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- Connect towfish to tow cable. See Figure 1.
- Connect power to isolation transceiver. See Figure 14.
- Start SeaLINK software. See Figure 8.
- Confirm communication w/ towfish
- Power-up towfish (Ctrl-O) if it is OFF. Commands are entered in the Terminal window of SeaLINK.
- Test communication by pressing spacebar (gets time) and entering the ‘d’ command (or click on the Extra info icon)
- Confirm GPS NMEA signal coming in from WinFrog (NMEA log field).
- The procedure to get the towfish in the water is a team effort. Safety of the team is just as important as the safety of the magnetometer.
Turn on main circuit to Maggie winch (Port side - Figure 9.) The drum winch also has a breaker type switch plus “Start” and “Stop” buttons (Figure 10.) The drum control handle/joystick is on top of the box with the on/off buttons (Figure 10.)
Releasing cable from the drum and operating the crane boom are done in tandem and good communication between all team members is important. It is not going to be explained exactly how to do each step; everyone may do it a bit differently. The result will be the same though. Slowly lower the towfish into the water. See Figure 12.
Deploy towfish so it is just below the surface of the water. This is done to equilibrate the towfish with the water temperature and to zero the pressure at the water line. Wait fifteen minutes. See Figure 13. Make sure the towfish is deployed far enough so that the cable does not jerk and snap in the waves. In heavier seas the Maggie may need to be let out a bit further - Zero the pressure sensor in SeaLINK. The pressure sensor is a Wheatstone bridge on a silicon diaphragm. The maximum pressure that this sensor can stand before potentially suffering damage is 2500psi (1725m of water). The sensor is an analog device that may drift with temperature and with time. It has a 0.5m sensitivity. For proper operation, the zero-level should be reset before every survey.
- Enter ‘p’ command (or click on the Zero Pressure icon).
- Exp 328 : at 11.5kts and 450m of cable deployed, the towfish was about 8.8m below the water surface. Minimal swells.
- Enter ‘p’ command (or click on the Zero Pressure icon).
- Slowly deploy the tow cable. Increase speed as the drag tension increases. Deploy the remainder of the cable leaving ~12in left on the winch (Figure 5.)
- Sync time source
- Either press the F7 key or click on the PC TIME icon.
Enter the layback distance (524m) by clicking on the Update the layback length icon (Unless layback is entered in WF.)
- Sync GPS w/ the towfish
- Either press the F8 key or click on the SYNC GPS icon.
- Click through any warning windows about the sync failing.
- Set the GPS position to append the magnetometer data by clicking on the Appending GPS position to Fish output icon.
- Turn on auto-tuning: COMMAND – Auto tuning on or enter x in the terminal window.
- Start cycling data by pressing on one of the frequency icons
- 0.3Hz cycling frequency should be fine for a general survey
- Check SeaLINK warning bar to ensure that the Signal Strength and Quality indicators are Excellent or Good or Acceptable and ensure that there is no high magnetic gradient (warning indicator is “OK”). See Table 1. for interpreting the warning bar.
- Start logging (saving data) by pressing the Toggle SeaLINK File Logging icon. Note that the file-logging indicator on the SeaLINK warning bar turns from red (OFF) to gray (ON), to indicate that data is being saved to disk.
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- To change the logging filename or directory, select File – Preferences and select the Output Streams tab. Use the Browse... buttons to select the save location for both the Raw Data log and the optional Geosoft XYZ Data Log.
- Data files;
- .mag
- SeaLINK raw data log that records the ASCII data generated by the magnetometer electronics. The file can be opened and viewed by any text editing program (ex. WordPad).
- .XYZ
- ASCII data format that can be easily imported into data processing and analysis software (ex. GeoSoft’s Oasis) or Microsoft Excel. File format is based on the industry standard GeoSoft XYZ format.
- .mag
