The Cuernos in Torres del Paine National Park, Chile, vertically exaggerated by a factor of two. |
The Cuernos, beautiful, without exaggeration. |
depth = seismic_velocity * two-way_traveltime / 2
The problem is that the velocity of the wave varies as it goes deeper (usually increases with depth as rocks become 'harder'); and, unless we are looking at perfect layercake stratigraphy (not that common!), it also changes laterally. So, if we want to look at the actual geological structures, without distortions due to varying velocities, we need to do a depth conversion and we need a 'velocity model' that roughly describes the spatial distribution of velocities. Precise velocity measurements often come from wells where depth is well known; less precise estimates can be backed out from the seismic recordings themselves, but the solution is often non-unique and multiple iterations are necessary to build a good velocity- and depth model. As a result, seismic reflection data is often interpreted with two-way traveltime on the vertical axis, without depth conversion; and not knowing the true vertical scale makes it easier to use vertical exaggeration with vengeance.
A recent paper, published in Marine and Petroleum Geology, shows that vertical exaggeration of seismic data is indeed very common. Simon Stewart of Heriot-Watt University has looked through 1437 papers published between 2006-2010 and found that 75% of the papers show seismic displays with vertical exaggeration of a factor larger than 2. Only 12% are shown with roughly equal horizontal and vertical scales.
Histogram of vertical exaggerations in 1437 papers. From Stewart (2011). |
The paper suggests that published seismic sections should be labeled with an estimate of the vertical exaggeration, in addition to the usual horizontal and vertical scales [I am guilty myself of not doing this as it should be done]. Even better, one can go further and create several versions of the figure with different vertical exaggerations. The cross section of a submarine lobe deposit below is a fine example of such a display. Showing only the version that was exaggerated vertically 25 times would suggest that this is a deposit at the base of a steep slope; the 1:1 figure at the top brings us back to reality and clearly shows that this morphology and stratigraphy are both extremely flat.
Dip section of a submarine lobe deposit, offshore Corsica. From Deptuck et al. (2008). |
References
Deptuck, M., Piper, D., Savoye, B., & Gervais, A. (2008). Dimensions and architecture of late Pleistocene submarine lobes off the northern margin of East Corsica Sedimentology, 55 (4), 869-898 DOI: 10.1111/j.1365-3091.2007.00926.x
Stewart, S. (2011). Vertical exaggeration of reflection seismic data in geoscience publications 2006–2010 Marine and Petroleum Geology, 28 (5), 959-965 DOI: 10.1016/j.marpetgeo.2010.10.003
A great post. Those references are great... I especially love that multi-scale figure from Deptuck. Definitely seems like a best practice (if there is such a thing).
ReplyDeleteThe figure is also a good reminder: 'flat' is a bit relative. As you point out, depositional dips are small: a delta front slopes at less than a degree, but can sustain a gravity flow.
Anyway, thanks for the insights.
Nice post. Although I have to say that stretched photo of the Cuernos massif looks awesome: like something from the cover of a lurid sci-fi novel.
ReplyDeleteThe stretched clouds look a little uncanny, but the rest of the stretched Cuernos shot is strongly reminiscent of an Albert Bierstadt painting, and therefore quite pleasing to my eye.
ReplyDeleteI was hesitant to vandalize the Cuernos the way I did... Glad to see that the photo is not that appalling.
ReplyDeleteNice post.
ReplyDeleteThis might be nonsense, but the stretched Cuernos mountains gives the 'impression' the photo was taken with a different (longer) focal length. Curiously, it seems to 'deepen' the subject, as well as elongate it.
Vertical stretching is done routinely (and perhaps haphazardly) not just with 2D seismic, but also with 3D objects (fault planes / patterns, surfaces, geobodies, etc). In these cases, I wonder if 3D objects, like the mountain photograph with a perception of depth, might be subject to the same curious visualization effect (illusion?).
This is a nice post on the difference between merely drawing on the data and actually interpreting it.
ReplyDelete@Ebianco: When I volume-interpret, i.e. interpret in 3D, I normally vertically exaggerate the data so I can see faults and subtle features better. However, when I display the interpretation to colleagues, I show them what it looks like at 1:1. Again, a geobody is a multiple-z-value amplitude extraction seen in 3D; like a fault, horizon or anything drawn on the seismic, it will display at the scale you choose.
Good point about 3D seismic - it didn't occur to me that the problem is pretty significant there as well. I just realized I rarely show the 1:1 version when displaying data in 3D.
ReplyDeleteI suspect a direct cause of naive layfolk concluding that California was imminently falling into the Pacific.
ReplyDeleteGreat article! When can we expect your WOGE#296?
ReplyDelete