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The underwater gravity meter operator must first be familar with the various procedures and adjustments described in this manual. It is strongly recommended that the operator practice reading the meter on land until satisfactory results are obtained. Operation at sea may present many new and unexpected problems. Most operating crews continue to improve substantially in speed, accuracy, and resourcefulness during their first several surveys. This section points out some of the major problems, and gives suggestions on how they may be solved
For water depths less than 100 feet, an Aframe or davit with a pulley or sheave can be used to lower and raise the underwater meter. A wireline or rope may be used for hoisting rather than using the electric control cable. The wireline is pulled in on a motor driven drum equipped with a clutch and brake, by use of a motor driven cathead, or with a hand winch in very shallow water. This method allows use of a separate light flexible electric control cable which is pulled in by hand. One crew member is usually given responsibility for paying out and recovering the electric cable and coiling it on the deck. This person must be careful not to become tangled in the cable and pulled overboard.
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In deeper water, or where there is coral or sharp rocks which could damage the control cable, it is recommended to use an armored cable strong enough to lift the underwater unit. A more sophisticated hoist, generally with sliprings for electrical connections, will be required. Such a hoist is expensive and can be used only on a fairly large boat, but will speed operations considerably.
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Hoists can be located on the sides, bow, or stern of the boat. Operating from the side reduces the chance of having the cable caught in the propeller, but ship roll can make operation more difficult. If a hoist is operated on the stern, a metal cage is usually put over the ship's propeller. A cage can be made so that it can be lifted when the boat is underway.
One major problem in underwater gravity surveying is broken electric wires inside the control cable. This problem can be particularly annoying because the broken electric conductors may appear to be broken only when the cable is stressed (when the gravity meter is on the sea floor). When the meter is brought up on deck, the broken wires often go back together, and the gravity meter appears to work. In such cases it is very helpful to have an extra cable. Some amount of extra cable slack should be put out even after the meter has reached the bottom. This will prevent pulling on the meter if the boat drifts off station somewhat. Twenty feet of slack may be typical on a shallow survey, but more than fifty feet may be required for deeper operations.
Ship vibrations can cause excessive meter drift, thereby reducing the accuracy of the readings. We recommend that the underwater unit be carried on a shockmounted platform on the ship deck. A simple and effective platform consists of a 4 foot square piece of 3/4 or 1 inch inch plywood supported by 3 or 4 old truck tires.
On hot sunny days the temperature of an unprotected gravity meter can exceed its operating temperature. The underwater unit can be covered with wet sacks between stations to prevent this from happening. The operator can monitor both the meter internal temperature and the "ambient" temperature (inside the inner sphere) from the computer display.
When making gravity readings the boat must be kept near the point where the meter was lowered. The simplest method usually is to anchor the boat while making the reading. Alternatively, a buoy may be placed and the boat maneuvered to stay near the buoy. If electronic navigation is available, it may be used to help maneuver the boat near the gravity station.
If the navigator notices that the boat is drifting off station, or the boat anchors are slipping, he must alert the meter operator immediately. The operator can then clamp and raise the meter before it is pulled by the cable.
After the boat has reached a station, check the computer display to confirm that the meter is in the CLAMP mode. Lower the meter and keep watching the display. Should the pressure sphere be leaking, a FLOOD WARNING message would appear on the screen. When the meter is on the bottom, a TILT WARNING message indicates the meter is tilted so much that it cannot be leveled. In this case, raise the meter and move it to a place where the bottom slope is not so great. Under good conditions an experienced operator can complete a gravity reading in 3 or 4 minutes.
Make the reading according to the "GRAVITY RECORDING" procedures outlined in the System Controller chapter. After the reading has been obtained, be sure to CLAMP the meter before hoisting to the surface.
The meter operator must be careful to avoid BACKLASH whenever changing the COUNTER READING. This requires that the counter ALWAYS be moved in the SAME direction (up or down) for at least 0.3 to 0.5 counter units before stopping the counter and taking a reading.
When the meter is back on the boat, a reference depth reading can be made. If the displayed meter depth is not zero, the parameter Depth_Zero_Offset can be edited to adjust the zero reference point. This parameter is in A/D bits representing voltage output from the depth gauge.
A standard spares kit is included with the Model U underwater meter.. This kit contains spares of the meter components considered most vulnerable to failure. The standard spares kit contains:
| 1 MAIN ELECTRONICS CARD | 1 PRESSURE GAUGE AMPLIFIER CARD |
| 3 POWER SUPPLY CARDS | MISC. CONNECTORS |
| 1 RS-232 TRANSLATOR CARD | MISC. ELECTRONIC CHIPS |
| 1 TRANSIENT CARD | DOW 111 SILICONE LUBRICANT |
| 1 LEVELING MOTOR WITH GEAR | SPARE INSTRUCTION MANUAL |
LaCoste and Romberg can provide training in operating the underwater gravity meter, either on site or at the company facilities in Austin, Texas.
Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 7
