An Australian-Canadian friend and former colleague in Australia, with many years of experience in gravity and magnetic data interpretation, used to tell me: “gravity doesn’t lie”.

What geophysicists and geologists measure with gravity data are actually variations in gravitational acceleration caused by a cumulative effect of mass deficit or mass surplus at Earth’s surface and within the subsurface. Most of the variation (say like >90%) in gravitational acceleration at the Earth’s surface is caused by topography, something which Pierre Bouguer found out the hard way during the 18^th century French Geodesic Mission. This expedition set off in 1735 to measure the length of a meridian at the equator in Ecuador (then Peru), in order to determine whether (as predicted by Newton) or not Earth was wider at the equator. The expedition suffered numerous problems, one of which was due to the gravitational effect of the Andes. What we now call Bouguer gravity data, is gravity data corrected for topographic gravity effects, and that thanks to 18^th century Pierre.

The Bouguer gravity shows the cumulative effect of density variations within the subsurface. These result from geological processes and it’s an interpreter’s job to interpret the geology reflected by the Bouguer gravity data. Importantly, Bouguer gravity data do not provide information on actual density, but only on density contrast. Even though a gravity map may look like a collection of all sorts of positive and negative anomalies (usually shown in a colour scale from red to blue, respectively), interpreting gravity data is not about mapping anomalies, it’s about mapping geology, yet another thing I learned from my Australian-Canadian friend. That is also the reason why gravity data interpretation is preferably done by geologists, ideally structural geologists (3D view!), with an understanding of gravity data acquisition and processing, rather than by geophysicists. This being said, geophysicists play a very important role in making sure the gravity data are acquired, processed and stitched correctly; they make sure the gravity data “do not lie”.

The cross-border GeoERA GeoConnect³d project applies a methodology in which, contrary to conventional mapping approaches (say, mapping stratigraphic horizons to a great level of detail and considering faults and discontinuities as pretty annoying), emphasis is put first on structures, unconformities and discontinuities in order to build a cross-border structural framework. The use of gravity data is ideal for constructing such a structural framework for the following reasons:

• Interpreting gravity data in map view doesn’t suffer from the uncertainties inherent to cross-section view approaches (e.g. seismic data interpretation) regarding fault trends and fault connectivity.
• Interpreting gravity data in map view immediately places features and areas into a broader geological context.
• Bouguer gravity is a summation of density contrasts in the subsurface from upper mantle to the surface. As such, it enables a bottom-up approach, showing large, deep structures to be included in a structural framework, not necessarily reflected in surface geology.
• Where/if available, high-resolution Bouguer gravity contains more high frequency signal allowing interpretation of shallow features, which can be used to gain information about fault dip, fault connectivity,…

Figure 1 shows the IGME geological map (A, D), an isostatic gravity image (gravity data with removal of gravitational effect of the compensating mass that supports topographic loads; WGM2012; B, C) and lineaments mapped from gravity data (C, D, E) for the Roer-to-Rhine area (R2R area) of GeoConnect³d. The main structures can be recognized easily in the gravity data. The Roer Valley Graben and the Upper Rhine Graben show up nicely, as well as their deepest parts, graben asymmetry, and change in asymmetry along the length of the grabens, and several left-steps and right-steps. Many of the mapped lineaments within and around the R2R area coincide with well-known faults such as graben-bounding faults, the curved Vittel fault in the south,… Moreover, showing the mapped lineaments on the IGME geological map (Fig. 1D) does make the geological map suddenly look more comprehensible, doesn’t it? That’s because many of the lineaments seem to offer some explanation for the distribution of the geological units.

However, several mapped lineaments or zones of lineaments, do not appear to have been mapped as faults or fault zones previously. This holds true in particular for a series of N-S lineaments.

I can now hear you think: “Perhaps gravity doesn’t lie, but surely these N-S lineaments are artifacts?”

When interpreting gravity and magnetic data one should always be wary of things like “border faults” and perfect “N-S faults”. These are often artifacts resulting from imperfections in sampling, stitching and gridding, and it is not that easy to determine whether or not a particular lineament is an artifact.

A comparison with the IGME geological map shows that the Eifel N-S zone is clearly reflected in the geology (Fig. 1D, 1E). Based on the gravity data, this zone widens and can be continued to the north across the Roer Valley Graben. Where the western and eastern branches cross the graben, the graben even shows a left-step (Fig. 1B, 1D, 1E), twice!

Also the Ourthe zone, the Leut – Bordière zone, and the northern half of the Donderslag zone (Fig. 1E) correspond to real geological features, of which the importance has been underestimated previously. Where observed, the Ourthe zone corresponds to a kink-like change in trend with displacement along several mapped faults (Dejonghe, 2008). The northeastern part of the Bordière fault is considered to form the eastern limit of the Brabant Massif (Legrand, 1968). The Leut fault and the Donderslag lineament affect the Carboniferous of the Campine Basin, and segments of these faults have been observed on seismic data, but different interpretations exist regarding fault traces (Langenaeker, 2000; Deckers et al., 2019). Gravity data show a connection between the Leut fault, the northeastern part of the Bordière fault and the western part Ourthe zone, whereas the Donderslag lineament can be recognized in gravity data, and traced to the NNE within the Roer Valley Graben where it coincides with a left-step of the graben (again!). More details on this small part of the R2R area, with cool pictures, will be provided in a future blog.

The geological reality of the Meuse zone, the southern half of the Donderslag zone, and the N-S lineament across Luxemburg (Fig. 1E) yet has to be investigated further. But gravity doesn’t lie, does it?

 

Tim Debacker
Vlaams Planbureau voor Omgeving (VPO)

 

References

Bonvalot, S., Balmino, G., Briais, A., M. Kuhn, Peyrefitte, A., Vales N., Biancale, R., Gabalda, G., Reinquin, F., Sarrailh, M., 2012. World Gravity Map. Commission for the Geological Map of the World. Eds. BGI-CGMW-CNES-IRD, Paris.

Deckers J., De Koninck R., Bos S., Broothaers M., Dirix K., Hambsch L., Lagrou, D., Lanckacker T., Matthijs, J., Rombaut B., Van Baelen K. & Van Haren T., 2019. Eindrapport Geologisch (G3D) en hydrogeologisch (H3D) 3D lagenmodel van Vlaanderen – versie 3. Studie uitgevoerd in opdracht van: VPO en VMM. 2018/RMA/R/1569.

Dejonghe, L., 2008. Le couloir de décrochement dextre de l’Ourthe dans l’axe Erezée – Saint-Hubert (Haute Ardenne, Belgique) et son implication sur le tracé des failles longitudinales. Geologica Belgica, 11/3-4, 151-165.

Langenaeker, V., 2000. The Campine Basin. Stratigraphy, structural geology, coalification and hydrocarbon potential for the Devonian to Jurassic. Aardkundige Mededelingen, 10, 1-142.

Legrand R., 1968: Le massif du Brabant. Toelichtende verhandelingen voor de geologische kaart en de mijnkaart van Belgie, 9, BGD.

 

 

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