Seismic reflection terminations
Seismic reflection terminations, also termed "lapout", are stratal discontinuities recognized on seismic sections that separate apparently conformable sedimentary units from non-conformable units, forming the basis of seismic sequence stratigraphy. These discontinuities, based on seismic reflection terminations, are indicators of depositional sequences and system tracts formed by one or more of the variables listed below :
- Tectonic subsidence: creates accommodation for sediment accumulation
- Eustatic sea level change: controls facies tract distribution
- Sediments supply: determines how much sediment is available to fill the basin
- Climate variations: effects the types of sediments eroded and/or deposited due to rainfall
Seismic reflections occur due to acoustic impedance variations in stratal packages at interface boundaries. Each unit has an acoustic impedance equal to the rock density multiplied by the seismic velocity. The intensity of the reflection is dependent on how much energy is reflected back at the interface boundary. For instance, the larger the acoustic impedance the stronger the reflection will be at the sediment interface boundary, causing a brighter reflection. In addition, even small variations in the acoustic impedance can create an interface boundary in reflection data which represents stratal surfaces and discontinuities. These interface boundaries parallel bedding planes and are therefore time stratigraphic horizons along surfaces. While the reflection termination patterns define a surface of stratigraphic discontinuity.
Acoustic Impedance= Density * Velocity
Reflection Coefficient (reflectivity strength)= (AI2 – AI1) / (AI2 + AI1)
Seismic Reflection Terminations
Reflection terminations indicate stratal discontinuities that separate depositional sequences and system tracts from one another. Several types of reflection terminations occur based on environmental indicators such as eustatic sea level change, climate variation, tectonic subsidence, or variations in sediment supply. Types of lapout surfaces are defined below:
- Truncation: Reflector termination due to erosion
- Toplap: Reflector termination at an overlying surface or upper boundary
- Baselap: Consists of Onlap or Downlap, and marks the base of an underlying surface
- Onlap: Reflector termination on surfaces with greater dips than that of the overlying beds; lapping onto a structural high
- Downlap: Reflector termination on surfaces which dip less than that of the overlying beds; lapping onto a structural low
- Offlap: Combination of Toplap and Downlap at both surface
Interpreting Seismic Reflections
In order to interpret seismic sections the vertical and horizontal resolution must be sharp enough to resolve a particular feature from another feature. In vertical resolution, this means the thickness of a bed must be large enough to resolve the reflection from the top to the base of the bed. Where as, horizontal resolution refers to identifying small scale structures in map view, such as channels or reefs.
The resolution of seismic data is often controlled by the amount of data collected during the acquisition phase and the processing technique used to remove excess noise and enhance the wavelets features. Often times the most prominent reflection events are due to major environmental or tectonic changes that create sharp impedance variations between underlying and overlying rock units. For instances:
- Weak reflection: Can be produced by shale overlain by another shale
- Strong Reflection: Can be produced by sand overlain by a shale
In the shale/shale case, a reflection would still be created but it would be difficult to interpret unless the resolution of the seismic data was very high. Whereas, if the units had a strong impedance difference, such as a sand/shale makeup the reflection would be more noticeable in potentially poor data.
An example of interpreting seismic reflector terminations is displayed in figure 4. Here the authors depict different facies packages that can be identified by mapping of the lapout features.
- Figure A and B are the same image. Figure A is the uninterpreted section showing dipping seismic reflectors. Figure B is the interpreted section showing sloping oblique clinoforms which flatten out down structure. This example shows downlap surfaces as the deposit progrades and truncated surfaces from the next deposited unit.
- Figure C and D are the same image. Figure C is the uninterpreted section with clear reflectors changing dip trajectories. The interpreted section in figure D clearly shows sigmoidal clinoforms with both onlap and downlap reflectors.
Figure 5 shows another example of a seismic section with clinoform oriented beds, exhibiting both downlap, onlap, and a truncation surface. (similar to figure 4)
Mapping of lapout surfaces can be useful in understanding timing of deposition within a basin and evidence of regressions or transgressions. The objective being to recognize patterns between lapout surfaces and tie them to facies distribution within sequence tracts. The common sequence tracts, as seen in figure 6 are the highstand system tract, lowstand system tract, and transgressive system tract.
Lapout surfaces showing progressive downlap are common indicators of sediment moving in a prograding manner, which is common in highstand system tracts. While lapout surfaces showing progressive onlap can indicate sediment moving towards the shoreline, possibly indicating the transgressive system tract.
Sequence boundaries, such as the transgressive flooding surface often show truncated lapout surfaces due to surface erosion.
- Bhattacharya, Janok. (2007) "Sequence Stratigraphy: History, Theory and Applications"
- Hart, Bruce (2011) "Introduction to Seismic Interpretation"
- Schroeder, F.W. (2004) "Seismic Reflections"
- Roberts, David; Bally, A.W. (2012) "Principles of Geologic Analysis"
- Mitchum, R.M.; Vail, P.R.; Thompson, S. "Seismic Stratigraphy and Global Changes of Sea Level, Part 2: The Depositional Sequence as a Basic Unit for Stratigraphic Analysis"
- Braz, J.; (2016) "Seismic expression of depositional elements associated with a strongly progradational shelf margin: northern Santos Basin, southeastern Brazil"