Airborne geophysics for watershed characterization

Bill Brown

ABSTRACT

Today’s airborne geophysical systems have the ability to make accurate, high-resolution measurements of the ground conductivity in three dimensions to give hydrogeologists, scientists, and environmental engineers tools to map subsurface water resources. These geophysical data will help develop a better understanding of surface and groundwater location and quality, contaminant flow and salt water encroachment. They contribute to improved watershed management practices, and aid efficient targeting of costly, high-resolution follow-up ground surveys and location of drilling sites.

Airborne survey systems can be used to rapidly characterize large watersheds. The benefits of airborne reconnaissance include minimal access issues and significant cost savings through fast data collection on largescale surveys. Airborne systems are multi-sensor platforms from which various types of other data are collected concurrently. In addition to the collection of electromagnetic data for ground conductivity measurement, magnetic and radiometric (gamma) are collected, as well as digital video, laser altitudes, and dual-frequency GPS.

Recent helicopter geophysical surveys undertaken by Fugro Airborne Surveys include:

  • finding fresh water resources – examples from Michigan and Brazil, Asia and Africa,
  • mapping the flow of contaminant plumes,
  • monitoring the fate of water produced as part of methane extraction practices,
  • measuring the extent of salt water invasion into the Florida Everglades,
  • assessing groundwater resources and recharge areas, and
  • locating brine leaks from abandoned oil wells.

Case studies show how the high resolution of Fugro’s multiple-frequency HEM systems allow for the creation of three-dimensional conductivity images of the soil and bedrock conditions. Surveys have been flown for federal departments such as the EPA, DOE and USGS as well as local governments and water authorities. One USGS case study demonstrates the effectiveness of an airborne survey in mapping the recharge and confined zones of the Edwards Aquifer, which provides drinking water to more than 1.5 million people in the area of San Antonio, Texas. The Marquette Michigan case study shows how airborne geophysics was instrumental in identifying new groundwater resources in a highly developed area, allowing the township to re-consider plans to pipe water from outside. Saltwater encroachment under a long stretch of coastline in the Florida Everglades was mapped efficiently, in several seasons. Brine from abandoned oil wells was mapped in west Texas, and acid drainage was mapped from abandoned coal mines in the Midwest.

Various types of data presentations and final products are used to integrate, present, and interpret these data. 2D and 3D images of ground conductivity, and maps of magnetic anomalies and/or variations in natural radioisotope geochemistry are powerful aids to interpretation and a comprehensive understanding of the soil characteristics, ground water location and flow, and bedrock structure and geology of an area - a vital part of any water exploration or land care management program.


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