The soil apparent electrical conductivity (ECa) has been used to map soil spatial variability and to relate it to the variability of various soil properties thus delineating zones of site-specific management. Most often, a single ECa survey is done and the generated ECa map is considered to infer the patterns of soil spatial variability. The objective of this work was to evaluate this assumption by comparing results of several time-lapse surveys at two fields with clay soils. The study site is located in SW Spain; clay content is high (>50 %) across the fields, Field I encompasses the 3-ha tillage experiment with 15 m by 180 m plots where the conventional tillage, the direct drilling and the minimum tillage are replicated. Field II is the direct-drilled 30 ha field. Three EM38DD surveys were done on 10/6/06, on 11/10/08, and on 11/20/08 using sensor coil orientations that yielded ECa values of both shallow and deep layers. Correlation coefficients between the data of different surveys varied from 0.22 to 0.96 for the field 1 and from 0.79 to 0.99 for the field 2. These differences could be attributed to the scale difference between the fields, to differences in data ranges that were much wider at the field II than at the field I, to higher variability in soil properties caused by different tillage in Field 1 as compared to field 2, and to topography variation which was more pronounced at the field II.  Map comparison methods have been used to characterize the observed EC patterns and compare them. Whereas time-lapse ECa patterns were similar at the field II, so that any of the surveys could be used to guide site specific management, time-lapse ECa patterns at the field I were substantially different, and several ECa surveys were needed to ascertain the general pattern of the field variability.