are calibrated in centimeters. The vertical pole is adjusted to touch any object within the frame. The coordinates of the point are recorded from three measurements read on the frame, the beam, and the elevation pole. The details around the point must be drawn by a diver hovering over portable 2-meter grids placed directly on the site materials. The simple 2-meter grids are divided into 20-centimeter squares, which are designated by numbers and letters marked on the sides of the grids. The measuring frame is used to fix the positions of the corners of the 2-meter grid. 8.9.2.5 Merifield Rosencrantz Method A simple method for surveying a small area under water to measure ground control points has been developed and tested by Merifield and Rosencrantz (1966). The method is simple, rapid, and minimizes the requirements on divers. Three divers are required to complete the survey. The survey procedure consists of the following operations: 1. Two guide lines spaced 4 meters apart are stretched over the site to facilitate placement, spacing, and rapid relocation of numbered control point marker stakes (weight markers for rock bottoms). For an area 15 meters by 30 meters, 11 markers are placed. 2. Distances between control point marker stakes are measured by tape and recorded on a plan of the approximate locations drawn on a plastic sheet mounted on a clipboard. Measurement of a complete triangular net (three sides of all triangles) eliminates the necessity of angular determinations. 3. Relative elevations are determined with a range pole and leveling device. The leveling device consists of a transparent plastic tube 1 centimeter in diameter and 10 meters long. A diver induces air into one end of the tube until it is almost filled. The air-water interface within the hose is held at the marker stake's highest elevation. Buoyed by the entrapped air, the hose forms a convex-upward arc. A second diver holds the opposite end of the hose against the range pole, which rests vertically atop an adjacent marker stake. The position of the air-water interface on the range pole indicates the difference in elevation between the two marker stakes. A third diver records the measurements and directs the movements of the other divers. 8.9.2.6 Photographic Method To improve mapping for detailed archeological studies, photographic towers may be utilized (Bass 1964, 1966, 1968; and Ryan and Bass 1962). The progress of excavation in each area can, therefore, be recorded with grid photographs taken through a hole in the top of the tower. Before objects can be traced onto the overall plan photographs taken by this method must be corrected for (1) difference in scale relative to elevations; (2) position of objects relative to their distance below the grids and lateral distance from the center of the grids; and (3) "pillowing" effect. Series of stereophoto pairs may be taken of sites for three-dimensional viewing under a stereo-viewer. More important, the parallax in the pairs can be measured with a micrometer, and the elevations of any object then calculated by the simple formula: 8.9.3 Archeological Sampling Archeological sampling requires considerable care and patience in the removal and recording of artifacts. The equipment necessary for the removal of heavy overburdens of mud and silt around an archeological site is large and cumbersome. The air lift, the "shovel" of underwater archeology, has been used on virtually every major excavation under water (Bass 1966). The air lift is a simple instrument (Figure 8-25), consisting of a rigid, or part rigid and part flexible tube, into which air is introduced at the lower end. Consequently, the air breaks into small bubbles and mixes with the water. The result is a gas-liquid mixture of less density than the liquid outside the pipe. Since the density of the mixture inside the pipe is less than the density of the liquid outside the pipe, the air-water mixture will rise until the pressure of the column of the mixture equals at its base the pressure of the water at the same level. The suction created at the lower end of the air displaces unconsolidated materials and carries them to the surface. Lifted matter is discharged at the surface for screening and inspection through a reinforced flexible rubber section of tubing onto a barge or nearby land, or directly into the water down current from the work site. In the latter case, a wire screen container fitted with a small cloth bag at its lower portion is used to prevent loss of artifacts (Bass 1966). The air lift, if used without discretion, can damage or lose important artifacts. Details of air lifts and excavation techniques are given by Dumas (1962), and Bass (1966). The success or failure of air lifting directly on the site is relative to a diver's ability to change the size of the mouth (footpiece) and vary the lift's power. With a small, 2-inch lift at low power, extremely delicate work can be undertaken. In some areas, to avoid the dumping of airlifted materials back onto the site, it is necessary to transport this material away from the area. A long horizontal pipe floated on the surface will accomplish this task. To facilitate movement of material in the horizontal portion of the pipe, it may be necessary to attach a water jet pump. A versatile instrument for removing muck and other overburden from a site is the underwater suction dredge. Similar to that used to recover gold, it uses the reverse injection method in which water, pumped under considerable pressure through a hose from above, is injected a few inches behind the mouth of a pipe (2 to 12 inches in diameter). A strong suction is created in the opening, and a current is forced along the pipe. A hose is attached to the pipe, and the sucked material is pumped to the surface for screening. The mouth of the dredge can be screened to prohibit the entrance of heavy bulky objects that might clog the hose. Although not as effective as the air lift in some ways, the dredge is apparently a more gentle technique and permits a fairly systematic excavation (Jewell 1964). An underwater dredge and surface sluice machine for screening materials and retrieval of artifacts, with sufficient power, can be used for shallow water work to depths of 30 feet to remove large amounts of overburden. A small water jet can also be incorporated into these units. Modifications of existing equipment would make the dredge and surface sluice machine highly adaptable for saturation diving operations with sluicing and screening being accomplished either in a dry compartment similar to that used for inert gas welding on submerged pipelines, or directly under water. Depth would not be a limiting factor. A high pressure water jet can be used to cut through unconsolidated overburden and force it away from the site. The system consists of a hose, pump, and nozzle. The recoilless nozzle, which sends a water jet backward as well as forward, is recommended. A standard fire hoze nozzle may be used if a diver compensates for the backward push of the water jet. The use of flotation gear is an inexpensive and effective method of lifting. Lifting bags are available in different sizes and forms ranging from large rubberized bags and metal tanks capable of lifting several tons to small plastic and rubberized nylon bags for lifting 50 to 500 pounds. Larger bags should be equipped with an air relief valve at the top. For archeological work, the smaller rubberized nylon bags are recommended; these self-venting bags with a lifting capacity of 100 pounds are useful in all underwater operations. Lifting devices are described further in Paragraph 7.1.5. 8.10 CAPTURE TECHNIQUES A wide variety of devices are utilized by scientists and commercial fishermen to aggregate, concentrate, or confine aquatic animals. Trawls, seines, traps, grabs, and dredges have been successfully observed by scuba-equipped scientists concerned with animal and gear behavior. High (1971) points out that divers can frequently play a critical role in the design and evaluation of trawls and related gear. This role may be in the form of basic design, Figure 8-27 Divers Inspecting the Cod operational observation in shallow water, or by the observation of animal behavior within the gear's influence. High and Beardsley participating in the Tektite II undersea program were able to directly observe fish near stationary traps 25 to 80 feet below the surface for up to eight hours per day (Figure 8-26) (Miller et al. 1971). Methods were found to alter catch rates and species captured. Divers from the National Marine Fisheries Service accurately estimated fish populations attracted to experimental submerged structures during studies to develop automated fishing platforms. These and other diver studies conducted near passive fishing gear usually posed few unique or unexpected problems. 8.10.1 Nets Nets vary as to size, purpose, materials, and methods fished. Size is a major concern as divers desiring to make direct observations of an active net (one which is being towed) can readily alter or distort small nets by swimming near or touching net components. Any net is considered large if direct diver contact does not appreciably influence its configuration or operation. Plankton nets typify small nets both in physical size and the light-weight web required to retain microorganisms. At the larger extreme, high seas tuna seines often are 3,600 feet long with 42 inch long meshes stretching 200 or more feet down into the water. Gill nets are designed to entangle fish attempting to push through the meshes. Webbing mesh and thread size varies as does the net length and depth, depending upon the size and species of fish sought. Gill nets use fine twine meshes hung vertically in the water Seines are similar to gill nets in that a wall of web is held open vertically in the water by opposing forces of a corkline and leadline. However, the seine is set in a circle to confine fish schools within the web rather than entangle the fish. Seines often have rings along the leadline through which a line or cable can be pulled to draw the bottom closed, sealing off the fishes' escape route. 8.10.3 Trawls Trawls are nets constructed like flattened cones or wind socks and are towed by one or two vessels. The net may be operated at the surface, in midwater, or across the seafloor. Specific designs vary widely depending upon the species sought. A 10 foot long plankton net having a 1/2 meter mouth opening may be towed at speeds up to 5 knots whereas a 200 foot long pelagic trawl opening 40 by 70 feet may filter water at 1 knot (Figure 8-27). Trawls may be opened horizontally by towing each wingtip from a separate vessel, by spreading the net with a rigid wooden or metal beam or with paired otterboards suspended in the water to shear out away from each other horizontally when towed. 8.10.4 Diving When Underway Diving on stationary gear such as traps, gill nets, and some seines presents only a few unique problems to the experienced diver. He can dive either inside or outside the net to observe animal behavior or carry out work assignments. A diver must be alert to the probability of loose diving gear including pull rods, valves, mask rims, knives, vest inflator mechanisms, and weight belt buckles becoming fouled in the web. Loose undulating web not under strain is a likely source of entanglement. Such entanglements can usually be cleared more readily by the buddy than by the fouled diver. Unless the fouled diver turns or spins around, he is not likely to be wrapped in the web. Sometimes a fouled diver must remove the tank, disengage the caught mesh, replace the tank assembly and continue the assign ment. Hazards and diver difficulties increase with the speed of active nets or their components. During early retrieval of purse seines, web, purse rings, and the purseline move slowly. Toward the end of the pursing and net retrieving sequence, these components move quickly through the water. Since divers usually lack communication with surface winch and line hauler operators, the divers must stay out of the bight of the line or the immediate path of the gear. Diving within the influence of a trawl or other devices towed from vessels underway does present unique hazards to divers. These include entrapment within the net, fouling, and being forced against bottom obstructions. If the device is moving slowly (under 11⁄2 knots), the diver may be able to swim alongside for short periods. At speeds up to about 2.5 knots he may hold onto large nets without seriously distorting them and be pulled through the water. Both of these methods require the diver to be in excellent physical condition and to be trained in this special form of research diving. High (1967) has described the methods used by divers to study trawls. Generally divers operating on large midwater or bottom trawls descend to the trawl by entering the water from the towing vessel and moving down the towing cables. Care must be exercised to avoid jamming broken cable strands into the diver's hand. This descent technique provides a direct route to the net with a minimum of energy and compressed air expended. Caution must be observed as the divers approach the turbulent water behind the otterboards, especially when the boards are in contact with the bottom. Clouds of sediment stirred up by the otterboard obscure portions of the bridles between the otterboard and the net so the diver must feel his way along the bridle. As an alternative, when visibility is good (25 feet horizontal), experienced divers may swim inboard of the otterboard just within the path of the oncoming trawl and wait for the bridles to clear the mud cloud or for the net to appear. Divers can make their observations while hanging directly onto the trawl and conveniently move hand over hand to all parts of the net. However, trawls having a stretched mesh size of less than 2 inches. (each side of the aperture 1 inch long) are difficult to hang onto. Distances between two points on the trawl can be estimated by pulling low stretch polypropylene twine taut between the points then cutting the line (Figure 8-28). The tied end will remain with Descending Down the Side of a Large Midwater Trawl, the Diver Pulls a String Tight to Estimate the Vertical Opening the trawl until retrieval when the line can be removed and measured. Small trawls and other moving gear can be observed without direct contact which may affect the system by using a separate tow line for the divers. A sea sled with a current deflecting shield can be towed parallel to the gear providing protection for divers who can then observe the gear at much greater speed than divers hanging directly on the gear. Under these conditions one diver is the pilot and his buddy the observer. The divers must be particularly careful to maintain proper breathing rhythms to prevent embolism in the event of sudden rising of the sea sled. The pilot should have a depth gauge mounted to be easily read at all times. He should continually monitor the gauge maintaining a constant depth or making necessary changes slowly. The use of a sea sled facilitates the use of a hard wire communications system between divers and the surface. Each diver must be alert to possible dangers in a bottom trawl's path. Some obstructions encountered will cause the trawl to stop momentarily then |