Abstract: A technique for determining the extent of underground contamination within a test area involves recording of magnetic measurements at each of a plurality of test sites to generate magnetic test data. The generated magnetic test data is grouped within one of a plurality of selected data ranges, and the grouped data is displayed as a function of the test area. The displayed test data may be compared with base data related to underground contamination to determine the extent of contamination within the test area. Generated data is grouped within the plurality of data ranges each having variation of less than 25 gammas, and multiple groupings of data may be displayed with similarly colored data within one of a plurality of levels. The displayed test data may be compared with the contamination level from one or more contamination locations within the test area, and contamination locations overlayed on a map of the test area with the grouped magnetic test data.