Gravity anomaly


2 1 10 mGal 3


Basement 2.7 g/cm3


2 km 2 1 5 km


Figure 6.2: Gravity modelling in practice. The hand-hold pointer is at the boundary between top basement and sediments; move it upwards through positions 1 to 3 and the calculated gravity anomaly is changed. The aim is a match with the observed gravity.


Gravity gradient High resolution of


shallowest top basement by gravity gradient


Gravity Gravity gradient 20 E


Figure 6.3: Synthetic model of a highly faulted basement in 3D and on a profile (location shown by large black arrows), with calculated gravity (Gz) and vertical gradient (Gzz). Inverted top basement obtained from inversion of calculated gravity Gz (1 E (Eotvos) = 10,000 mGal/m = 10-9


second-2


Low resolution of deepest top basement


Basement model Gravity Inverted basement


0 2


[km]


6 4


Basement 2.7 g/cm3


Sediments 2.2 g/cm3 20 mGal )


(Parker inversion; GEOSOFT Oasis Montaj).


Sediments 2.2 g/cm3


3


observed modelled


the gravity anomaly itself. Te top basement is heavily faulted at all depths and its appearance on the gravity data illustrates the decaying resolution capability with increasing depths: all peaks forming the shallowest part of the basement high are clearly visible on the gravity gradient, which as such provides an excellent tool for top basement mapping. However, fault blocks in the deeper part of the basin (left section of the profile) are smeared together. Te same loss of detail occurs when inverting the calculated gravity for top basement, solely based on the pre-defined density contrast between sediments and basement. Nevertheless, the gravity data give a clear idea of the shallowest and deepest parts of the basin as well as the main structural trends, which are related to those of the overlying sedimentary strata. Tis is useful for 2D seismic survey planning, both ensuring a suitable angle of the lines to the structures and focusing on the most interesting parts of a basin. Te level of detail unravelled depends on the quality of


the gravity data, as noise causes artificial anomalies to merge with real ones. For offshore areas, gravity data from satellite altimetry covering the entire world is freely available, or in enhanced quality from contractors. Onshore, the options are airborne or ground gravity data. In well explored areas, better gravity data should be


available, as shown in Figure 6.4 from the Gjesvær Low in the south-western Barents Sea. Te almost triangular depression


10 km


235


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