The Earth Story

Seismic Interpretation: Learning to Read between the Lines

Ever wondered how geologists study rocks buried thousands of metres below the surface? Or how oil and gas companies find hydrocarbons when drilling under the sea floor? The answer lies in the study of subsurface reflections known as seismic. These surveys have revolutionised our understanding of underlying strata and have been instrumental in many of the world’s oil and gas discoveries. The seismic section below is from offshore Morocco and comprises numerous reflections representing changes in the subsurface. Contrary to popular belief the different reflections do not necessarily reflect changes in lithology (rock type), and are in fact the product of changing densities and velocities between two horizons.

For example, if a seismic wave hits the surface between a shale and a sandstone, then the differences in acoustic impedance (density x velocity) of the two layers will cause a reflection. The stronger the difference between the two layers the stronger (and brighter) the reflection will be. Going from a hard rock to a soft rock will produce a negative reflection (in most cases blue) whereas a transition from soft to hard will give a positive (normally red) response.

However, a seismic wave hitting a cemented horizon within a sandstone (the pore space being infilled with cement increases the velocity and the density of the rock) will also create a reflection, despite the rock type remaining the same.

Furthermore, because the reflection strength and polarity are dependent on the relationship between the two layers there is no hard and fast rule as to what lithologies are being imaged without well data or prior knowledge of the area.

Looking at the seismic section below you can see that the data has been annotated with the time in seconds along the vertical axis. This is the time it takes for a reflection to reach a horizon and return to the surface, and is called the Two Way Travel Time (TWTT). In areas of very complex geology or where companies wish to increase the accuracy TWTT can be converted to depth, removing any artifacts produced by velocity anomalies.

So what does seismic tell us? It can highlight structures within the subsurface such as faults or folds, as well hinting at the environment of deposition. With this it is possible to start narrowing down the lithologies involved and a rough geological history can be constructed. As can be seen below an entire cross section through the geology can be inferred, although this takes time, practice and a fair bit of patience!

References: http://aapgbull.geoscienceworld.org/content/94/5/615.abstract

Further Reading: http://www.aapg.org/publications/news/explorer/column/articleid/2471/what-is-seismic-interpretation http://140.115.21.141/download/courses/sequence_strat/10_seismic_stratigraphy.pdf

Image Credit: Dunlap et al, 2010 http://aapgbull.geoscienceworld.org/content/94/5/615.abstract

The Main Himalayan Thrust

These beautiful figures come from a just-published manuscript. They combine the work of several authors who have studied the 2015 Gorkha quake that struck Nepal. Analyses of that quake and the position of the aftershocks have illustrated the deep structure of the main Himalayan Thrust fault, the main fault on which the Himalayan Mountains are growing, at depth beneath Nepal and India.

India is basically being pushed beneath Nepal to build the Himalayas and Tibet. As it moves, it is sliding down along a major thrust fault with the high Himalayas in the hanging wall and India itself in the foot wall. This fault has a complicated structure at depth, with rocks folded and faulted in-between and other previous portions of the fault stranded in the mountains.

As shown in these cross sections, the fault actually has an area where it is almost flat, before it ramps up to the surface. This complicated series of jumps in deep fault surfaces is thought to be common around the world. As these faults ramp upwards, they fold the rocks on top of them and create hills and mountains out of those folds. This cross section gives a good example then for how faults create mountain ranges when continents collide.

The Gorkha quake itself was perhaps another illustration of how complex these fault zones are. Although most of the aftershocks took place on the Main Himalayan Thrust, the main shock actually was located just above the major fault. In other words, there are probably other, smaller faults all along this boundary that take up portions of the stress and rupture at different times, leaving the continent shredded into slivers.

The Main Central Thrust (MCT) is another, older thrust fault on which part of the high Himalayas were pushed up. It is not seismically active in the same way that the MHT is, and in fact in many places only shows up as boundaries between metamorphic rocks. The STD is the South Tibetan Detachment, an actual normal fault in the highest part of the Himalayas that marks the boundary between metamorphosed rocks in the mountains and sedimentary rocks in the highest part of the range. The STD actually outcrops on Mount Everest and marks the boundary between crystalline rocks on the mountain’s face and limestones at the top.

As the Main Himalayan Thrust approaches the surface, it splits into multiple separate thrust faults, several of which are marked on these cross sections.

-JBB

Original paper and image credit: https://doi.org/10.1016/j.epsl.2017.05.032

Other papers cited: https://doi.org/10.1130/G38077.1 http://onlinelibrary.wiley.com/doi/10.1002/2016GL068083/full http://www.nature.com/ngeo/journal/v9/n2/full/ngeo2623.html_ _

Reconstructing a cross section This gif was created by a geologist to show how she can take maps and cross sections of deformed, folded, and faulted layers of sedimentary rocks and undeform them to recognize the original stratigraphy and to calculate how much deformation has happened.

The lower pink layer serves as a breakaway fault, something we call a “decollement”, a weak layer that allows the rocks above it to break loose. As the rocks are squeezed from the side, that layer forms a flat fault that breaks upward repeatedly, stacking sedimentary layers into a pile called a Duplex Structure and folding the rocks above.

When folded and faulted layers are mapped in the field, cross sections can then be “undeformed” by pulling apart the fault deformation. A key check in these exercises is to make sure the length of each bed stays constant across every step. A setup where this works is called a balanced cross section and it indicates that the layers broke and bent along the faults in a normal pattern, as seen here - if the lengths of the beds are changing, then either you’ve misunderstood the tectonics or something has happened internally to disrupt one of the sedimentary layers. 

If a geoscientist can complete the calculation, as done in this animation, she can figure out how much the area has been shortened. In this case, the total shortening shown in this gif is about 40% (the folded package is about 60% as wide as the original package).

Image credit: aace “the period” geo “at” insert term for google’s standard mail system then a normal dot and then follow that up with the com, and I’m writing all this deliberately to avoid automated email finding programs.

Read more: http://pubs.usgs.gov/of/1998/ofr-98-0034/BC.pdf http://bit.ly/1VCL0mS http://www.neckers.siu.edu/pinter/pdf/suppeexercise.pdf

Geology of New England, 1826 #tbt

I was so excited when I found this image that I wasted time I should have been working on an abstract. Oops. This was just too neat.

This is a gigantic geologic cross section of New England drawn about 130 years before the acceptance of plate tectonics. It starts on the left in Boston Harbor and ends in the right at Lake Erie. Some noteable places like Boston, Syracuse, and Utica give context for the cross section.

There’s so much content on here that gives insight to how early geologists viewed this continent – rocks that were gradually folded, single units of shale or greywacke running across half the section, missing faults, but mostly granite/gneiss bodies rising upwards inexplicably to make up the cores of mountain ranges. The Old Red Sandstone, one of the most famous units from Great Britain, even makes an appearance. Today we understand that plate tectonic processes have often caused those igneous cores to be thrust upwards by faults and that some of the igneous rocks west of Boston actually are an accreted island arc terrane.

This file was published last week and made available under a creative commons license from the collections of the New York Public Library. Look closer and see what else you can find.

-JBB

Image credit: The Miriam and Ira D. Wallach Division of Art, Prints and Photographs: Print Collection, The New York Public Library. "Geological profile extending from the Atlantic to Lake Erie." New York Public Library Digital Collections. http://digitalcollections.nypl.org/items/510d47d9-7f0e-a3d9-e040-e00a18064a99

THE ENTIRE SCIENCE OF GEOLOGY IN A SINGLE CROSS-SECTION

This tremendous geopanorama is a simplified “world of geology” as drawn by pioneering geologist Thomas Webster (1773 – 1844) with the pictures of animals and fossils added by Dr Buckland. It was published by Webster and Buckland in independent publications in 1836 and 1937.

Entitled “Ideal Section of a Portion of the Earth's Crust, Inteded to Shew the Order of Deposition of the Stratified Rocks, with Their Relations to the Unstratified Rocks” this is possibly the earliest integrated geologic theory of the earth: Steno’s Law of Superposition, William Smith’s use of fossils to identify geologic strata, and observed relations within igneous and volcanic terrains are all synthesized into a single, idealized model. Even dinosaurs are there, though the word “dinosaur” had yet to be invented. And the Dodo Bird (noted on the section as not found alive since 1691) is demonstrated as an example of a newly extinct species.

Consider: this is the state of the science of geology when the young Charles Darwin returned from his voyage in the HMS Beagle in 1836; this is Darwin’s world of geology! What I find inspiring as a modern geologist is that, given this artistically superb cross-section, I can easily go to places on the earth today that demonstrate all these features: the powers of geologic observations documented by this map are recognizable and modern. And it would not be a difficulty for any of us today to continue the section deeper into the earth, connecting it with modern plate tectonic boundaries and terranes. Or could we? Perhaps never again will a complete compilation of the knowledge base of geologic science be capable of documentation in single, simple but so elegant image.

Annie R.

I downloaded Webster’s version from http://www.pop-pervert.com/files/geology_panorama.html who seems to think he got it off Flickr somewhere. The geologic panorama of Webster and Buckland, translated into German and re-drawn apparently by Heinrich Berghaus is owned by the David Rumsy Map Collection, and can be examined in supreme detail at: http://www.davidrumsey.com/luna/servlet/detail/RUMSEY~8~1~1516~160054:Idealer-Durchschnitt-eines-Theils-D

On-line information on this map is scarce; some further sources include. http://www.oldworldauctions.com/archives/detail/140-063.htm https://archives.aber.ac.uk/index.php/webster-thomas-1773-1844-geologist;isaar