Tools And Techniques For The Application Of Highresolution Aeromagnetic Surveys To Engineering Problems

Modern high-resolution airborne magnetic surveys can be a practical tool for engineering and<br>environmental applications. Surveys conducted by fixed-wing aircraft over gentle topography can<br>detect magnetic point sources separated by 150 meters or linear magnetic sources that extend 150 meters<br>in at least one direction; examples include well casings, geologic unit boundaries, faults, and pipelines.<br>For smaller targets, and in areas of rugged topography, helicopter surveys are required.<br>Careful planning is the key to a successful high-resolution aeromagnetic survey. Flight-line<br>spacing should be no greater than the minimum terrain clearance in order to avoid undersampling of the<br>magnetic anomalies. Flight lines should be flown perpendicular to the expected strike of sources of<br>interest.<br>Even after standard data corrections are applied, variations in terrain clearance can result in<br>uneven resolution of features across the survey area. Equivalent source continuation of the data can be<br>used to evaluate the magnetic field on a surface that is more reasonable for interpretation than the<br>surface on which the data were collected. This is demonstrated on helicopter magnetic data for a<br>hazardous waste site in Tennessee. Phase-shifts of magnetic anomalies due to the local direction of the<br>geomagnetic field vector can be corrected using a reduction-to-the-pole filtering operation. In many<br>cases, this has the effect of moving the anomaly peaks directly over the sources, as is demonstrated<br>using aeromagnetic data over an oil field in Oklahoma.<br>At the interpretation stage, various techniques and software tools are available for extracting<br>geologic and cultural information from the data. The magnetic fields of shallow sources can be<br>separated from those of deeper sources using a process known as matched bandpass filtering. Three<br>methods for locating magnetic sources in three dimensions and identifying properties of the sources are<br>demonstrated on aeromagnetic data for Tucson, Arizona.