Introduction

The interlace between glaciers and ice sheets and the marine environment includes grounded tide-water glaciers together with floating ice shelves and glacier tongues which are dynamically a part of the parent ice mass. In some cases, therefore, the ice-ocean interface is simply the terminal ice cliffs of tide-water glaciers. In others, it includes vertical clifis together with the base of floating ice shelves and fast-flowing outlet glacier tongues. The interface is an important location lor the loss of mass from the glacier system in the form of icebergs, meltwater and sediments. Associated with this transfer of mass are a number of glaciological, oceanographic and sedimentary processes, together with the suite of sedimentary forms and fades that result.

Direct observations of the ice ocean interface, and of the ice-proximal marine and sedimentary environment, are important in the specification of the morphology and dynamics of the system. Several models of the ice ocean interface and its oceanographic and sedimentary regime have been proposed e.g. (Reference PowellPowell, 1981, Reference Powell, Dowdeswell and Scourse1990), but detailed direct observations in modern glacier-influenced waters are relatively few (e.g. Reference Oliver, Oconnor and WatsonOliver and others, 1978; Reference Klepsvik and FossumKlepsvik and Fossum. 1980; Reference StocktonStockton. 1983). This is because the nature of the interface imposes severe logistical constrains on our ability to make detailed observations in locations proximal to glacial tide-water dills or beneath floating ice shelves, Tide-water glacier termini are usually in longitudinal tension, many transverse crevasses are present as a result, and the calving of icebergs occurs frequently, providing a significant hazard (Fig. 1.) The Underside of floating ice shelves, and the ocean waters and sediments that lie beneath them, are also inaccessible, but have been instrumented occasionally by drilling through the ice shelf above (e.g. Reference Zotikov, Zagorodnov and RaikovskyZotikov and others. 1980: Reference Nicholls, Makinson and RobinsonNicholls and others. 1991).

Fig. 1.

(a) tghe tide-water ice cliff of a teprature valley glaceir in southeast Alaska. (b) Ice-slab calving event. The iceberg is about 40 m high and 100 m across.

We have used remotely operated vehicles (ROVs). carrying packages of scientific instruments, as a means of examining tide-water glacier cliffs, and the oceanographic and sedimentary environments proximal to them, in southeast Alaska. Antarctica and Last Greenland. we have also deployed ROVs beneath the margins of floating ice shelves and reached the grounding line of floating glacier tongues in Antarctica. In this paper we describe the specifications of ROVs and associated umbilieals used in these investigations, and consider their methods of deployment and operation and the instrument packages carried aboard these vehicles. We also give examples of images we have obtained of the ice-occan interface and ice-proximal sediments from ROVs. together with records of salinity, temperature and water-turbidity measurements close to tide-water glacier margins.

Remotely Operated Vehicles (ROVs)

ROV specifications

We have used several types of Phantom ROV (manufacturer Deep Ocean Engineering during operations around the ice-ocean interlace (Fig. 2). The ROV is mounted within a crash frame and is approximately 0.63 m high, 1.5 m long and 0.85 m wide. Weight is dependent on the modifications made to individual ROVs weighing between about 20 60kg. Flotation can be added to ROV frame as required to provide neutral buoyancy in water, and to balance any asymmetry in the positioning of scientific instruments about ihe frame. Readjustment is then made after trial immersion. The number and power of the counter-rotating, torque-balanced thrusters which propel the ROY are dependent on individual ROV configuration. A typical configuration would he two to four forward thrusters and two vertical/transverse (vertrans) thrusters. yielding 50 75 kg of forward thrust and la 15-20kg of lateral and vertical thrust. This configuration would give a maximum speed of about 4 knots. Power requirments are 115 or 220 V AC at 50-60 Hz and 6 kVA. Maximum operating depth is about 500 m.

Fig. 2.

(a) Configured submersible ROV prior to launch, with side-scan sonar fish. CTD meter and camera systems mounted on its external frame, (b) ROV deployment through sea ice from Ihe deck of CSS Hudson in Kangerdlugssuaq Fjord, East Greenland.

The ROV is controlled by joystick(s). which udjusi the balance between forward and venrans thrusters. Read-outs of heading anil depth are av ailable to ihe pilot, in addition to video camera output w ben v isual l onlat l is made wiih underwater objects. Transponder ranging svsiems mav also be used as a navigational aid. The flying of an ROY requires considerable experience, and professional RON’ pilots from companies servicing the offshore oil and gas industries can be hired. if needed. The pilot and control centre are normally located in ihe parent vessel in drv. heated accommodation. We have also operated RON * from sea-ice platforms using vehicles with hydraulic arms for deployment. I he control centre for sea-ice operations is housed in a healed cabin or trailer.

Umbilicals

The umbilical connecting an ROV to its control centre has several functions: (i) for piloting signals and the supply of power: (ii) for the transfer of data from the ROV to the control Centre in real time; and to the (iii) as a means of vehicle recovery. Some ROVs can be radio-controlled without an umbilical: however. during dives in glacimarine environments the ROV can also ice walls oi other large objects which can block communication signals. The ROV can also become so positioned dial recovcrv bv hauling on the umbilical. is the best safety precaution. Length and inleni.il configuration ol die umbilical are dependent on the operational and scientific requirements in anv specific siudy. However, the following configuration would be typical ol our ROV investigations of tide-water ice clifis, the margins of ice shelves and the ice-proximal marine environment.

The diameier of and footprint on the sea door is dependent on umbilical length, water depth and current conditions (Fig.3). For example, a 640 m umbilical would have an operating footprint of 400-500 m diameter in 100 m of water. The umbilical weights about 30 kg per 100 m in air. It is usually neutrally or positively buoyanl in water, which helps avoid tangles on the sea floor, especially around boulder fields which are relatively common in ice-proximal environments. However, a buoyant umbilical may tangle on underside protuberances of sea ice. icebergs, ice shelves and floating glacier tongues, and care is needed during descents and ascents. The weight and diameter of the umbilical, together with the current regime, define the amount of drag thai will he caused. As drag on the umbilical increases, the ROY operating footprint decreases as die umbilical becomes progressively more curved (Fig. 3) An alternative method of ROV deployment, especially in strong Currents and or where the sea floor is a principal object of interest, is to deploy the umbilical vertically downward to the floor. and then to fly the ROV horizontal outward from this position.

Fig. 3.

Schematic diagram of the deployment of a submerisible ROV from a parent surface craft close to tide-water ice diffs. Note that the length of the umbilical constains the area that can be examind. by the ROV In this example water depth is 150 m and the shaded area represent the “footprints” on the sea floor which can be examined by the ROV while the parent vessel is stationary.

The configuration of the umbilical varies with the nature of each operation, but uuarmourcd umbilicals. which are most useful for operation from smaller vessels and ship’s launches (< 20 m long), are typically designed with a breaking strain of about 8t. Inside a watertight sheath the umbilical has cables for power, control of the ROV thruslers, and any links between instruments and data-logging facilities on the parent vessel. The number of cables, especially shielded twisted pairs, will usually be the limiting criterion for the number of sensors operating on the ROV at one lime for real-time displays of data. A video are photographic camera and associated light connection is supplied as standard in most umbilicals, and connections for manipulator arms are also common. When “off-the-shelf” umbilicals are used, the simplest configuration is to deploy scientific instruments which log internally on the ROV and. therefore. require no separate cable within the umbilical. However, for scientific operations there are considerable advantages in having real-time information on the variation of environmental parameters such as water salinity. turbidity and temperature, and from imaging systems such as side-scan sonar. Real-time displays allow instant evaluation of conditions in process studies and facilitate adjustment of flight strategies during a dive. All cables for real-time data logging ai the surface require appropriate terminations and watertight seals for connection to the umbilical, and the umbilical to have both coaxial and. increasingly, fibre-optic cables available for ROV to surface data transfer.

ROV Deployment and Navigation

ROV-Mounted Sensors for Investigating Ice, Ocean and Sediments

A first-order constraint on the scientific equipment to be deploved to be a basic ROV is its size and weight, The equipment must be capable of being mounted on the ROV frame, and preferably inside the frame for protection against collision, The Phantom is relatively well suited io the attachment of scientific equipment because of its open frame (Fig. 2a). Weight considerations are important to the speed and manocuvrability of the ROV, and also to deployment and recovery from smaller launches where lifting gear, such as winches, may be unavailable. The power draw of each scientific instrument, and the type of data logging (internal to the sensor or via cable to a remote unit). are further considerations, which are dependent in pari on the umbilical configuration, and may constrain the combination of sensors that can be deployed on any single sortie.

For the investigation of glacier ice. and the marine waters and sea-floor sediments located proximal to and beneath tide-water glaciers and ice shelves, a number of types of instrument are appropriate for deployment on ROVs. These can be divided into: (i) imaging systems to view ice and the sea floor (e.g. black-and-white and colour video cameras, still cameras, side-scan sonar, forward-looking sonar); (ii) manipulators for direct sampling of marine waters and sea-floor sediments; (iii) sensors for measuring water-column properties (e.g. salinity, temperature, sediment concentration, dissolved oxygen content, current velocities); and (iv) geophysical equipment Ibr sub-bottom profiling.

Of the imaging systems, the video camera(s) allow the pilot a real-time display forward of the ROV in clear water. The images are usually recorded on VCR continuously. It is also possible, if contrast is sufficient, to digitally reproduce still images from a “frozen” videotape frame. For keeping records of locations and events we use one stereo channel on the videotape to record a digital time-code and the other channel for audio description Of images and any other data of note. We commonly use a colour and a low-light black-and-white video camera. The Latter is most useful where there is high turbidity in the water column, as occurs in temperate glacier environments with large stream discharges. Still photographs can be taken using underwater cameras (Stereo cameras are also available) and synchronised flash units. Because aperture and focus need to be pre-set before diving, it is useful initially to use polaroid slide film that can be processed immediately in the field, so settings can he adjusted. Once favourable settings for a dive site are found, then regular, bulk slide film or print film can be used in the camera.

Where conditions of high turbidity or rapid attenuration of light away from the ROV occur, visual images from the video cameras do not allow long-range viewing. To compensate, it is possible to use a forward-looking sonar, with real-time scanning image displayed in the control centre. The display can also be recorded on a VCR if required. With such images one can determine exact distances from ice faces or grounding lines when they are beyond viewing range. Large boulders or banks on the sea floor on the can also be detected and avoided while flying the ROV in “blind” conditions of high water-column turbidity. Side-scan sonars can be used to map the sea floor, but care must be taken to fly the ROV at constant height above the sea lloor and at a constant speed. Pitch and yaw of the ROV can also be problems. We have experienced problems with side-scan signal interference from the ROV thrusters; shielded twisted pair cables should be used to ensure good analogue records.

Manipulator arms are available commercially for sampling water and bottom sediment. A small bucket dredge has been custom-made for sampling bottom sediment; it can collect samples of about 5-6 cm³ in volume. which retain primary sedimentary structures. Six samples can be collected on one dive using ibis instrument.

Water-column properties are critical lor process Studies, and most types of sensors used to standard oceanographic surveys can be configured imo the ROV system. Caution is needed to ensure minimum disturbance from thrusters. Perhaps she most difficult measurements are of current velocity. The ROV must be Stationary and its orientation must be known in order to obtain a precise current-velocity vector. It is possible to process the magnetic-compass signal from die ROV with electromagnetic-current meter voltages to produce a real time current velocity and direction read-out in ihe control centre. To increase the number of sensors that can be used simultaneously with a finite number of cables in the umbilical. it is also possible to “daisy-chain” data streams up the umbiltcal and then separatc them ai the surface.

Examples of ROV Observations of Ice, Ocean and Sediments

Several images digitized front black-aud-white video tapes acquired from ROV Operations in the cool-lemperale glacimarine setting of soutbeast Alaska are shown in Figure 4. The first image is of sea-floor sediments at ihe gronnding line where the glacier is in contact with the top of a morainal bank (Fig, 4a). Note that the glacier is overriding the sediment, since clasts can be seen under the glacier through the ice. A second image illustrates the submarine ice-cliff face with dispersed englacial debris and gas bubbles (Fig. 4b). The front of the morainal bank has steep slopes at the angle of repose of the open-framework gravel. Boulders have rolled down from ihe bank to become isolated blocks in pro-bank muds (Fig. 4c and d). it is also possible to obtain images of sediment in the walls of turbidity-current channels to observ e die internal nature of sea-floor sediment. These data allow the spatial pattern of debris within ice. and the spatial patterns of ice-proximal sedimentation dial result from the release of this sedhnent, to be observed and modelled in more detail than has been possible previously.

Fig. 4.

Images of the grounding-line enviorment of tid-water glacier termini in Alaska, obtain usind pause frames of S-VHS video tapes recorded by a submersible ROV (a) Close-up of the grounding line(0.6-1.0 m in lenght on the image) where marainal bank sediment is piled against the ice face. The grounding line is marked by arows. The ice is also overridding the top of the morrianal bank and it is suffciently clear that overridden sediment (bottom right) can been seen through it.(b) Close-up of the ice face above the grounding line.objects englacigal debris partic1es. up to 20-30 mm across. (c) chaotic sediment on the morainal bank front where boulders, up to 0.3 m across, and of angular-to-rounded shape are stacked at the angle of response. The view is looking obliquely along the bank front The slope best illustrated at the top right where the black area is waler, is the real foreslope of the bank. (d) Gravel in the ara immediatly beyond the morainal bank showing boulders (to over Iam across) that have rolled down bankfront. and other clasts ol are from iceberg rafting. The background shows a smoother surface represting glacimarine mud.

A second set of images, taken from an ROV deployed in Antarctic waters, shows the submarine environment close lo the grounding line of a small floaling ice tongue (Fig. 5). These are among the first images obtained from such an environment, The floating tongue of Mackay Glacier is about 4 km long. Access to ihe underside of the tongue was from the side, where ice thickness is about 220 m. Using the ROV, we found that water depth at the grounding the ranged from 90 lo 130 m. Debris can be seen within the basal ice (Fig. 5a and b). This is an observation of some significance, confirming that sediment can be present at the base of ice shelves close to the grounding line. This englacial debris may (lieu contribute to the sediment load of calved icebergs, or he melted out al the ice-shelf base before caking lakes place (cf. Reference Drewry and CooperDrewry and Cooper. 1981: Reference Dowdeswell, Murray, Dowdeswall and ScourseDowdesweil and Murray. 1990), Glacimarine diamicton (sandy, pebbly mud of heterogenous grain-size) is also deposited by rockfall and grainfall from the rain-out of glacial debris as it is melted out from the underside of the Mackay Glacier tongue, This diamicton immediately drapes fluted Subglacial till that has been exposed by retreat of the grounding line (Fig. 5c). There is also a diverse epifauna close to the grounding line (Fig. 5d).

Fig. 5.

images of the grounding-line enviornment at Mackay Glacier, a floating glacier tongue of an outlut glacier of the East Antarctic ice sheet. acquired from ROV-mounted cameras. (a) Vertical ice face above the grounding line showing basel debris-rich ice with sediment of a heterogeneous, diamiclic grain size and individual clasls tens of cm across. Layers arc subhorizontal and subparallel with the glacier bed. (b) The grounding line, marked by arrows, showing basal debris-rich layers dipping down to the bed (i.e. to the left). Clasts in the ice are tens of cm across, (c) Fluted suhglacial till with marine diamicton (sometimes known as waterlain till) draping its surface. The flute is about 0.5 m high. (d) Diverse epifauna present close to the grounding line, primarily on hard grounds.

In addition to images of the ice-ocean-sediment interface, a variety of quantitative oceanographic data can be acquired from the suite of instruments available to be carried aboard ROVs. An example of quantitative oceanographic data obtained using an ROV is given in Figure 6. The ROV obtained these data w ithin 50 m of the month of a subglacial stream channel entering marine waters at tin- grounding line of an Alaskan glacier. The record shows changes in water temperature, salinity, density and back-scatter (relative turbidity) as the ROV dives down. Note the relatively low salinity, temperature and density in the meltwear-derived waters between 0 and 7 in (Fig. 6). The strong temperature and salinity gradient at a depth of about 7-9 is known as the pyenoeline. I There is also a spike ol increased water turbidity above die pycnoclinc. ihe high back-scatter and water salinity and densiiy in the lowest part of the w ater column probably rcpreseni a turbidity curreni close to the sea floor. These detailed data on the salinily and temperature structure of the water Column beneath ice shelves and at tide-water glacier ice cliffs. can be used to constrain and calibrate physical models of ocean circulation and mixing in these environments e.g. Reference Josberger and Martin.Josberger and Martin, 1981.

Fig. 6.

Oceanographic data collected in front of a small subglacial stream portal from ROV -mounted equipment at about 50 m Jimn a tide-water ice cliff in Alaska. Parameters were measured using CTD and optical back-scatterance instruments on the ROV Hatk-scatterance (B) is a measure of relative suspended sediment concentrations in the Water column. Water density (D) is a function of salinity (S) and temperature (T). Water depth at the point mini of sampling is only 17 m because a morainal bank has bank has built up to that level against the ice face from the deeper basin poor.

Concluding Remarks

Submersible remotely operated vehicles (ROVs) are valuable research tools for data collection in dangerous or inaccessible environments associated with glaciers terminating in the sea (e.g. at calving tide-water ice cliffs and beneath floating ice tongues and shelves).

ROVs can he operated from relatively safe distances (hundreds of metres); they can also descend to considerably greater depths (hundreds rather than tens of metres) than scuba diving permits. They can provide data on glacier grounding-line and sea-floor morphology and water-column characteristics (e.g. salinity, turbidity, current velocity).

ROVs can be fitted with a variety of oceanographic sensors, imaging sensors, tracking devices, and water and sediment samplers, making them extremely versatile research instruments for obtaining qualitative and quantitative data for investigating glacial sedimentary processes and ice and sea-floor morphology in logistically difficult environments.