Encyclopedia of Marine Geosciences

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| Editors: Jan Harff, Martin Meschede, Sven Petersen, Jörn Thiede

Bottom-Simulating Seismic Reflectors (BSRs)

  • Jürgen MienertEmail author
  • Stefan Bünz
Living reference work entry
DOI: https://doi.org/10.1007/978-94-007-6644-0_133-3

Keywords

Instantaneous Frequency Impedance Contrast Methane Hydrate Blake Ridge Polygonal Fault 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Definition

A seismic reflection occurring in the upper few hundred meters of marine sediments mimicking the seafloor, crosscutting sediment layers, and showing a phase reversal is known as a “bottom-simulating reflector.” Such a gas hydrate-related BSR originates from a large impedance contrast between a layer of gas-hydrated sediment above and a free gas layer below. A diagenetic-related BSR occurs at the opal-A/opal-CT transition zone, lies often deep and outside the base of the gas hydrate stability zone, shows no phase reversal, and does not always mimic the seafloor.

Introduction

The intent of this article is to describe the two most commonly observed bottom-simulating reflectors (BSRs). The term BSR stems from their principal characteristic that these reflectors mimic the seafloor topography in marine seismic reflection data thereby crosscutting sedimentary strata. BSRs are known to occur in continental margin sediments in regions of gas hydrate and free gas (Shipley et al., 1979) and/or in siliceous ooze (diatoms, radiolaria, silica sponges, silicoflagellates) bearing sedimentary formations (Hein et al., 1978). The largest silica contribution comes from diatoms (Holland and Turekian, 2003), while the largest methane contribution derives from methane-producing Archaea in sub-seafloor sediments (e.g., Kotelnikova, 2002).

Diagenesis-Related BSR

Siliceous ooze occurs in a wide range of sedimentary basins indicating times of higher ocean productivity, for example, along equatorial, polar, or coastal upwelling regions (Holland and Turekian, 2003). With increasing burial depth, the temperature and pressure increases, resulting in the dissolution of the siliceous skeletons. The dissolution process causes a collapse of the siliceous framework and a geochemical reaction that converts amorphous opal-A into opal-CT (e.g., Hesse, 1989; Knauth, 1994). Deep-sea drilling project studies allowed documenting the chemical, mineralogical, and structural changes occurring during this process with increasing depth (e.g., Hurd and Birdwhistell, 1983). As a consequence of the skeleton dissolution, an interface develops where the opal-CT formation causes an increase in density and compressional-wave velocity and a decrease in porosity and permeability (e.g., Tribble et al., 1992). If the seismic impedance contrast between opal-A (lower density and velocity and higher porosity and permeability) and opal-CT becomes large enough, a seismic reflector with a positive polarity occurs (Fig. 1) (Berndt et al., 2004). It is believed that the temperature increase with burial depth is the main parameter controlling the opal-A/opal-CT transition aside from the time since burial, type of surrounding sediment material, and interstitial waters (e.g., Hein et al., 1978). The opal-A/opal-CT diagenesis causes a volume reduction of as much as 30–40 % (Davies and Cartwright, 2002). Since large areas of siliceous ooze exist in sedimentary formations, a diagenetic BSR (Fig. 1) often shows a very large lateral extent, which is uncommon for gas hydrate-/free gas-related BSRs. Moreover, diagenetic BSRs are believed to develop at greater depth below the seafloor, at temperatures (35–50 °C) where hydrate is normally no longer stable (Berndt et al., 2004).
Fig. 1

Reflection seismic profile from the mid-Norwegian margin siliceous sedimentary formation showing an example of an opal-A/opal-CT BSR. The BSR and amplitudes are offset by polygonal faults. The origin of BSR 2 is speculative and may be related to transformation from smectite to illite at higher temperatures (Figure is from Berndt et al., 2004, Fig. 4)

Opal-A/opal-CT BSRs have been reported from many areas containing siliceous sediments such as from the mid-Norwegian margin (e.g., Brekke, 2000; Berndt et al., 2004) (Fig. 1). Often, polygonal faults occur in conjunction with diagenetic BSRs as they preferably form in similar types of sediments (Fig. 1) (Cartwright and Dewhurst, 1998; Davies and Cartwright, 2002). Polygonal faults show an interruption and vertical offset of continuous reflections leading to short reflection segments. The BSR may be difficult to identify if it runs parallel to the strata, because normally it also does not show a reduced instantaneous frequency (Berndt et al., 2004). The BSR has a strong amplitude, has an apparent polarity that is positive, lies deeper than a gas hydrate-related BSR, and is often interrupted by polygonal faults.

Gas Hydrate-Related BSR

Gas hydrates occur as an icelike substance composed of water molecules forming a rigid lattice of cages that trap a guest molecule (Sloan, 2003). The predominant guest molecule in the submarine environment is methane, but also hydrates containing high-order hydrocarbons, carbon dioxide, hydrogen sulfide, or other gas may exist. Gas hydrates occur naturally in the pore space of different types of marine sediments, where appropriate high-pressure and low-temperature conditions exist and there is an adequate supply of gas and water (Fig. 2) (Kvenvolden, 1993; Rempel and Buffett, 1997; Sloan, 2003). Those requirements confine marine gas hydrates to the upper few hundred meters of the shallow geosphere of continental margins, where biogenic processes produce sufficient amounts of methane gas. The gas hydrate-related BSR detected on marine seismic reflection data commonly corresponds to the base of the gas hydrate stability zone (GHSZ, Fig. 2). It is the result of an acoustic impedance contrast between hydrate-bearing sediments (increase in compressional-wave velocity) and free gas trapped in the sediments underneath (decrease in velocity) gas hydrates (Hyndman and Spence, 1992; Bünz et al., 2003). As a consequence, the hydrate-related BSR has reversed polarity (compared to the seafloor reflection) and is often accompanied by high-reflection amplitudes. The gassy sediments beneath the hydrate-bearing sediments also show on the instantaneous frequency attribute as the free gas predominantly attenuates the high-frequency content of the seismic signal (Berndt et al., 2004).
Fig. 2

Reflection seismic profile of a hydrate/free gas BSR that follows the sub-bottom depths and coincides with the predicted depth of the base of the gas hydrate stability zone (schematic diagram, right). The gas hydrate stability zone is shown as a function of water temperature, pressure, and geothermal gradient

The BSR shows not always as a reflection proper because gas beneath the hydrated sediments accumulates only in places where rock properties of the host rock have high enough permeability. The seismic reflection shows higher amplitudes preferentially in areas where appreciable amounts of gas accumulate beneath the GHSZ (Figs. 3 and 4). Thus, whether the BSR is a true reflection in its own right on the seismic data is mainly the result of the frequency bandwidth of the acquisition system (Wood et al., 2002). High-frequency seismic acquisition systems often image gas accumulations along layers. The BSR is then identified as the envelope of amplitude increases that crosscuts stratigraphic boundaries (Figs. 2, 3, and 4). The BSR generally lies shallower than a diagenetic BSR, has a smaller areal extent, and might often show in patches over a larger area.
Fig. 3

High-resolution 3D P-Cable seismic data showing higher amplitudes beneath the upper regional unconformity (URU) in the SW-Barents Sea. This high-amplitude reflection crosscuts sedimentary reflections with reversed polarity and is interpreted as BSR at the base of the GHSZ

Fig. 4

High-resolution 3D P-Cable seismic data showing higher amplitudes preferentially in areas where appreciable amounts of gas accumulate beneath the GHSZ

Assumptions on the possible presence of gas hydrate-/free gas-related BSRs along continental margins are based largely on modeling the GHSZ (Fig. 2) (Dickens and Quinby-Hunt, 1997; Zatsepina and Buffett, 1998). The modeling and thus the theoretical potential for the existence of a BSR are mainly based on water depth (pressure), seafloor temperature, and the geothermal gradient (Fig. 2). As these parameters are largely controlled by water depth from the shelf to the deep sea, the methane hydrate BSR shows two pinch-out zones, one at the shallow water depth where pressure becomes too low for hydrates to be stable and one toward the mid-ocean ridge where heat flow becomes too high. Thus, if one describes these two end members, one may still use the terminology BSR but should be clear that the BSR does not parallel the seafloor.

Seismic Character of BSRs

BSRs related to hydrate/free gas phase transitions can be distinguished from opal-A/opal-CT phase transitions based on their phase polarity. While diagenetic BSRs have the same positive phase as the seafloor reflection (Fig. 1), the hydrate/free gas BSRs show a phase reversal (Fig. 2). This reversed polarity is due to the negative impedance contrast between hydrated sediments above (higher density and velocity) and gas-saturated sediments below (Hyndman and Spence, 1992). However, caution is necessary if the phase of the BSR is the only criterion used. Gas trapped beneath a diagenetic BSR may also result in a phase reversal. As a consequence one should use several criteria such as instantaneous frequency, phase reversal, and GHSZ modeling to predict the BSR depth. Frequencies are useful as additional criteria for gas hydrate BSRs. A shift from higher frequencies in the gas hydrate zone to lower frequencies in the free gas zone (Berndt et al., 2004) beneath the BSR may be used.

Dynamics of BSRs

Both the diagenetic- and the hydrate-related BSR may be used to evaluate the thermal state or reconstruct the thermal development of a sedimentary basin (Grevemeyer and Villinger, 2001; Nouzé et al., 2009). Particularly, the hydrate-related BSR is widely used as a proxy for heat flow in marine sediments. On a small scale, BSR-derived heat flow changes may often be associated with structures that focus warm fluids from deep sediments like mud volcanoes (Depreiter et al., 2005) or chimneys (Rajan et al., 2013). In such instance, the BSR may not mimic the seafloor if lateral variations in heat flow or changes in gas compositions exist. Generally, an increase in heat flow causes a shoaling of a BSR, whereas an increase of higher-order hydrocarbons causes a deepening. Hence, the depth of the BSR may vary greatly as, for example, in the Barents Sea depending on the contribution and thus the amount of thermogenic gases migrating into the GHSZ (Chand et al., 2008; Rajan et al., 2013).

More recently, BSR observations and hydrate stability zone modeling of the upper pinch-out zone on continental margins have been used to assess past and contemporary changes in hydrate stability through warming of ocean bottom water (Vogt and Jung, 2002; Mienert et al., 2005; Biastoch et al., 2011; Ferré et al., 2012; Phrampus and Hornbach, 2012).

Inferred former positions of a BSR are often referred to as paleo-BSR. One of the best examples for a paleo-BSR can be found on the Blake Ridge, approx. 450 km offshore Georgia on the East Coast of the United States (Hornbach et al., 2003). This BSR formed when erosion by strong contour currents on the eastern flank of the Blake Ridge removed the top sediments of a hydrated formation causing an adjustment of the hydrate/free gas boundary by moving it deeper. The free gas layer beneath the former BSR crystallized into a newly formed concentrated layer of hydrates causing both a density and velocity increase. Beneath this paleo-BSR, the new BSR formed with free gas underneath the base of the gas hydrate stability zone (BGHSZ). Though the timing of the readjustment is unknown, it presents one good example for the dynamic behavior (in this case deepening) of a BSR due to erosion of sediments and a drop in seafloor temperature.

Summary and Conclusions

Bottom-simulating reflectors (BSRs) occur in a wide range of sediments in the world oceans. Such creation of BSRs involves the existence of free gas and water in the pore space of sediments under low temperature and high pressure, forming hydrates beneath the ocean floor. The depth of the BSR defines the base of the gas hydrate stability zone (BGHSZ) under which free gas accumulates. Free gas becomes trapped beneath the hydrate-charged layer causing a distinct impedance contrast and a seismic reflection of reversed (negative) polarity if compared to the seafloor. The GHSZ depends on temperature and pressure and to a lesser degree on salinity and gas composition (thermogenic, biogenic). The thickness of the GHSZ decreases toward the upper continental margins (lower pressure) and sedimented ocean ridges (higher temperature). The second type of BSRs concentrates in regions of siliceous ocean sediments. Here, increases in temperature with burial depth result in dissolution of siliceous skeletons, which in turn creates an interface with higher porosity and permeability above and lower values beneath the interface. If the contrast becomes large enough, a seismic reflector occurs but with positive polarity (no phase reversal). As a consequence, diagenetic BSRs occur commonly deeper, show no phase reversal, and exist over large areas at the opal-A/opal-CT interface in ocean sediments.

Cross-References

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Copyright information

© Springer Science+Business Media Dordrecht 2014

Authors and Affiliations

  1. 1.Centre for Arctic Gas Hydrate, Environment and Climate (CAGE), UiT The Arctic University of NorwayTromsøNorway