Preface: Most people acquainted with the drilling program are aware that complicated operations take place at sea almost routinely. What is actually behind some of them, however, is rarely conveyed by typical reports from JOIDES Resolution, or published operational summaries. The following is an attempt to explain what one such operation was really like.
The ship has arrived at its target. For nine days it has toiled eastward against stiff headwinds, rough seas, and the Agulhas current. Now, finally, the ship slows and comes to a halt, seemingly in the middle of nowhere. On the horizon, blue on blue, in all directions.
Cape Town, last port of call, is a memory and a distant hope. There, the ship will return from this same spot some 7 weeks hence, this time with the winds, the seas, and the currents chasing it from behind. In the meantime, this ship, JOIDES Resolution, will drill a hole in the seafloor. In the middle of the ship stands a derrick, and above the main deck is a rack of pipe which will be lifted stand by stand into the derrick, then strung together, end on end, and suspended through a hole - a "moon pool" - in the middle of the ship. When it touches the seafloor, and works its way in a little bit, the whole string of pipe will be turned some sixty times per minute, allowing a massive coring bit at the end of the pipe to cut into the rock, making a hole. A hole in the bit allows uncut rock to pass into the pipe. The rock inside the pipe is periodically hauled to the surface in core barrels which are passed by wire up and down inside the pipe. When the rock reaches the deck, the actual cargo of this ship, a team of scientists, can examine it.
Today, though, an incredibly small target must be found. Ten years past, this same ship drilled a hole at this very same spot. The hole, known simply by its number, 735B, was then drilled to a depth of just more than 500 m. At the top, it is only 13 inches in diameter; it is narrower still for most of its length. The mixing of units of measurement in the last two sentences is characteristic of the partnership between scientists and drillers on this vessel. Scientists think metric, and cast their results in kilograms, grams, meters, and centimeters. Drillers use tons, pounds, feet, and inches. In any case, the target is officially 13 inches across, slightly larger than the diameter of a coring bit. And the hole underneath is officially 500.7 meters deep. Ask anyone.
Some modern technology makes it possible to find the hole. The ship is equipped with GPS, which stands for "global positioning system". In order to be able to talk to people out here, there is a language of alphabetical acronyms that has to be learned very quickly. In this narrative, GPS is the first. The principle of GPS is simple. The ship is in constant communication with a network of high-altitude satellites, the position of each being precisely known. At least seven satellites are usually above the horizon anywhere on Earth, enabling continuous precision navigation. By ranging on the satellites, the position of the ship can be precisely and continuously determined, to within a few meters, even when the ship is moving. The old business of navigating by stars, sun, and moon is nearly a thing of the past. The sextant, which is a beautiful instrument, still holds a place of honor in the heart of the navigator, but it does not come close to allowing the kind of precision required to locate a 13-inch hole in the ground beneath 700 m of water.
Ten years ago, before GPS, there was another positioning system which depended on a smaller number of satellites moving in lower orbits. These only appeared on the local horizon erratically, perhaps several dozen times a day. Receiving equipment had to "lock on" to those itinerant satellites, and measure relative position using shifts in frequency recorded prior to and just after the time of the satellite's nearest approach. At least three satellites had to be above the horizon at the same time to provide an unambiguous "fix", and this only happened a few times a day. If a ship like this one stayed on one spot long enough, an average position, taken over several days, could be calculated. This was just about as good as any of the instantaneous GPS measurements of today. On this day, using GPS, JOIDES Resolution will be slowly positioned over the original averaged satellite position of the hole as measured ten years ago. The ship is equipped with a dozen thrusters, fore and aft, which allow it to be pushed sideways, forward, or backward when it is on station. These are in addition to the main twin screws, which provide forward propulsion when underway. Under way, most of the thrusters are raised above the hull and not used. On station, all the thrusters and the screws are used. The first stage of locating the hole is therefore to lower the thrusters into the water. Then forward, backward, sideways, adjusting constantly for winds, waves and currents, the ship is moved as close to the old position of the hole as possible. A beacon is dropped, this to send a sound signal to the ship, to be used as a fixed local navigational reference in the search for the hole.
Now we come to the second acronym, DP. This stands for "dynamic positioning". Aft of the bridge is a room with a bank of computers. The computers are used to monitor the location of the ship with respect to the beacon. The ship can be held directly over the beacon, with the same computers instructing each of the thrusters when, which way, and how far to push the ship in order to keep it in one place. This is an unending battle with the elements. On calm days, the thrusters merely need to tweak the ship one way or another every now and then. Their rumble against the hull is rare, even reassuring. People can sleep. On rough days, the thrusters are in nearly constant turmoil, and with the added ship motion, sleep can be elusive.
Above the computer room is the DP shack. The two are linked by a tight spiral staircase made of steel. The DP shack is the nerve center for the operation which is about to ensue. The ship has to be held in one position long enough to provide a GPS location for the position of the beacon on the sea floor. This turns out to be some distance, not very far, from the last known position of the hole, calculated from satellite fixes ten years ago. The computers are instructed to move the ship over to the old position, offsetting by a fixed distance and direction from the beacon. Ideally, the hole should be directly under the ship, but it still has to be found. Even GPS cannot place the center of the derrick precisely over a 13 inch hole. We will have to investigate the seafloor itself to find the hole.
Now comes the third acronym, VIT. This stands for "vibration-isolated television". This is essentially a pressure-cased video camera on an ugly dinged-up metal frame which can be lowered along the drill pipe to the sea floor. The frame serves as both carrier and protection for the camera, first of all to prevent it from banging against the pipe, but also if it should chance to run into rocks when near the bottom. This seems a bit more than isolation of vibration, but judging from the condition of the frame itself, it has more than served its purpose. The video signal is sent along an electrical cable to monitors in the DP shack. If the hole is not immediately visible, an "expanding-box" search pattern will begin, with the box widening by ten meters at each pass until it is found.
What exactly does the hole look like? Well, to be honest, this account has fibbed a bit up to this point. There is actually a piece of equipment above the seafloor and over the hole called a "hard rock base" or, in the local parlance, HRB.
This consists of a heavy metal box about 4 meters on a side with a hole in the middle. The box is filled with cement to keep it from moving. Above the box is a funnel, about 3 meters in diameter. The box and its funnel were constructed on deck and lowered to the seafloor when drilling began here ten years ago. The end of the drill string has to pass through the funnel to reach the hole. Since coring bits wear out after days of drilling in hard, crystalline rock, the funnel has to be used repeatedly to find the hole, in order that the bits can be changed and the hole continued. This was done 13 times the last time the ship was here. The funnel and its box are actually what we are searching for. The question now is, whether after a ten year break and a change in navigational systems, can the ship get back over a hard rock base in the middle of the Indian Ocean?
Besides the VIT camera, there is one other tool to assist in the search. This is the color-scanning sonar tool which is also mounted on the VIT frame. This tool rotates a pencil of sound, the reflections of which are presented on another monitor in the DP shack. High-standing objects, like the guide-base and its funnel, show up as bright spots with shadows behind them on the monitor. The display is highlighted by false colors, yellows and oranges being "warm" colors, to indicate lots of reflected sound, blue and green being "cool" colors, indicating little reflected sound. Black areas are acoustic shadows where sound return is completely blocked by an intervening object. Rock outcrops show up as bright yellow and orange strips. Tonight, we expect the seafloor around the guide base to be virtually flat, and therefore that the base's bright yellow spot and trailing shadow will show up quickly. The only difficulty is that the VIT frame itself can spin around the pipe in bottom currents, and it will also orient itself in the direction of ship's motion. At the outset, at least, the sonar scanning tool will provide range, but not direction, of objects on the sea floor.
The DP shack also contains a large bank of computing equipment. There are some tall chairs and a chart table crowded behind the computers, and a small desk is tucked away in one corner. Phones above the chart table allow communication with the drillers on the rig floor. The computers are imposing, but truth to tell this room is an anachronism. Most of the equipment is 20 years old, and takes up much more space than far more powerful positioning computers on modern vessels. Parts for it are no longer manufactured, therefore these particular computers are sustained by spare boards and parts scavenged from other systems which have been retired. The equipment here is slated for replacement during a dry dock in 1999.
On this evening, the DP shack is dimly lit, with the monitors providing much of the light. Half a-dozen people are present, not all of whom count. One who does count is Ray Frank, who will supervise the expanding-box search if it is necessary. Ray's job seems to consist of running between one computer display or another, adjusting knobs, reading dials, and punching coordinates into the DP system. He is very busy. Two people who don't really count are the co chief scientists, Henry Dick and Jim Natland. They have given themselves the job of obtaining recordings of the VIT and Mesotech displays, and providing voice-over during the survey. They were both present for the previous drilling and, as instigators of this current expedition, are keenly interested in finding the hole and beginning new drilling as soon as possible. Staff Scientist Jay Miller, who acts as a kind of executive officer within the group of scientists on board, is also present. He counts for a bit more than the co-chief scientists, since he has hooked up a laptop computer to record positioning coordinates as the survey progresses.
As the camera is lowered through the water, only the drill pipe can be seen. Occasionally, scaly rust from the pipe falls past. Once, a startled fish darts by. Finally, near the target depth, the camera is slowed, and a murky bottom comes into view. The guide base is not in sight. The color scanning sonar reveals some very low ridges and swales, and a couple of bright spots, one of which might be the guide base. The bottom is very flat.
As usual, not everything works perfectly. A beacon attached to the VIT frame, which was to provide a separate reference for the position of the camera, fails. There will be no navigated survey of the approach to the hole. The ship's position will be precisely known, but not that of the camera. There are urgent discussions, and a number of attempts to interrogate the beacon, but to no avail. Jay, no longer counting, turns off his computer.
Finally, the expanding-box search begins. In order to view the most seafloor possible, the camera is held about 20 meters off the bottom. The view of the seafloor is murky from this distance, black, white, and shades of gray, but not color. There are thin patches of brightly reflective sediment, and some outcrops with narrow fissures and cracks. But there is nothing to make of the geological character of the seafloor. To enable that, the camera needs to be at least ten meters lower. Ten years before, a 200 m-square box was surveyed prior to landing the guide base. Jay Miller has a summary map from that survey with him in the DP shack. It shows stretches of outcrop, patches of sediment, and the orientation of structural features in the rocks. The search this night will provide no such information.
There is an expectation that the guide base will be immediately apparent when the camera passes over it. One bright spot and shadow on the Mesotech scanner draws comment, but simply to move over directly to it is not a good strategy, since if it turns out to be something else, then the next nearest bright spot will be further away. The slow-and-steady expanding box survey is the best way to proceed. The bottom passes slowly beneath the ship, its indistinct image revealing rock and sediment, but no more. Everyone starts to settle into the routine. Some non-essential people nod with sleep.
Then, after only 40 minutes, a straight line appears on the camera monitor. There are no straight lines on the seafloor! That must be the guide base! Lower the camera. Yes, there it is. The base's outlines separate from the gloom, the shape of the funnel emerges. The bright spot and shadow on the color-scanning sonar enlarge. The co-chief scientists quietly shake hands and offer congratulations to the others. The ship drifts slowly over the funnel. The light for the camera reveals an old friend. It looks kind of weather- beaten. Ten years under water can cause a lot of rusting. The bright white paint on the base is pretty patchy. Still, the concentric black and white rings inside the funnel show up well. When precisely over the funnel, the hole itself can be seen beneath the neck of the funnel, slightly offset from being perfectly centered.
So this is how you find a 13-inch hole on the seafloor.
Now, all that is left is to re-enter the hole. The VIT frame is pulled up a bit and the drill string lowered into the funnel. It barely touches the sloping side of the funnel before passing through its neck and then into the hole. After ten years, we are there.
Outside of the DP shack, many of the scientists have been watching the re- entry on remote monitors. One of them, Jonathan Snow, has a printout of a digital photograph of Table Mountain, which dominates the scenery from Cape Town harbor. A digital camera. Another sign of the modern age. There is an aerial tram which goes to the summit of Table Mountain, 1067 meters above the city. Snow also has a picture of the city from that vantage. What are we going to do next? As far as the drillers are concerned, we are going to drill a hole that may eventually be twice as deep as Table Mountain is tall. The hole is already almost half the height of Table Mountain. We have the rest of the leg, until we have to leave for Cape Town, to drill it deeper.
Two days later, laboratory officer Dennis Graham, who supervises the technical support team on board JOIDES Resolution, provides a plotted diagram of GPS positions of the ship over the hole. Each position is accurate, but since the ship moves around in the wind and currents, and has to be pushed back over the hole by the DP computers and the thrusters, the plot shows a cluster of points. The final "official" position of the hole is given by the average location of all of those points. It turns out to be 5 meters, or just over 16 feet, from the average position obtained ten years before.