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You are in:  Home » Publications » Reports » Report 16 » Appendix 7

SCAR Report No 16,

Appendix 7

SEISMIC STRATIGRAPHY AND SEDIMENT PROPERTIES:
A NEED FOR GROUND TRUTH
Anders Solheim

Background

In a regional sense, the Antarctic continental margin (at least the accessible parts of it) has an extensive coverage of seismic data, acquired by a number of countries over the last 20 years or so (Cooper et al., 1995). This includes both multi-channel seismic data (MCS, more than 200,000 km) and single channel data (SCS, at least an equal amount, probably more), using a variety of tools and seismic configurations. The interpretation of these data, however, suffers greatly from the sparcity of “ground truth”. Sediment sampling campaigns around Antarctica have mainly involved conventional piston- and gravity coring, usually limited to the upper 10 m sediment depth. Most commonly core recovery on the continental shelf is much less than 10 m, recovering the Holocene, while overcompacted tills from the last glacial maximum prevent further penetration. Given a seismic resolution of 5-10 m at best, it is clear that piston coring and related techniques have a limited potential for ground-truthing seismic data.

In addition to the CIROS-1 and MSSTS-1 drilling projects in the Ross Sea (Barrett, 1986, 1989), drilling operations offshore Antarctica are limited to a few DSDP / ODP legs, of which only four (Legs 28, 35, 113 and 119) drilled on the Antarctic margin (Hayes, Frakes et al., 1975; Hollister, Craddock, et al., 1976; Barker, Kennett, et al., 1988; Barron, Larsen, et al, 1989). Of these again, only Legs 28 and 119 drilled on the continental shelf, in the Ross Sea and Prydz Bay, respectively. Including the CIROS-1 and MSSTs-1 sites, a total of 10 wells are drilled on the Antarctic shelf, confined to two areas. Considering the size of Antarctica, and the fact that is is surrounded by continental shelves, the difficulties and discrepancies in seismic interpretation are not surprising.

Another problem is the relatively poor core recovery of the few boreholes that do exist. An exception is the CIROS-1 borehole, drilled from fast sea ice, in which a core recovery of 98% was obtained (Barrett, 1989). The DSDP / ODP sites have recoveries varying roughly between 20 and 60%. The recovery problems mostly relate to the diamictic character of the glacial sediment encountered around Antarctica (as well as in other glacial continental margins). On the shelf, the diamictons are often heavily overcompacted, enforcing the need for rotary coring, which often severely disturbes the recovered sediments. Such disturbance makes them useless for physical property measurements and, consequently, for calculations of acoustic impedance logs to create synthetic seismograms.

Therefore, in future work there is a great need for:

• Increased number of drillholes.
• Smaller, more flexible and less costly operations than ODP type drilling.
• Improved coring techniques which give udisturbed samples, adequate for geotechnical testing.
• Downhole logging of all drilled wells.

Drilling plans are treated under other parts of the workshop agenda, but it should be mentioned that ODP drilling is proposed by a Detailed Planning Group (DPG) for a four year campaign, starting in 1998. The DPG proposed drilling legs in all of the five main “ANTOSTRAT areas”; the Antarctic Peninsula, Ross Sea, Wilkes Land, Prydz Bay and the Weddell Sea. Until now, however, only the Antarctic Peninsula proposal is approved by ODP, and is scheduled for February - April 1998, as Leg 178. Shallow, light-weight drilling, using a mobile rig to mount over the side of an oceanographic vessel, has been tested over the last two years by a Norwegian group. This equipment is currently designed to diamond drill to depths of around 50 mbsf and obtain 4.5 cm diameter cores. Further development will take place over the next couple of years. The most recently scheduled drilling operation in the Antarctic is the Cape Roberts Project (Barrett et al., 1995), planned to start drilling from fast sea ice in the Ross Sea during the austral summer of 1998.

Physical Property Measurements; Parameters And Techniques

Age, biostratigraphy and lithology represent of course crucial pieces of information for an interpretation of paleo-environments and paleoclimate. These are to a large extent dependent on the study of core samples. There are many examples that interpretations of lithology and paleoenvironment based on seismic data are far from straightforeward, even using very high resolution tools. Often acoustic variations are not associated with any significant lithologic change, when drilled (e.g. Stoker, 1997). In the Barents Sea, structures which could be interpreted as a buried moraine topography from the seismic records turned out to be slabs of glacitectonized Mesozoic bedrock when subsequently drilled (Gataullin & Polyak, 1997). The boundary between pre-glacial and glacial deposits in dipping continental margin sequences is another important environmental boundary which is difficult or impossible to determine by seismic data alone.

However, to tie various borehole information, for instance ages, to seismic records and thereby extend the borehole information regionally, require sediment physical properties data. Seismic velocity information is necessary to enable conversion from two-way travel time to sediment depth (and vice versa). Wet bulk density is needed in addition to the velocities, to compute acoustic impedance logs and construct sythetic seismograms, a tool of great importance for the correlation between sediment cores and seismic sections.

Downhole Logging

Physical property measurements are particularly well suited for down-hole logging. Not only can this give continuous records, but the results will also be much closer to real, in-situ conditions. Logging tools used in ODP operations have been adapted from industry-standard tools to fit the 3.8 inch drillstring bore used aboard the “Joides Resolution”. However, a wide range of logging tools are available for “slim-hole” operations and can be used in holes with diameters down to two inches (BPB Slimline Services, 1996). There are no minimum limitations to hole depth. Any hole longer than the tool length (usually 1-4 m) can be logged if the hole conditions are adequate. Although open holes give the most reliable results, many tools can also be used with relatively good results in cased holes. The slim-hole logging possibilities are particularly interesting for alternative (to ODP) drilling operations on the Antarctic continental margin.

A number of different sensors, combined in a variety of ways, are available. For studies of Cenozoic, glacial sequences the most essential parameters to measure are P-wave velocity, bulk density, lithology and magnetic properties (susceptibility and polarity). These are all standard parameters measured in ODP Logging operations. With the possible exception of magnetic polarity, slim-hole tools are available for all these parameters. Acoustic velocity can be measured with a vertical resolution of 20 cm, magnetic susceptibility with a resolution of 25 mm, to mention a couple of examples. Lithology and porosity information can be obtained from natural gamma tools combined with neutron tools. These are only examples, but the essential point is that most physical sediment properties (as well as lithological information) can be measured or extracted from downhole logging.

In shallow holes, primarily in unlithified sediments, unstable hole conditions may be a severe limiting factor for downhole logging. Another important point is cost. The company BPB Slimline Services, which is a world leader in slim-hole logging operations, estimates operating charges of USD 1600 - 2200 per day for equipment and crew (2 persons). Freight, travel, mobilisation, demobilisation, etc., would accrue. For a 50 days cruise a total cost of USD 100.000 - 150.000 is likely. However, as logging provides continuous data from near in-situ conditions, this investment may prove to be “good value for money”. Furthermore, careful correlation between downhole logs and ship / shore based measurements may help finding the correct stratigraphic position for cores in poor-recovery holes.

Shipboard And Shore Based Measurements

For correlation with seismic data, a MST (Multi-Sensor Track; velocity, density, magnetic susceptibility and natural gamma ray activity) provides the most useful set of measurements. Dependent on core recovery, the MST provides near-continuous measurements of acoustic impedance information for construction of synthetic seismograms. Parameters like susceptibility, natural gamma intensity and bulk density often vary in response to climatic variations, and because of its high stratigraphic resolution (1-3 cm) the MST has therefore proved to be an important tool for quick studies of climate variability which can be carried out immediately after core retrieval. An important aspect of MST measurements is that they are non-destructive to the core. Since poor contact between sediment and core liner introduces a serious source of error, an MST system which also can measure split cores is to be preferred. Given a good MST set-up, other index property measurements on discrete samples can be kept to a minimum, for calibration/checking purposes vs. the MST.

Geotechnical tests of whole-round core sections from the Antarctic margin have only been carried out on material from Prydz Bay, cored during ODP Leg 119 (Solheim et al, 1991). Such testing is important for an evaluation of the loading history of a sediment. This is particularly important in relation to discussions of glacial impact on the continental shelf. Consolidation tests can verify whether seismic sequence boundaries result from erosion and can also provide stimates of minimum past overburden, whether caused by sediment or ice. The usefulness of geotechnical testing is, however, totally dependent on a non-disturbed character of the core. This is often a major problem in rotary coring, like the XCB or RCB techniques used by ODP in hard, glacial diamicts.

Suggestions And Topics To Discuss

• ANTOSTRAT related efforts over the next five years should focus on ground truthing existing seismic stratigraphy data rather than acquiring more seismic data. Seismic acquisition should primarily be carried out as site surveys prior to drilling, and/or to tie drill-sites to regional seismic lines. An exception could be to acquire high resolution data in areas which are mainly covered by older, low-resolution data.
• As ODP activity will be limited, in particular on the continental shelf, alternative drilling platforms and/or techniques must be sought. Options are drilling from fast ice (like the Cape Roberts Project), smaller drilling vessels, and mobile rigs to deploy from other vessels. (Other?)
• The most flexible and time-economic option, given the present-day technology, is to use a smaller, ice strengthened drill ship. On the other hand, this is an expensive option.
• At present, to use mobile rigs deployed from oceanographic vessels in Antarctica, requires willingness to take the risk of investing time in testing and development of technology. This is important, however, and somebody has to take this risk(?).
• Coring techniques must be designed to maximize core recovery and minimize core disturbance. Such techniques may be time consuming, but must be preferred over techniques resulting in many, but poor quality sites.
• Downhole logging should always be an integrated part of a drilling campaign.
• After MST investigations, carefully selected whole-round core samples should be cut off, sealed and stored for later geotechnical testing at all sites where past loading history is of interest.
• To follow the above suggestions may requires multi-national efforts. Is this realistic to any volume, or should the focus rather be on smaller operations, feasible under each individual country’s’ resources?

References

Barker, P.F., Kennett, J.P., et al., 1988: Proc. ODP, Init. Repts., 113: College Station, TX (Ocean Drilling Program).
Barrett, P.J. (Ed.),1986: Antarctic Cenozoic history from the MSSTS-1 drillhole, McMurdo Sound. DSIR Bull. N.Z., 237, 174pp.
Barrett, P.J. (Ed.),1989: Antarctic Cenozoic history from the CIROS-1 drillhole, McMurdo Sound. DSIR Bull. N.Z., 245, 251pp
Barrett, P.J., Henrys, S.A., Bartek, L.R., Brancolini, G., Busetti, M., Davey, F.J., Hannah, M.J. & Pyne, A.R., 1995: Geology of the margin of the Victoria Land Basin off Cape Roberts, southwest Ross Sea, in Geology and seismic stratigraphy of the Antarctic margin. Antarctic Research Series, Vol. 68, edited by A. K. Cooper, P. F. Barker, & G. Brancolini, 183-207, AGU, Washington, D.C.
Barron, J., Larsen, B., et al, 1989: Proc. ODP, Init. Repts., 119: College Station, TX (Ocean Drilling Program).
BPB Slimline Services, 1996: Unpublished catalogue of available tools.
Cooper, A.K., Barker, P.F. & Brancolini, G. (Eds.), 1995:Geology and seismic stratigraphy of the Antarctic margin. Antarctic Research Series, Vol. 68, 301pp., atlas, CD-ROMs, AGU, Washington, D.C.
Gataullin, V. & Polyak, L.,1997: Glaciotectonic features, southeastern Barents Sea. In: Davies, T.A., Josenhans, H., Polyak, L., Solheim, A., Cooper, A., Bell, T., Stoker, M. & Stravers, J. (Eds.) "Seismic Atlas of Glacimarine Features". Chapman & Hall, England, in press.
Hayes, D.E., Frakes, L.A., et al., 1975: Init. Repts. DSDP, 28: Washington (U.S.Govt. Printing Office).
Hollister, C.D., Craddock, C., et al., 1976: Init. Repts. DSDP, 35: Washington (U.S.Govt. Printing Office).
Stoker, M.S., 1997: Seismic-stratigraphic record of glaciation on the Hebridean margin, north-west Britain. In: Davies, T.A., Josenhans, H., Polyak, L., Solheim, A., Cooper, A., Bell, T., Stoker, M. & Stravers, J. (Eds.) "Seismic Atlas of Glacimarine Features". Chapman & Hall, England, in press.