Our services primarily involve the integrated interpretation of gravity, magnetics, and seismic data. We also offer quality control and survey design for gravity/magnetics/FTG, as well as QC of data processing projects.
We are experts in 2-D and 3-D modeling around salt, carbonates, overthrust and other complex geological environments, using gravity and FTG/Falcon data sets, oftentimes tightly integrated with seismic velocities, closely coupled with magnetic data sets.
Integrated 3-D Modeling & Inversion
In addition to the use of gravity and magnetic modeling and inversion to assist with understanding the structural tectonics of an area, we spend a tremendous effort on building better 3-D models in and around complex salt features, carbonate reefs, volcanics, etc.
The following illustrates the workflow commonly used during careful, integrated (seismic/gravity/well control) 3-D modeling studies.
The process for using gravity inversion and modeling to aid the interpretation of complex salt features can involve multiple phases throughout an imaging project, such as:
- Careful build of canopy salt using multiple salt body construction - often involving top salt 1, 2, 3 and base salt 1, 2, 3 over the lifespan of the seismic imaging effort
- Comparison of difference gravity from alternative salt interpretations
- Compare with and without deep autochthonous salt
- 3-D gravity inversion modeling to derive alternative base of salt at each stage and each salt body level (various levels of base salt canopy, multiple tree branches of salt canopy, top salt roller, etc.)
- Inversion of sedimentary densities, holding well-imaged seismic base of salt fixed – transform alternative densities back to velocity & compare with seismic velocity model.
An example integrated (seismic & gravity) 3-D salt model is shown below, where magenta regions indicate salt from seismic acceptable to gravity, dark blue indicates salt added by gravity inversion, and white indicates salt removed by gravity inversion. Interesting areas show deeper keels or possible feeders (dark blue) and areas suggesting less salt (white), or higher density in these areas than the density of typical salt. Although the results are shown in cross section below, all work is done in 3-D.
Once the salt geometry has been finalized through a careful seismic and gravity integration effort, there is often a remaining gravity difference between the observed gravity and the model gravity. We like to say at this point: “ignore the gravity at your peril”! There is clearly something still missing in the model that should satisfy the remaining difference gravity field. We then invoke a gravity inversion using the sediment density as our inversion variable, to understand one possible reason for the remaining gravity field differences.
In the example below, the salt geometry (canopy and authochthonous / roller level) was finalized and held fixed during subsequent gravity inversions. The sedimentary densities had been created from a density vs. depth function derived from a study of density logs (upper left cross section). We then invert the gravity data by allowing the sediment densities to be altered. On the lower right we see the amount of density change that is required to match the remaining gravity field differences. An area of higher density is predicted to lie just beneath the central canopy, possibly indicating a salt weld that juxtaposed younger rocks on the left against older, more dense rocks beneath the primary canopy.
At this point, we can convert the inverted density cube back to apparent velocity to directly overlay and compare with the then-current seismic velocity model. In this way, we can study alternative velocities beneath and around the salt canopy, driven by our gravity inversion results and closely constrained by seismic control. Gravity difference maps before and after inversion for apparent density are shown on the lower left. Our results indicate a much-improved difference gravity field using geologically plausible alternative model densities.
2-D and 3-D Structural Modeling
The example shown below illustrates a 2-D model in progress over Eastern Canada where depth to magnetic basement estimates are first used to create a basement surface, integrated with seismic constraints. This is used as the top of crust for gravity inversion to derive depth to mid-crust and Moho. 2-D modeling is then employed to investigate sedimentary structure, and test alternative density of crust and mantle. A series of cross section models are then used to develop a regional tectonic framework and provide constraints on full three-dimensional modeling at both regional to prospect scales, to fully understand exploration targets.
Integrated Basement Studies
One key to integrated geophysical methods is constructing a correct depth to basement - particularly if your seismic imaging of basement is challenging and/or if the crystalline basement is a deeper surface beneath a shallower acoustic basement. We have developed a new depth to magnetic basement tool – MagDepth™, which has numerous methods built in for computing magnetic depth to source, with a key emphasis on properly computing depth with the thickness of the magnetic crust as a built-in constraint. This is especially critical for thin-crust environments, where much of today’s exploration efforts are focused. See Flanagan and Bain (2013) for more information.
In parallel with depth to magnetic basement, we apply gravity inversion methods to derive depth to high-density basement. This surface is often strongly associated with seismic–acoustic basement, while magnetic / crystalline basement can be a deeper, underlying tectonic framework that is setting up and underpinning shallower acoustic / high-density basement structures.
The example shown below is the depth to magnetic basement over a key area offshore Newfoundland, from our Eastern Canada Regional Crustal Study.
In addition to depth to magnetic basement, we also provide “Curie” depth mapping, where we determine the depth to the bottom of the magnetic crust using the magnetic data, generally over large regional study areas. This assists us in our depth to magnetic basement work by providing us with an initial magnetic crust thickness, and plays well into basin and thermal modeling, as we can derive an estimated heat flow from our Curie depth mapping results.
“Labrador Sea – The extent of continental and oceanic-crust and the timing of the onset of sea-floor spreading”, by J. A. Chalmers and K. H. Laursen, in Marine and Petroleum Geology 12 (1995)
“The oceanic crustal structure at the extinct, slow to ultra-slow Labrador Sea spreading center”, by Delescluse, Funck, Dehler and Louden, in Journal of Geophysical Research (2015)