Geophysical Prospecting, 2003, 51, 261–272 Decorrugation and removal of directional trends of magnetic fields by the wavelet transform: application to archaeological areas Maurizio Fedi ∗ and Giovanni Florio Dipartimento di Scienze della Terra, Universit` a Federico II di Napoli, Largo San Marcellino 10, 80138 Napoli, Italy Received May 2001, revision accepted January 2003 ABSTRACT The problem of removing directional trends frequently occurs in the processing of magnetic data and also in the subsequent steps of data interpretation. The so-called corrugations are typical directional trends occurring in levelled data, which may be removed in several ways. Classical techniques are based on high-pass filtering of the data and successively filtering these transformed data with directional cosine filters. Other linear features are due to real sources, such as pipelines in shallow surveys or dike swarms in regional surveys. They should, nevertheless, be considered as noise, due to the fact that their effect is strong and tends to hide the field features related to structures of more interest. We deal with both kinds of problem, presenting the results of a study in an archaeological area of southern Italy. Decorrugation of magnetic field anomalies is performed using a method based on the excellent space–frequency local- ization properties of wavelet bases, allowing a very sharp filtering of the field along a selected direction. We compare this technique with the classical one in a synthetic case and find that the wavelet decorrugation is simpler and produces low distortion maps. Besides the field decorrugation, the wavelet approach was also shown to be useful in the subsequent enhancement of the measured field. In fact, we show that the wavelet analysis offers a unique framework where various filtering problems (directional, isotropic, global or local as well) may be easily solved. As regards the archaeological case, strong noisy effects from elongated sources (pipelines) were successfully removed in a sharp and local way. INTRODUCTION Aeromagnetic data are normally measured along lines, possi- bly parallel, with a constant line spacing. Data are related not only to the magnetization variations due to geological struc- tures, but also to temporal variations of the field. These lat- ter must be removed before any interpretation of magnetic anomalies can be attempted. To do this, two methods are typ- ically used, namely the base-station measurement subtraction (BSS) method and the correction technique known as level- Paper presented at the 61st EAGE Conference – Geophysical Division, Helsinki, Finland, June 1999. ∗ E-mail: fedi@unina.it ling. The BSS method involves measurement of the temporal variation of the field at a reference base-station and the use of that information to correct the measurements. The con- ductivity is assumed uniform in the survey area, otherwise residual effects of temporal variations may still occur. Level- ling consists of estimating and removing the temporal vari- ations, using the information provided by additional mea- surement lines. These are known as tie-lines and they run transversely with respect to the survey lines. Data from the two perpendicular directions are compared at each crossover point and their differences are modelled as piece-wise lin- ear trends between them, so as to reduce the crossover er- rors to zero. Obviously, crossover differences depend not only on time variations but also possibly on other errors, such as those related to the horizontal and vertical positioning of the C  2003 European Association of Geoscientists & Engineers 261