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Seaf loor Geodesy by the Year 2 0 0 0 A. N. Spiess Scripps Institution of Oceanography University of California, San Diego LaJolla, California 92093 INTRODUCTION Other papers in this group make it clear that high technology geodesy, with particular relevance to geodynamic problems, is a well- advanced maturing 'field of endeavor as applied to terrestrial situations. Lasers,::satellites, high precision clocks,-advanced signal processing and data reduction capabilities are moving us into second and third generation versions of'~'systems brought to operation initially in the 1970s. Unlike the~terrestrial situation, however, 'seafloor geodesy in the geodynamic context still only consists of paper analyses, 'workshop reports, and the beginnings of testing of a few potential system elements at sea. Nevertheless, the principal message of this paper is that by the year 2000 we will be discussing at least a few real multi-year data sets and using them to constrain our models of the structure and dynamics of the crust beneath the sea - its genesis, its evolution as it moves away from the mid-ocean ridges, its destruction in the trenches, and the effects of its interaction with continents and islands. things: If this prediction is to be realized, we need to achieve three Development of some new elements of undersea technology. 2. Application of a variety of systems at a few interesting initial sites. 3. Programmatic support frameworks for subsequent long periods of observation. This paper will discuss each of these three topics briefly in the order listed. TECHNOLOGY A discussion of technology must start with some measurement goals. Seafloor geodynamic problems are such that first generation systems would be useful if they could produce accuracies of a few centimeters for measurements of horizontal or vertical position change, strain measurements of a few parts in a million, and tilt determinations in the 100
101 range-of a few microradians. Beyond that, whatever means are used to make these measurements must be compatible with the seafloor environment and capable to being used to monitor changes over periods of many years. An encouraging aspect of the technological problem is that a large number of options are potentially available. Some of these involve the possibility of fairly direct transfer of well-established land techniques, while others are derived from undersea developments related to other goals. At least five differences between subaerial and subsea circumstances drive the design and development of seafloor geodetic systems (NAS, 1983~: Electromagnetic radiation at frequencies high enough to be "useful is highly absorbed in the sea because of the electrical conductivity of seawater. The only useful range is the optical one, but even there, one's capabilities are limited to tens, or perhaps a few hundred meters. 2. Acoustic energy of appropriate wavelength can be transmitted effectively over distances of many kilometers. 3. Ambient pressure at the seafloor is not only large, it is relatively free of large amplitude short wavelength or short time scale irregularities. 4. The seafloor environment is quite stable in terms of temperature and sediment water content. 5. Access to the seafloor is much more difficult than for most terrestrial sites. These considerations lead to some immediate conclusions about the most fruitful directions that system developments are likely to take. First, acoustic systems will dominate for measurement of horizontal or slanting distance measurements involving distances of more than about 100 m. At the same time, shorter range systems now in use on land, but that suffer from environmentally induced monument motion problems should behave better in the deep sea. Finally, one must strive for simplicity in system installation and maintenance. The techniques that should be most readily transferred from the land are those that operate effectively over rather short distances. Mechanical or laser strain measuring systems, as well as both short and long baseline tiltmeters (Agnew, 1987) also fall in this category. They will benefit from having substantially reduced "noise" due to monument instabilities. Some of them, however, introduce collateral requirements for substantial power and for continuity of operation. They also introduce challenging installation problems, as will be discussed below.
102 Laser ranging through seawater (as opposed to systems in which the path is contained in a pipe or optical transmission fiber) would be limited to ranges of at most a few hundred meters by the attenuation of light in seawater - about 200 decibels per kilometer in clear water. Speed of propagation of light in seawater is only known to about a part in 104. The change of speed with temperature and salinity (Stanley, 1971) is small, however; thus, as long as the laser power levels are modest enough that they do not create appreciable localized temperature increases, they can be useful in detecting path length changes with time. The localized heating problem prevents one from overpowering the high attentuation by brute force introduction of massive transmitted power. Borehole oriented approaches using strain or tilt measuring devices installed in deep-sea drilling program holes should have no problem. Similarly, well-logging methods used to detect changes in the cross- sectional shapes of holes or to document orientation of breakouts in order to infer the nature of stresses in the uppermost parts of the crust should operate as well at sea as on land once there are established systems for doing wireline re-entry into deep-sea drill holes (Langseth & Spiess, 1987~. One new class of systems would involve the use of pressure measurements to determine the distance of the seafloor below the sea surface. With present state of the art in measurement of temperature and salinity as a function of depth, and knowledge of how to convert these measurements to water density (Saunders, 1981), one should be able to convert pressure measurements to depths with accuracy of a part in 105 (Reid, 1984~. Quartz crystal pressure gauges offer the best possibility as far as accuracy is concerned (Irish and Snodgrass, 1971), although these come in a variety of configurations, some of which actually utilize mechanical elements (e.g., Bourdon tubes and bellows) as intermediate links and thus suffer from drift problems that degrade their performance in this context (Watts and Kontoyiannis, 1986; Wearn & Larson, 1984; Busse, 19879. The major new class of systems entering geodesy are those utilizing underwater sound transmission. They are based on over 50 years of ocean acoustics research and development (primarily oriented toward submarine detection) (Eckart, 1968; Urick, 1975) but with a long history of application in marine geology and geophysics (Spiels, 1987~. Early discussion of their usefulness in geodesy occurred at a symposium in 1966 (Speiss, 1966), although at that time the goals set were not as stringent as those being discussed here. The principal limitation on use of sound propagation through seawater for distance measurement is our ability to know the velocity with which to convert travel time to distance. In this, one has a choice between using a direct measuring sound velocity meter or measuring pressure, temperature and salinity and converting these, via empirical relationships based on laboratory data into sound velocity.
103 At the present time the latter approach is only good to a few parts in 105 (Lovett, 1978~. The direct measurement method relies on the fact that, even for geodetic purposes, sound propagation in seawater is essentially non-dispersive (Urick, 1975~. One can thus use a small, dimensionally stable device, operating in the megahertz frequency range, to measure travel times, calibrate it in pure water (for which the relationship between pressure, temperature, and sound speed are known to a part in 106 (Greenspan, 19721), and use the resulting velocity determination for systems operating as low as 10 kHz. The best such meter devised to date had its motivation in geodetic application and is capable of a little better than a part in 105 (McIntyre and Boegeman, 1986). Clearly, there is room for technological improvement in this area. The limit on sound velocity measurement capability translates directly into a distance measuring accuracy limit of the same amount, 1 in 105. This breaks down at the short range end at distances of only a few meters, at which there is difficulty in determining the locations of the acoustic centers of the transducers. At the long range end it simply does not meet our goal of a maximum of a few centimeters uncertainty beyond about 10 km. There is, however, a fundamental environmental constraint. Since the sound velocity field must be determined experimentally by moving one, or perhaps a few j point measuring instruments through it, there can be dynamic situations in which the spatial variations of water temperature and salinity can vary rapidly enough with time that the sound velocity averaged over long travel paths may not, in a practical sense, be knowable to the same accuracy as the individual measurements. Particularly in shallow coastal waters the effective accuracy may not be better than a part in 103, while in the open ocean, paths including near surface water may only be good to a part in 104. Good understanding of the local physical oceanography, leading to proper distribution of measurements and averaging can probably make some improvement on these numbers. Near- bottom deep water may often show microstructure at the level of a part in 105 (Spiels, 19809. In any event, every site at which acoustic measurements are made must include, at the same time, a determination of the sound velocity structure, including some evaluation of the magnitude and time scales of its variability. One other aspect of the environment arises because water motion velocities are not usually negligible compared with the velocity of sound (1500 m/sec). For accuracies of a part in 105 one must know the along track component of water velocity to 1.5 cm/see (0.03 knot). In this case, it is thus preferable to measure round trip travel time by using reflectors or echo repeaters, since, under those conditions, the ratio of water velocity to propagation velocity only enters as its square, and all realistic deep ocean currents become negligible (NAS, 1983).
104 Acoustic systems take on three forms, depending on the path lengths over which one works. For distances of a few hundred meters one should be able to achieve a part in 105 (few mm) accuracy using direct two-way transmission between fixed bottom units by operating in the 100 kHz (15 mm wavelength) regime. Over such short ranges it should be possible to have very good knowledge of the sound velocity structure along the path as well. As path lengths approach a kilometer or more one is forced to somewhat lower frequencies by the fact that attenuation is less (Fisher and Simmons, 19779. One is also forced to abandon the direct path approach because, in nearly isothermal water (typical of deeper situations) the sound velocity increases with depth because of increasing pressure. Under these circumstances sound rays curve upward (Spiels, 1966) and, unless one places the acoustic elements on towers (with the added complexity of having to account for tower tilt), no direct path between transponders will exist. Under these circumstances an intermediate, near-bottom towed vehicle can be used to range simultaneously on three or more transponders from many different locations in the area being surveyed. A large number of observations are then used to determine the internal geometry of the transponder array, with the advantage of averaging over acoustic paths traversing differing portions of the area and thus averaging over the time-varying aspects of the sound velocity field. Computer simulations of this type of system, using sets of 300 observations distributed through a four transponder array having a 2 km radius resulted in baseline length errors of 1 to 2 cm-when errors having a Gaussian distribution with 10 cm standard deviation were inserted into the range data (Spiels, 1985a). The third type of acoustic system under development is designed to relate points on the seafloor to vehicles at the sea surface, with the goal of tying from the surface vehicle to points on land using GPS technology. Composite systems of this kind will support determination of baselines of lengths of hundreds of km,;and first generation versions are expected to have uncertainties of the order of a few cm (Spiels, 1985b). The difficulty of making the surface-to-bottom tie is that it must encompass the more rapidly changing complex uppermost layers of the ocean. As pointed out initially by Bender (1982), this effect can be mitigated if one uses a set of three seafloor transponders and operates chose to the point for which all three travel times are equal. Under these conditions the uncertainty due to lack of knowledge of the sound velocity is proportional to the distance one is away from the central point (Spiels, 1985b). For example? if one can operate within 100 m of the center, one only needs the sound velocity to within a part in 104 to achieve centimeter accuracy. This simple picture is degraded by the existence of horizontal gradients of sound velocity across the region
105 traversed by the sound paths. Fortunately, the only oceanographic effects that can produce gradients large enough to create problems in the open sea (away from major fronts) are those associated with the higher frequency internal waves (periods of tens of minutes to a few hours),-and these can be reduced by averaging over periods of a day or so . Conventional commercially available transponders capable of operating over km ranges operate by having a circuit that recognizes an incoming pulse and then transmits a pulse at some other frequency. This process, for systems operating in the 5 to 20 kHz regime, introduces timing uncertainties equivalent to a meter or more uncertainty in range (NAS, 1983~. A method for overcoming this problem is to use some version of a signal re-transmitting system such that the phase relationships are maintained between the incoming and outgoing signals. One method that has been developed to implement such a system has been to build a transponder that contains a digital shift register delay line having a number of microsecond steps capable of holding several milliseconds of signal. A spread spectrum coded timing signal is then transmitted with the transponder interrogation. The transponder continuously digitizes the incoming acoustic energy and puts the samples into the delay line. Upon recognition of the interrogation, the transponder shifts from listening to transmitting and sends out the contents of the delay line. The outgoing acoustic waveform thus is delayed by an amount known to within a microsecond and the returning signal retains its phase relationship with the original signal (Spiess et al., 1980~. Transponders of this type have been built and tested at sea in March of 1988, showing timing uncertainties having an individual pulse standard deviation of 10 psec, corresponding to ranging uncertainties of less than 1 cm. All of the various systems described or implied above have a common need with respect to capabilities of installing, tending, and removing complex equipment in the deep ocean. Such capabilities start with requirements on ships that must support any on-site activity, as well as transporting the necessary people and equipment to the area. These functions have been recognized and included among the ship performance requirements developed by the ocean science community through studies and committees, particularly those sponsored through UNOLS (University National Ocean Laboratory System)(UNOLS, 1986~. Characteristics of particular importance in this context include good seakeeping, transfer of cumbersome heavy loads from the ship to the seafloor, adequate on- board storage space, and dynamic positioning. No individual ships in our present academic ocean research fleet meet all of these requirements, although two of them (Knorr and Melville) are scheduled for major upgrading during the coming year that will put them close to being adequate for these purposes. Beyond that, one new ship is programmed for construction starting this year with Navy funding and a second is in the preliminary design phase with NSF support.
106 Four types of systems are in various stages of development relative to performing the kinds of tasks involved in setting up and maintaining seafloor geodetic devices on the deep seafloor: manned submersibles, tethered neutrally buoyant swimming vehicles (conventional Remote Operated Vehicles - ROV's), cable supported dynamically positioned devices, and seafloor supported manipulative systems (tractors, etc.~. The only one of the four for which there is substantial deep-sea operating experience is the manned submersible category. Alvin, operated for the research community by Woods Hole Oceanographic Institution (WHOI) has been used for many years to carry out manipulative tasks, primarily in the contexts of marine geology and benthic biology (particularly hydrothermal vent-related experiments). The French submersible Nautile has recently been used to position a nearly neutrally buoyant winch assembly in a deep-sea drilling program borehole re-entry cone (Langseth and Spiess, 1987~. These craft have the advantage of a man actually on the site, although that often is a disadvantage in that safety consideration places substantial limits on the kinds of objects one is willing to handle. Since these craft operate at close to neutral buoyancy, they have limited load handling capabilities. Overall, they can carry out some of the necessary tasks, but their deployment and safety considerations limit their usefulness. ROY's have been in use in shallow water for many years, particularly as complements to diving operations in offshore oil and gas field development. These systems rely primarily on small, nearly neutrally buoyant vehicles positioned in the water by vertical and horizontal thrust propeller systems (Wernli, 1984~. They have proven quite useful for both observation and manipulation. Since the actual work vehicle is coupled by a neutrally buoyant wire to the ship or to an intermediate, cable supported "garage", these units are fairly effectively decoupled from the heaving motions of their surface support ship. Thus, they trade stability for load handling capability. Two such systems are emerging for deep water use by the ocean science community. One of these is the Argo-Jason system, (Ballard, 1982; Yoerger and Harris, 1986) under development at WHOI, with the- other built by ISE (Langseth and Spiess, 1987) to support Canadian research efforts. Both of these are pushing a major technological advance - the use of fiber optic information transmission links built into the long main strain cables that support the ''garage" unit from which the neutrally buoyant work vehicle operates. The high data rates expected from these telemetry systems will allow operation of full bandwidth television systems for viewing the vehicle's surroundings and the tasks they are carrying out. Cable supported dynamically positioned systems have the capability of working with heavier loads albeit without as effective decoupling from the motion of the surface suspension point as the conventional ROV's. One system of this type has moved into operating condition for ocean research in the past year, developed with NSF support at Scripps
107 Institution of Oceanography (SIO). Its primary function is to place instruments accurately on the seafloor (including into drill holes) and either monitor their outputs (providing a telemetry link to the supporting surface ship) or release then for later recall or recovery. This device, being directly cable supported, can carry loads in excess of 1000 kg negative buoyancy (Spiess et al., 1987~. The fourth category - bottom-supported manipulative devices - seem best adapted to many of the tasks envisioned in the implementation of seafloor geodetic observations. Resting on the seafloor, such devices, although cable connected to a surface ship for power and control, can be almost completely decoupled from ship motion. At the same time they are able to manipulate heavy objects or carry out fine scale assembly operations using their reaction against the (relatively) solid seafloor. One device of this type is emerging from the development stage at SIO- RUM III (Anderson and Horn, 1984) is scheduled to carry out its first deep-sea tasks this summer, installing precision transponder mounts and seafloor hydrophore assemblies in basins off southern California. Summarizing the technological situation, it appears that both a wide range of instruments, and the capabilities for installing and monitoring them, could be available for conduct of deep seafloor geodetic observations within the next year or two. INITIAL OPERATIONS Initial use of systems of these kinds present two requirements. They should address interesting geodynamic problems, and they should be at sites conveniently located relative to operating bases so that it is easy to make frequent visits to carry out developmental, as well as routine observational functions. As new terrestrial geodetic systems evolved, the tectonically active southern and central California areas provided an appropriate laboratory region meeting these requirements. Our present understanding (Minster and Jordan, 1984) implies that the offshore regions adjacent to the southern California coast are equally interesting and appropriate. It is clear that there are localized areas of tectonic activity in the Continental Borderland, and that, in order to understand fully the present day dynamics of the North American/Pacific Plate boundary, we must have deep ocean reference points beyond the Channel Islands. Localized networks utilizing seafloor strain measurements and short range direct path acoustic systems could have their initial trials within the borderland itself. Some sites should be shallow enough that decreasing temperature with depth would counter increasing pressure to produce sound velocity gradients leading to downward, rather than upward, refraction. This would in turn allow fairly useful ranges for direct linkages between near-bottom points. In these regions, however, there would have to be intensive measurement of sound velocity to cope with changing oceanographic conditions.
108 Composite GPS/acoustic systems would play a most significant role by establishing reference points in the deep ocean, not only off the southern California borderland, but off the central California coast as well. Ties between these points and the well-established VLBI sites in northeastern California (e.g., Owens Valley) and in Arizona (Yuma) would provide the necessary constraints on the contemporary total motion between the Pacific and North American plates, as well as satisfying the requirements of being available for re-occupation with minimal ship operating cost. For many of the approaches mentioned in the previous section, the most exciting and fruitful zones for initial application would be at the crests of the mid-ocean ridges and rises (NRC, 19889. Geodetic measurements of all kinds would provide very useful constraints on models of lithosphere formation, volcanic activity and hydrothermal circulation. The most logistically convenient, well-studied site for initial implementation of such studies would be on the East Pacific Rise at 21° north latitude (Normark, 1980; RISE Group, 1980~. That location is, however, within waters controlled by Mexico and there have occasionally been delays in obtaining clearances to re-occupy the site. it may be possible to generate a continuous program there, similar to those set up on land by the seismologists, by helping develop Mexican scientific community interest in seafloor geodesy. The other two immediate choices are the Juan de Fuca spreading axis off the Canadian and northwestern U.S. coast and the East Pacific Rise at 13° N. If it were not for a restrictive weather window, the Juan de Fuca site would be a very desirable one; however, operations there with our normal research ships in support of development of new approaches would be limited to the summer months with resulting constraints on coordinating ship schedules with other research activities. The 13° N site's disadvantage is its distance (about 1500 miles) from the nearest major oceanographic research ship operating base. Other than that, it is an attractive location since it has a stretch of well-developed simple structure including hydrothermal activity, but with examples of overlapping rift zone features (Sempere, 1986) nearby. The pros and cons of these locations will be debated over the coming year and, hopefully, the beginnings of geodetic site occupations will take place early in the 1990s. PROGRAMMATIC CONSIDERATIONS Successful geodetic programs have one essential requirement - a long-term commitment by the participants - both individual leaders and, even more important, sponsoring agencies. In the U.S. the two groups that have maintained -the necessary commitment over the years on land have been the U.S. Geological Survey and the National Geodetic Survey (surviving the multi-step transformation into a NOAA element from the Coast and Geodetic Survey). Neither of these groups has been particularly aggressive in attempting to develop an oceanic capability.
109 This lack of push probably arises because applications of marine geodesy do not fit logically into their missions as developed in the terrestrial environment. The one exception is in the USGS mission of understanding earthquake phenomena. Here, there would be clear gains in having reference points on the oceanic side of the San Andreas complex, but even more exciting is the prospect of being able to measure convergence across the Aleutian Trench or uplift along the Vancouver Island/Washington continental slopes. The only agency that has shown consistent support for development of oceanic capabilities has been the NASA Geodynamics program. It recognized a number of years ago (Walter, 1983) that space-based techniques (particularly GPS) could have a more nearly global capability if it were possible to make ties from continental sites into ocean areas. This extension has largely taken place by occupation of island locations. While these are particularly convenient for VLBI and satellite laser ranging operations, the program also recognized the desirability of being able to establish reference points in important places where no islands were available, most obviously on the seaward flanks of the major trenches on the ocean margins. It is hoped that this program will continue, at least to a point at which USGS and/or NOAA will pick up the challenge. The other logical agency that might support oceanic geodetic activity is the National Science Foundation. Geodesy in general has had scant support from that quarter, in part because geodetic impacts on basic earth sciences have not been well established until rather recently. The gradual development of our awareness of the many facets of plate tectonics, and the fact that nearly all plate boundaries lie in the ocean, give considerable emphasis to the desirability of including research in ocean floor geodesy within the NSF Ocean Sciences purview. A problem in this is that, while de facto NSF provides continuous support for various aspects of ocean science, the general pattern for administering its ocean floor research grants (except deep-sea drilling) has been based on short-term commitments. This pattern is, however, being somewhat altered and some portion of its new initiatives are targeted for coordinated research activities focused on longer term goals. Within this context, the RIDGE initiative (NRC, 1988) does provide a logical context for ocean geodesy as a part of ocean science research. CONCLUS ION AND ACKNOWLEDGEMENTS Although there are many steps yet to be taken along the way, it appears that, by the year 2000, there may very well be the technology and the programmatic commitment for significant seafloor geodetic activities. The research and application opportunities are there both in relation to our basic understanding of the crust of the earth beneath the sea, and the context of related impact on man's activities.
llQ The technology discussed in Section II is quite diverse and its development has been carried out by a larger number of contributors than even the list of references implies. Ihe authors geodedy-oriented activities have been supported primarily by NASA, but with small inputs from the Office of Naval Research, NQAA, and NSF.
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