New approach to directional filtering of near‐surface magnetic data

Topographical anomalies or compressed areas of soil caused by ploughing or more significant relic features such as ridge and furrow produce a response in near‐surface magnetic surveys that are usually identified by their repetitive, linear pattern. While they are accurate recordings of the subsurface magnetic properties and micro‐topographical features of the site, it is often the anomalies due to deeper features that are the primary focus of the survey. These target anomalies can be masked by the near‐surface pattern and it is therefore often preferable to remove these from the final presentation of the data. Two routines used for removing these features are common in commercial processing software. These are the directional pass/reject and cosine‐taper filters. While these filtering techniques can dramatically improve the clarity of the data image, it is shown here that they make significant changes to the data that remain. As interpretation of near‐surface magnetic data moves beyond image analysis towards more quantitative methods, it is important to ensure that the final processed data set represents as close as possible the response to the subsurface features of interest. Here an alternative filtering routine dependant on both the azimuth and power‐content of the anomalies is proposed that overcomes the problems encountered by the traditional techniques. It is shown that patterns of agricultural linear anomalies can be removed from the data without significantly changing the properties of the desired responses and therefore quantitative interpretation can be subsequently carried out without the data being significantly compromised by the choice of previous processing techniques.