POSMAG: an ad hoc GPS positioned ground magnetic surveying system

Abstract

The advent of small, cheap, and highly portable Global Positioning Systems (GPS), together with recent advances in ground magnetometer technology, has allowed the development of an economic GPS positioned ground magnetic surveying system. Like their larger sophisticated counterparts in the aeromagnetic arena, these systems facilitate the collection of magnetic data without the need for any navigation tool other than the GPS. Unlike their counterparts, however, they may be constructed at minimal cost, with minimal research and development, and with a very short lead time. A simple marriage of off the shelf components has resulted in the creation of POSMAG, a GPS positioned ground magnetic system. POSMAG can acquire magnetic and positional information in real time. It is based on a Garmin GPS unit and a Gem GSM- 19f Overhauser magnetometer. Data is collected in a "hands free" operation, at normal walking speeds, with high along line spatial resolution, and on arbitrarily placed survey lines. GPS data is corrected in a pseudorange differential fashion with post processing and combined, with the magnetic data on a time basis. Positional accuracies averaging 8 m have been achieved. The system is used in all reconnaissance applications, particularly in rugged areas where grids are prohibitively expensive to establish. High along line resolution, and speed of operation make POSMAG a cheap and effective tool. Areas previously not amenable to assessment with the ground magnetic method are now covered effectively and economically. This paper describes the features and limitations of the equipment used in the POSMAG system. Results of tests on the accuracy of GPS positioning with rudimentary post processing are given. Finally, case studies are presented to illustrate the uses of the system in both standard grid and reconnaissance applications.