Gravity a


b high G low


Figure 6.4: Gravity anomalies from a 20-year-old survey (a) are compared to recent results. What was hard to see in 1994 is clearly identified on a modern composite data representation (b), when the small sedimentary basin Gjesvær Low (G) was found to be a continuation of the Nordkapp Basin (SWNB), south- western Barents Sea. 3D shows the vertical gravity gradient draped over the gravity anomalies (GEOSOFT Oasis Montaj).


was identified as a prolongation of the salt-bearing Nordkapp Basin by gravity and only later confirmed by seismic.


6.1.2 Modelling of Complex Salt Structures


Te development of a realistic model for some salt diapirs can be a hard nut to crack for seismic imaging, but gravity data can help to define the edge and base of the salt. Figure 6.5 gives an example from the ‘Uranus’ salt dome in the Norwegian Nordkapp Basin, which was interpreted by combined seismic and gravity gradiometry. Te seismic profile illustrates the lack of reflectors at the base of the salt and presents minimum and maximum salt extension models from the pre-well interpretation. Well data proved the minimum


Gravity 0 [mGal ] 45


model wrong and, after the acquisition of new gravity and gravity gradiometry data, the density model was developed using sedimentary boundaries and top salt from seismic interpretation and densities from wells. Salt forms a negative density contrast against surrounding rocks, thus generating a negative gravity anomaly. Te vertical gravity gradient, Gzz, senses the shallowest part of the salt, as can be seen by the way the salt ridge lines up with the lowest values of Gzz (dark green and blue). Te gravity Gz is wider and smoother as it is also sensitive to the deeper parts of the salt body. Tis difference in sensitivity is used to address different depth regions during the inversion of Gz and Gzz in order to yield thickness estimates of cap rock, depth to base salt and thickness of mother salt.


Entire salt body by wide negative gravity


Figure 6.5: Salt model derived by integrated seismic and gravity interpretation (modified from Stadtler et al., 2014 and Hokstad et al., 2011).


Gravity gradient 0 [E] 90


Shallow salt by gravity gradient; negative anomaly at top of salt ridge


Well path


Pre-well minimum model


Salt model salt 2.8 g/cm3 mother salt


Gravity model range


236


Pre-well maximum model


Density model 2.1 5 km 1 km


Modified from Johansen et al., 1994


Olesen et al. (2010)


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