GNGTS 2013 - Atti del 32° Convegno Nazionale

[Eq. (7)] for gravity fields, by first suppressing the highest frequencies, (upward continuation), can help to remove that noise. Case history: the Venelin-Aksakov fault (Bulgaria). The VUC method combined with horizontal derivative was applied to an anomalous gravity profile (Stavrev and Reid, 2010) in the area of the Dolna Kamchia depression (Bulgaria), across the Venelin-Aksakov fault, at the western bound of the depression. The Dolna (lower) Kamchia depression lies in Eastern Bulgaria along the lower course of the Kamchia river. This depression is elongated west to east and runs into the adjoining Black Sea shelf, according also to geophysical data. It belongs geologically to the most northeastern part of the Balkan orogenic system. The Venelin-Aksakov fault is the western bound of the depression. An intense gravity gradient zone (see Figs. 5 and 6 in Stevrev and Reid, 2010) indicates a steep contact structure with north-to-south orientation. Exploratory wells west and east of the line of the gravity gradient zone indicate a considerable difference in the depth to the boundary between the low-density tertiary-upper Cretaceous layers and the denser lower Cretaceous and deeper sediment layers, the depth to high-density strata increases by a factor of 10, going from the western elevated horst structure to the eastern depression. Core samples fromwells in the Dolna Kamchia depression show also a significant difference in the density of tertiary-upper Cretaceous layers and lower Cretaceous-Jurassic-Triassic layers. The analysed Bouguer gravity profile was extracted across the gradient axis of the gravity anomaly map shown in Fig. 2a and the geologic data available for the area strongly suggest that this is a contact of considerable depth extent. We performed VUC on the anomaly gravity profile in Fig. 2b obtaining the vertical section showed in Fig. 2c. Then we calculated the horizontal derivative of the VUC result, following Eq. (2), and we extracted from the section profiles corresponding to increasing continuation heights (see Fig. 2d and the related key table). For a vertical contact, the derivative maximum values, related to different continuation heights, are along the same vertical line, whereas for dipping contacts, the maxima gradually move in the direction of dip. Furthermore the absolute values of the horizontal derivative maxima decrease as the height continuation height increases, as one could obviously expect. In fact the resulting signal is related to progressively deeper sectors of the discontinuity, if increasing continuation heights are analysed. In the case shown in Fig. 2c the position of the maxima slightly shifts in the direction of dipping, but the shape of the derivative function is skewed in the direction of dipping. This is in agreement with other geophysical and geological data, suggesting a steep and abrupt contact between tertiary-upper Cretaceous layers and lower Cretaceous-Jurassic-Triassic layers. Conclusions. In this paperwe introduce a new tool, theVUC (VolumeUpwardContinuation), for the well-known procedure of combining upward continuation and vertical and horizontal derivatives of potential field data. The VUC by itself yields good results especially in case of draped-to-level upward continuation and when dealing with the continuation of severely truncated anomalies. One advantage of the VUC tool is that it yields a volume upward continuation, and thus it may be used to extract in a simple way the continuation on any kind of surface and also to multiple levels. We may in fact define all the three kinds of continuation problems ( level-to-level, level- to-draped and draped-to-level ) by simply picking up the sought surface on the inverted 3D magnetization model and then transforming the corresponding magnetization data into upward continued data, through Eqs. (5) and (6). So it can be usefully placed side by side to common upward continuation techniques. In the gravity case, moreover, the VUC absorbs also the vertical derivative in its result, fulfilling a remarkable feature, already examined by many authors who tested its efficiency if 148 GNGTS 2013 S essione 3.2