Geol 135 Sedimentation
J Bret Bennington
Updated 12/99

Seismic Stratigraphy

Sequence stratigraphy could not have been developed without extensive information on the stratigraphic patterns in the subsurface of continental shelves and other depositional basins over thousands of meters of depth and hundreds of kilometers of distance. Large scale images of the subsurface revealed the unconformities and packages of strata characteristic of sequence boundaries, sequences and systems tracts.

Most of this information came out of the intensive search for petroleum reservoirs during the last several decades using subsurface imaging from seismic reflection profiles. These profiles are based on the generation of low frequency sound waves which travel through the subsurface and are reflected by surfaces within the rocks. Reflected waves are recorded by microphones at the surface and this information is used to generate a subsurface profile.

How to generate a seismic profile

1. Explosives or a "vibraseis" truck generate shock waves at the surface. At sea a compressed air gun is used.
2. Shock waves travel through the interior strata. Waves that encounter a boundary between different materials are partially reflected because of differences in sonic velocity and density (sonic velocity X density = acoustic impedance). High acoustic impedance = large amount of reflection, meaning that rock layers with very different mineral density or porosity characteristics generate strong reflections.
3. Microphones called geophones (hydrophones at sea) record reflected sound waves.
4. Computer processing determines the two-way travel time of each relector surface.
5. A vertical profile of reflectors is drawn as a sinusoidal trace. Reflection peaks are filled in to make them visible. Put together into successive columns, the vertical profiles create a cross section with visible reflection planes.

With current digital signal processing technology it is possible to image the subsurface in some detail down to a depth of 5 km.

Multiple seismic profiles can be intersected to produce a 3-dimensional profile of the subsurface. Computer interpolation of a 3-D seismic survey results in a large scale, continuous 3-D image of the subsurface that can be used to study the areal patterns of stratal units - something that is difficult, if not impossible, to do with surface outcrop data alone.

Seismic profiles related to geologic cross sections

At first glance, seismic profiles seem to be synonomous with geologic cross sections. The two are certainly related; one can identify layers, unconformities, faults, folds, and other geologic features on a seismic profile. However, tthere are some important differences that must be kept in mind when interpreting seismic profiles.

1. Scale - a typical seismic wave has a frequency of 100 Hz, which translates to a wavelength of about 15 m, which is the lower limit to resolution of layers in a seismic profile. Geologists typically focus on beds that are an order of magnitude or more thinner than this. The units defined by reflectors are not individual beds, but packages of strata.
2. Different beds or packages of beds will not show up on a seismic profile if there is insufficient contrast in acoustic impedance. For example, sandstones and conglomerates would not be resolved.
3. The lithology of layers resolved in a seismic profile can only be broadly guessed at, unless drill cores are available from the subsurface that can be correlated with the seismic section.
4. Depth on a seismic profile is given as two-way travel time, rather than as thickness. Travel time is partly a function of thickness, but it is also a function of acoustic impedance, therefore seismic profiles distort true thickness. Also, angles of layering and of faults shown in the profile are distorted. If the acoustic characteristics of the different layers in a profile can be determined from drill hole data, then travel time can be converted to an estimated depth in meters.

Interpreting seismic profiles

Continuous reflectors - these are strong lines on a seismic profile that mark significant, widespread changes in lithology, denoting a widespread change in depositional environment in the basin. Changes in tectonic setting, climate, and / or sea level create strong reflectors, which often are associated with parasequence, systems tract, and sequence boundaries.

Clinoforms - these are inclined surfaces on reflectors bounding stratal packages. It is tempting to relate clinoforms to depositional processes such as progradation, but again, there is a problem with scale. Clinoform slopes are very steep - much steeper than most depositional slopes, which are generally less than a few degrees. Only in carbonate environments are steep slopes common (for example, slopes of 25° can develop in front of reef crests), however, not all clinoforms can be related to carbonate environments. It is likely that differential compaction explains the development of some clinoforms in clastic settings, where mud-rich sediments will compact more than sand-rich sediments, causing a downwarping of the distal end of stratal packages.

Erosional truncation and unconformities - these do not create reflectors themselves, rather, they are revealed by reflector terminations. Generally, some angular discordance is needed between the reflectors and the unconformity for the unconformity to be resolved; the greater the angle, the better. Minor episodes of erosion and unconformity generation may not show up on a seismic profile unless the relief of erosion exceedes the resolution depth of seismic imaging.

Reflector relationships

Onlap - the successive deposition of stratal packages toward the shoreline, often progressively covering an erosional surface. Onlap occurs during transgression as depositional environments backstep shoreward.

Downlap - the successive depositon of stratal packages over underlying strata toward the basin center. This is generally a progradational pattern, occurring during relative sea level fall as sediment packages build farther out into the basin.

Toplap - the pattern made by the deposition of a horizontal strong reflector above a succession of downlapped or inclined packages of strata.

Offlap - a pattern of stratal packages and their reflectors the both prograde and aggrade, building upward and outward into the basin.

Structural features

Folds and faults can be recognized on seismic profiles. Fault surfaces do not show up as distinct reflectors. Even if there is a distinct difference in acoustic impedance across the fault, the generally high angle of many faults (>45°) results in a weak reflection signal to the surface. Generally, faults are recognized as disruptions running through a vertical sequence of horizontal reflectors.

Seismic facies

Some general information as to the nature of the rock in a seismic profile can be gleaned from the patterns of the reflectors.

Continuous reflectors - suggest sedimentary strata deposited in a relatively stable environment that changes periodically through time. Example: continental shelf

Discontinuous reflectors - suggest sedimentary strata deposited in regionally heterogeneous environments. Terrestrial and shallow water carbonate depositional environments tend to produce discontinuous reflectors.

Chaotic reflectors - suggest crystalline rock such as evaporites, igneous, or metamorphic bedrock.