Technical Note 25


Where we are now:

The current ODP site survey data guidelines (approved by PCOM in August 1994) require swath-mapped bathymetry, photographic or video data, a regional magnetic anomaly survey and rock sampling for tectonic window drill sites. In addition, the current site survey data guidelines recommend, under certain circumstances, several additional data types: gravity, OBS microseismicity, side looking sonar, high resolution seismic reflection or 3.5 kHz, and deep penetration or surface ship refraction. Proponents are expected to submit "recommended" data types to the ODP Data Bank if such data already exists, but they are not expected to acquire such data if it does not. The seismic data are intended as regional rather than site-specific data types, and, in fact, need not cross the site at all. The high resolution seismic reflection or 3.5 kHz data are intended for use in the selection of backup sites in sediment ponds in the event of failure of the bare rock drilling equipment. Deep penetration seismic reflection or surface-source refraction data are recommended in cases where it is possible to identify and survey an adjacent or conjugate piece of undismembered crust that is expected to have nearly the same crustal structure as that possessed by the targeted crust prior to tectonic disruption.

Each of the "required" data types for tectonic window drilling was available for the Hess and MARK drill sites. In each case the visual data came from submersible dives, typically with one dive actually crossing the drill site. The visual data were presented as interpretive sketches with structures and lithologies indicated as symbols along a profile or map-view track line. The Hess data package included two additional "recommended" data sets: gravity and deep penetration seismic reflection. The MARK data package included the following "recommended" data types: gravity, deep penetration seismic reflection and side looking sonar (nearby but not over the sites). In the case of MARK, only two potential drill sites were documented with site-specific data.

In general, data of the sort submitted for Hess and MARK, in combination with a modest amount of Resolution VIT surveying, proved to be sufficient for finding drillable sites of high scientific interest for single-bit shallow (<200m) penetration sites with unsupported spud-in. No substantive adjustments of existing site survey procedures would be needed to drill more holes of this sort, although it would be prudent to document a larger number of potential drill sites than was done for MARK or Hess.

However, both Hess and MARK experienced great difficulty and limited success at finding spots in which to place a guide base and drill a deep hole. It would appear that successful a priori identification of deeply drillable sites in tectonically dismembered terrains involves three-dimensional characterization of surface and subsurface microenvironments around candidate drill sites of a better quality than the community has yet accomplished. The engineers at this workshop have described their problems at MARK and Hess in two categories: (1) to put down the guide base, (2) to "dig the hole." The remainder of this section discusses ways in which better site characterization might help with these two types of problems. Pre drilling techniques for seafloor characterization can help find the right place to put down the guide base. Pre-drilling techniques for subsurface characterization can help find the right place to "dig the hole." An iterative process, involving close coordination between the drilling and surveying communities, will be needed to find the most useful combination of data acquisition, processing, display and interpretation strategies to reliably find deeply drillable sites.

The remainder of this section deals only with site-specific data, with the expectation that the regional geological and geophysical setting is already well characterized by broader scale surveys.

Seafloor site characterization:

Visual data (typically from a submersible) and high quality swath-mapped bathymetry (typically from a hull-mounted multibeam echosounder) are recognized as absolute prerequisites for siting any borehole in a tectonically dismembered terrain. In addition, the Hess and MARK experiences have shown the importance of detailed pre-drilling characterization of the local seafloor slope and sediment cover around prospective hard-rock guide base sites.

At this workshop, the engineers asked the surveying community to find and document hard-rock guide base targets 100 m in diameter, throughout which the seafloor slope is less than 20° and the sediment cover is less than 1 m. It is clear that such sites are uncommon in areas like Hess or MARK. But it is also clear that we have not come close to exhausting every available strategy for finding and documenting such sites.

Seafloor Slope: In typical oceanic water depths, the footprint of a single beam of a hull-mounted multibeam echosounder is more than an order of magnitude larger than ODP's bare rock guide base. Thus multibeam bathymetric maps are not an adequate database on which to identify locations where the seafloor slope will be flatter than the ~20° maximum slope on which a guide base can be placed. Better slope information can be obtained through a combination of several techniques:

* nearbottom-towed bathymetric mapping sonars: Higher resolution bathymetric maps can be produced with a near-bottom towed multibeam echosounder (such as that under development for the MPL/SIO Deep Tow) or near-bottom towed interferometric swath-mapping sonar (such as that on SeaMARC CL). Such data will not single-handedly document an appropriately flat area, but it could be used to rule out large areas that are not worth consideration as prospective guide base sites.

* quantitative stereo photogrammetry: It is possible to produce extremely fine-scale (decimeter vertical) resolution microtopographic data from digital stereo pairs of vertically incident photographs shot from a near-bottom towed vehicle. To complete such an analysis for a 100-m-diameter photomosaic would be labor intensive, but the resulting map would certainly provide the desired documentation of seafloor slope.

* submersible water depth measurements: Submersibles typically measure their depth with a pressure sensor, and their altitude above the seafloor with a high-frequency echosounder. These two measurements can be summed to produce a high-resolution bathymetric profile along the submersible track. If a potential drill site were criss-crossed with a network of well-navigated submersible dives, a high quality bathymetric or slope map could be produced. Even in the case of a single dive traverse, accurate seafloor slopes can be calculated for those portions of the dive where the submersible is driving directly upslope (typically a large fraction of the time for most dives), provided that the submersible navigation is of high quality. The submersible data for MARK and Hess, as submitted to the ODP Data Bank, were not presented in such a way that seafloor slopes could be accurately or precisely computed.

* Geocompass: The Geocompass, developed by Jeff Karson and associates, is a kind of underwater Brunton compass. When held in the submersible's claw, and placed against a seafloor surface, it can measure the dip and strike of the surface. Whereas the techniques described above have the problem of integrating seafloor slope over a larger area than the guide base footprint, the Geocompass has the opposite problem: it measures slope over a tiny area compared to the guide base footprint. In the site characterization context, Geocompass measurements can be useful as spot checks or ground truth for the broader-scale slope-measuring techniques. Geocompass measurements could also be very valuable in calibrating engineers' eyes for the interpretation of submersible videos: "that's what a 17° slope looks like."

Sediment Cover: A fundamental problem is that a site can exhibit absolutely no penetration on a hull-mounted 3.5 kHz subbottom profiler, and yet have too much sediment to emplace a hard-rock guide base. Improved knowledge of the distribution of sediment cover could be obtained through several existing or viable techniques:

* photomosaicking from a towed vehicle or ROV: A reasonably large area can be photographed with vertically incident cameras mounted on a towed vehicle or remotely operated vehicle. If these data are photomosaicked, an accurate map of outcrop locations can be produced. A photography campaign with towed vehicle or ROV produces many times more areal coverage of outcrop location map than does the same amount of shiptime devoted to submersible diving.

* quantitative photoanalysis of visual data: On the typical interpretive sketch of dive observations, with lithologies and structures represented as symbols along a profile or track chart, the importance of outcrop is quantitatively over-represented. This is partly an unintended side effect of trying to represent all important observations, and partly a reflection of the observer's visceral sense of a dive in which the pilot probably sped over sediment ponds and lingered over outcrops. If, instead, the percent sediment cover is estimated from the still photographs every 15 or 30 seconds, and then graphed as percent sediment cover versus distance along track, it becomes more obvious just how little outcrop there is in a specific area.

* sediment measuring rod: Using its claw, a submersible could stick a calibrated rod into the sediment and measure sediment thicknesses up to about a meter. Areas with more than a meter of sediment are not interesting anyway. This method is primitive, and time consuming, but cheap and effective.

* subbottom profiler on towed vehicle or ROV: A typical subbottom profiler on a near-bottom towed vehicle would be a wide-beam 3.5 kHz or 4.5 kHz downward-looking sonar. It's not clear whether such a sonar, which might have a 50-100 m footprint on the seafloor, could help much amid the complex microtopography and depositional microenvironments of a tectonically dismembered terrain. Certainly, such a tool could be used to eliminate some areas that clearly have too much sediment. New developments in near-bottom towed subbottom-profiling sonars, including parametric sonars and swept-frequency sonars, may be able to produce the requisite combination of narrow beam width and fine resolution.

* subbottom profiler on submersible: Although we are not aware of any examples where this has been done, we think it should be possible to mount a subbottom profiler on a submersible. One could envision a frequency-agile, downward-looking sonar with a choice of frequencies between 1 and 10 kHz, so that the diving scientists could choose what trade-off to make between resolution and penetration. The advantages of such a system are: (1) it would produce profiles of use for science, as well as for ODP site-characterizaton; (2) it would not require additional expertise on the part of the scientific party, as would, for example, the percussive or pinger-type refraction experiments described below; (3) it would not require special dives or a modification of the dive track. Disadvantages are: (1) the extra sonar would consume battery power and thus shorten the dive; (2) the system would probably not penetrate very well if the sediment were a rubbly mixture containing a substantial fraction of coarse clasts.

Predrilling Subsurface Site Characterization

Marine seismic experiments that use bottom sources and receivers to investigate the oceanic crust to shallow depths (a few meters to 100s of meters) provide the ocean drilling community with a promising new technology for evaluating the drillability and structural context of a hard rock/offset drilling site. Unfractured volcanics are known to have a seismic velocity of 4-5 km/s; velocities as low as 2-3 km/s are a direct result of increased porosity resulting from voids, cracks and fractures. This suggests that a relationship may be inferred between the drillability of a formation and its bulk seismic velocity (as opposed to that measured in hand specimens). On-bottom seismics might also help determine the structural context of a drill site. For example, consider the two alternative hypotheses for the emplacement of peridotite in the western wall of the MARK area proposed by Karson and Cannat (see Fig. 6 of Leg 153 Scientific Prospectus.) If the seismic velocity of the serpentinized harzburgite differs from that of the surrounding rocks, it might be possible to determine whether the peridotite is an isolated body (has a bottom).

On-bottom seismic refraction and reflection experiments have the potential for addressing the following issues: (1) the thickness of sediment and/or talus overlying hard rock to an accuracy of meters and (2) lateral and vertical variations in seismic velocity resulting from changes in lithology and/or bulk porosity, the latter being affected by the distribution and density of cracks, fractures, and faults. Shallow geophysical experiments conducted on land over the past several decades have proven that high-frequency (>100 Hz) refraction and reflection experiments are a cost-effective and reliable method for measuring the thickness of overburden and the geometry of the interface between bedrock and overburden. The principal advantages of a bottom-source, bottom-receiver experiment are (1) the higher frequency content of the refracted energy permits the resolution of smaller-scale features, (2) the ability to observe crustal refraction at near-zero offset, allowing the imaging of near seafloor structures and (3) a modest but significant improvement in the accuracy of the travel time data.

On-bottom percussive or pinger-type sources. To image structure on a scale of meters, land-based experiments often utilize a sledgehammer (or shotgun-type) source and a receiving array of 12-24 geophones deployed at intervals of meters (maximum source-receiver offset of 100-300 m). The dominant frequency of such a source may vary between 100 and 1000 Hz. Typical objectives include the thickness and composition of overburden (e.g., sediment or talus) and the topography of the contact between overburden and bedrock. The thickness of overburden can be measured to within 1-3 m and the dip of the contact can be constrained to within a few degrees. The depth of penetration of these techniques is limited to <100 m. The technology for conducting such experiments on the seafloor is not currently available; however, Macdonald conducted a similar type of experiment more than a decade ago on the East Pacific Rise at 21°N. It is anticipated that a well-navigated submersible would be required to position the sources and receivers. Receivers may include existing ocean-bottom hydrophones or scaled down receivers deployed for short intervals from a submersible; the latter would require design and manufacturing. Refraction experiments using a pinger source and hydrophone streamer draped on the seafloor have also been used to characterize the uppermost sediments in deep-ocean settings, suggesting that a similar experiment may be feasible at a bare rock site.

On-bottom explosive sources. To image structure on a scale of tens of meters over distances of hundreds of meters to kilometers, one can use on-bottom explosive sources and receivers (ocean bottom seismographs). This technology already exists within the marine seismology community and it has been used successfully to image the shallowmost crust of the East Pacific Rise [Christeson et al., 1992]. The depth of penetration of these techniques is 1-2 km. Experiments in one, two, and three dimensions are possible and would be conducted from a typical research vessel.

Reconnaissance experiments around already-drilled sites would be useful to determine: (1) which seismic methods may be used to assess drillability, and (2) whether there is a resolvable seismic signature to crustal structures such as fractured versus unfractured gabbro and peridotite, hydrothermal mineralization zones, and serpentinized regions.

Navigation and Site Marking

Drillable targets in tectonically dismembered terrains may be very small (<50 m diameter). To maximize the chances that the JOIDES Resolution will be able to re-occupy the exact spot identified during pre-drilling site characterization surveys, the best possible navigational techniques must be routinely used, and a range of site-marking techniques should be implemented.

All high resolution, near bottom data should be transponder navigated, and transponder nets should be tied to GPS. In areas where repeated survey and drilling cruises are anticipated, a long-life transponder network should be established and maintained. All surface ship data should be GPS navigated.

Long-term coordination between site survey proponents and ODP/TAMU should be encouraged. In particular, site survey investigators should be provided with advice and materials with which to mark candidate drill sites. Appropriate markers could include passive sonar reflectors, passive visual markers or acoustic beacons, depending on the terrain, the anticipated time before possible drilling, the numbers of anticipated sites, etc.

Drillship procedures

ODP/TAMU needs to acquire the equipment and expertise to navigate the JOIDES Resolution and the VIT-camera relative to the same long-baseline acoustic navigation networks used for site survey navigation.

VIT video data is useful for geological mapping, as well as for operations. The utility of VIT data could be maximized by improving the quality of VIT navigation and image quality. The navigation system for the VIT camera system should provide (a) realtime display of camera and ship position as track charts on an X-Y or lat/long grid, (b) logging of camera navigation data, (c) software to acquire and integrate short-baseline navigation, long-baseline navigation, and GPS navigation, (d) software to post-process DP data to provide the best possible post facto camera track chart.

Image quality of the video could be improved with a more modern video camera, better lights and better lighting/camera geometry. A pan, tilt and zoom option is desirable.

Some aspects of these recommendations will be addressed by a scheduled, funded upgrade of the JOIDES Resolution navigation system.

Site Survey Funding

The site-specific survey data required to reliably identify deeply drillable sites in tectonically dismembered terrains will seldom be produced as a by-product of independent science-driven survey cruises. Dedicated submersible dives and perhaps dedicated cruises may be required to produce the requisite density of near-bottom observations, measurements and samples to adequately characterize the surface and subsurface microenvironments of candidate sites.

Such dives and such surveys may not be able to compete successfully for funding as world-class science in their own right. We feel that the funding structures of ODP member nations should include mechanisms to support site specific surveys whose main contribution is to prepare the ground for drilling, rather than to directly reveal primary truths about earth processes.

To Need for an Engineering Leg

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