Seismic geomorphology

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Overview

Geomorphology is study of landforms, its processes and sediments at the earth surface. Such processes and sediments are produced by sedimentation and erosion. Surface geology helps in observing current or past landforms that are exposed in the sediments on the surface. There is no direct way to observe landforms that are buried in the sediments deep in the earth. Drilling does help, but large number of points of information are needed to deduce any information. Here with seismic data, landforms can be observed that were produced in past and now are buried deep.

Seismic Geomorphology helps examine buried landforms (geomorphological features) using seismic data as tool. Let’s examine these disciplines separately to make sense of the composite word.[1] Reflection seismology (or seismic reflection) is a method using sonic waves to assess the properties of the earth's subsurface from reflected seismic waves (Figure 1). In this method a controlled seismic source of energy , usually a dynamite or vibroseis is deployed (Figure 2). This way the energy penetrates the ground and gets reflected from subsurface layer, creating an image. The data gets processed and the final product looks like an image of subsurface (Figure 1). Geomorphology is science of earth forms, where earth's sub-surface features are studied, including their origin, composition, history and impact of human civilization. Geomorphology concentrates primarily on Quaternary (Pleistocene and Holocene) features[2]. Earth’s landforms reflect the local and regional balance between hydrologic and tectonic processes.[2]

As earlier stated, there are no direct means to observe buried paleolandforms and seismic is one of the key methods that is able to delineate such features in the subsurface. Seismic is usually a 2D image, unless its 3D data. By combining both the sciences of geomorphology and seismic a third dimension to the two dimensional seismic images[3] (Figure 3). Figure 3 is an excellent example of making a three dimensional sense from 2D seismic sections by Posamentier and Kolla 2003[3].

See also: https://en.wikipedia.org/wiki/Geomorphology

Figure -1 Seismic reflection data, this is how seismic reflection data looks like
Figure 2 Seismic reflection outlines. The figure illustrates the seismic waves being reflected by subsurface stratigraphy or landforms[4]

History

Seismic geomorphology is relatively a new science and started in 1970s when better quality two dimensional seismic data became available to oil industry (Figure 2). A basic assumption, that seismic reflections are roughly the geological time lines, gave birth to a new science of seismic stratigraphy (Vail et. al. 1977). Petrel Vail, Bob Mitchum and S. Thomson, in AAPG Memoir 28, recognized that the relationship of reflection packages to each other. They observed that the relationship of strata can be classified according to their geometry and mutual relationship such as top lap, onlap, truncation, etc.

1980s saw a huge improvement in subsurface imaging technology in terms of subsurface imaging and availability. This era also saw advent of three dimensional seismic data, which is acquired in closely gridded 2D seismic data and processed in way that it becomes a 3D volume. 3D data enables us to look into the data as horizon slices, time slices, which provides a plan view.This plan view can highlight the geomorphological features. (Figure 7)

The discipline of seismic geomorphology is in its early stages of development , but its rapidly growing in due to advancements in seismic imaging technologies resulting in high-quality three dimensional seismic data.[5] In recent past an in depth book on Seismic Geomorphology—Applications to Hydrocarbon Exploration and Production, was published in 2006.[5] Figure 4.

Figure -3 Making three dimensional geomorphological sense from 2D seismic data
Figure 4 Cover page of the Seismic Geomorphology book

Seismic attributes and geomorphological processes/ features

Various seismic attributes[6] can be extracted from seismic data and combined together to get insights in depositional systems (Figure 7 and Figure 5).[1] These attributes can be related to amplitude, structure, dip, curvature roughness or 3D perspective. The attributes can be co-rendered to get more insights into various features. Attributes are a powerful tool when viewed in time slices[7] or horizon slices.

Figure 5 Examples of Pleistocene meanderloop cutoffs in the De Soto Canyon area of the Gulf of Mexico. A, B) Cutoff meander loops of a high-sinuosity leveed channel, forming two oxbows. C) Seismic profile illustrating a section view of the meander cutoff. (Posamentier 2003)
Figure 6 Mass Debri flows interpreted from seismic
Figure 7 shows seismic horizon slice and various seismic attributes to give insights into deposition systems.Source: Seismic geomorphology - an overview, H. W. Posamentier, R. J. Davies, J. A. Cartwright and L. Wood
Figure 8 Geomorphological processes covered by seismic method

Geomorphological Processes

Land forms sizes can be a million square kilometers to meter sized features, therefor seismic method has it limitations when applied to the discipline of geomorphology. Seismic data is acquired in usually in 2 D or 3D, and most of the data is acquired by E&P industry with primary focus on hydrocarbons and covers a limited geographical area. Below are examples of landforms that can be described by seismic method.

Geomorphological processes that can be delineated by seismic

(a) Fluvial processes such meandering channels, oxbow lakes as shown in Figure 5.

(b) Tectonic processes such as parts of orogenic belts, mountain belts, sediment depocenters, various types of basins including passive margins and foreland basins

(c) Igneous processes such as SDRs(Seaward Dipping Reflectors), which indicate magmatic activity, rejuvinating the landscape in the longer run[2]. Volcanic cones build considerable amount of new topography. whether the new material is denser or less dense than the rock it displaces. (c) Glacial processes, can be visible in the form of U shaped valley profiles profile.

(d) Hillslope processes such debriflows, salt deposits.

(e) Marine processes results in mass debri flows (Figure 6)[8], turbidites

Geomorphological processes that are hard to image with seismic data

There are many processes which are hard to imaged/delineated with seismic data include (a) Aeolian processes resulting in deserts.[2](b) biological processes result in beaver dams, fallen tree, burrowing by animals.(c) small scale marine deposits associate with waves.

Scales in Geomorphology and seismic imaging constraints

Geomorphological processes dominate at different spatial and temporal scales and range from a meter size to ten of millions of sq kilometers[2]. Seismic method can only cover process that fit within the limitation of the data. Figure 8, provides provides a visual understanding . Process in range of :-

  1. ~10 Million km2 , ~1 Million  km2 , ~100 thousand km2 Seismic can not cover. Its too large and seismic survey can not cover such a large area.
  2. ~10 thousand km2 - 2D Seismic may cover by combining various seismic lines yest its not easy
  3. ~1 thousand km2 - 2D Seismic can cover. Also 3D mega survey, by stitching various surveys, can delineate these process and related features.
  4. ~100 km2, ~10 km2 , ~1 km2 2D & 3D Seismic can cover.
  5. Meter-sized features - Too fine for seismic to image by current seismic technology. Seismic can describe cover few square meters of features.

References

  1. 1.0 1.1 Seismic geomorphology - an overview H. W. Posamentier, R. J. Davies, J. A. Cartwright and L. Wood Geological Society, London, Special Publications, 277, 1-14, 1 January 2007,https://doi.org/10.1144/GSL.SP.2007.277.01.01
  2. 2.0 2.1 2.2 2.3 2.4 https://en.wikipedia.org/wiki/Geomorphology
  3. 3.0 3.1 Posamentier, Henry W., and Venkatarathnan Kolla. "Seismic geomorphology and stratigraphy of depositional elements in deep-water settings." Journal of sedimentary research 73.3 (2003): 367-388.
  4. https://en.wikipedia.org/wiki/Reflection_seismology
  5. 5.0 5.1 POSAMENTlER, H. W. "Seismic geomorphology-an overview HW POSAMENTlER, 1 RJ DAVlES, 2 JA CARTWRlGHT3 & L. WOOD4." Seismic geomorphology: applications to hydrocarbon exploration and production 277 (2007): 1.
  6. https://en.wikipedia.org/wiki/Seismic_attribute
  7. Time slice
  8. Sawyer, Derek E., et al. "Seismic geomorphology, lithology, and evolution of the late Pleistocene Mars-Ursa turbidite region, Mississippi Canyon area, northern Gulf of Mexico." AAPG bulletin91.2 (2007): 215-234.