnels and other excavations. Electromagnetic and magnetic surveys can identify rock types and locate these structures.
Case Study  Planning and Monitoring Pipeline Construction
Permafrost
The presence of permafrost, and its thickness are major concerns for construction and remediation projects in the Arctic. Permafrost renders the overburden very resistive, thus making an excellent target for airborne electromagnetic surveys.
The three dimensional nature of multi-frequency EM can be used to calculate the thickness of the permafrost under many conditions. |
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Pipeline Location
 The location of pipelines often becomes uncertain with the passage of time, and re-locating them is often necessary for safety in seismic surveys or construction, or for site remediation. Even small metallic or metal-reinforced pipelines can be quickly located with airborne magnetic and/or electromagnetic surveys.
The cathodic electrical currents used to protect pipelines from corrosion can be measured from the air, to help locate the lines, and to detect areas of current leakage.
Airborne EM surveys can also be used to map the soil conditions around a pipeline or power line. Highly conductive soils can increase the rate of corrosion of a pipeline, and location of these zones can assist in construction or maintenance planning. Power line protective grounding can be designed more easily if a map of ground conductivity has been prepared from an airborne EM survey.
Landslide Investigation
Landslides often occur when loose material slides along a slip surface defined by a change in soil type. Airborne EM has been successfully used to map these hazardous zones, by detecting the change in conductivity at the slip surfaces. Changes in conductivity can also be detected where porosity or water saturation contrasts exist.
Case History Landslide Assessment (PDF, 136 Kb)
Bathymetry
Acoustic (ship-borne) measurement of water depth is relatively slow, and can be difficult or impossible in conditions such as tidal zones or through permanent ice. Measurement of water depth from the air using scanning LIDAR or electromagnetic surveys can provide water depth estimates quickly and cost effectively.
Case History Sydney Harbour Bathymetry (PDF, 772 Kb)
Paleochannels
These ancient riverbeds may provide aggregates for construction, and often channel fresh groundwater flow, but the porous soil may allow seepage of contaminant plumes or floodwaters under levees. Airborne EM surveys can be used to locate and map these channels, and provide an estimate of the thickness.
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