1887

Abstract

We present a new stable method for interpreting the basement relief of a sedimentary basin which delineates<br>sharp discontinuities in the basement relief and incorporates any law known a priori for the spatial variation of<br>the density contrast. The subsurface region containing the basin is discretized into a grid of juxtaposed<br>elementary prisms whose density contrasts are the parameters to be estimated. It is imposed that any vertical<br>line intersect the basement relief only once and that the mass deficiency be concentrated near the Earth's<br>surface, subject to the observed gravity anomaly being fitted within the experimental errors. In addition, upper<br>and lower bounds on the density contrast of each prism are introduced a priori (one of the bounds being zero)<br>and the method assigns to each elementary prism a density contrast which is close to either bound. The<br>basement relief is, therefore, delineated by the contact between the prisms with null and nonnull estimated<br>density contrasts, the latter occupying the upper part of the discretized region. Thus, the method is stabilized by<br>introducing constraints favoring solutions having the attributes (shared by most sedimentary basins) of being<br>an isolated compact source with lateral borders dipping either vertically or toward the basin center and having<br>horizontal dimensions much greater than its largest vertical dimension. Arbitrary laws of spatial variations of<br>the density contrast, if known a priori, may be incorporated to the problem by assigning suitable values to the<br>nonnull bound of each prism. The proposed method differs from previous stable methods by using no<br>smoothness constraint on the interface to be estimated. As a result, it may be applied not only to intracratonic<br>sag basins where the basement relief is essentially smooth, but also to rift basins whose basements present<br>discontinuities caused by faults. The method's utility in mapping such basements were demonstrated in tests<br>using synthetic data produced by simulated rift basins. The method mapped with good precision a sequence<br>of step faults which are close to each other and present small vertical slips, a feature particularly difficult to<br>detect from gravity data only. The method was also able to map isolated discontinuities with large vertical<br>throw. The method was applied to the gravity data from Recôncavo Basin, Brazil. The results showed close<br>agreement with known geological structures of the basin. It also demonstrated the method's ability to map a<br>sequence of alternating terraces and structural lows, which could not be detected just from the inspection of<br>the gravity anomaly.

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/content/papers/10.3997/2214-4609-pdb.215.sbgf063
1999-08-15
2024-03-29
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