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SHALLOW MARINE MAGNETOMETER SURVEY TECHNIQUES AND RESULTS G-877 COMPARISON TO G-881

There are many applications for shallow marine magnetic magnetometers including pipeline and cable location, wreck search, UXO and archeological investigations. Techniques for finding wrecks and estimating their size have improved greatly over the last few years. New hardware and software tools have increased the detection efficiency of the marine magnetometer. This brief review will show some of the results of recent surveys and compare the output of the G-881 (Cesium) and G-877 (Proton) Marine Magnetometer systems.

Basic specifications for the 881 and 877 are 0.02nT @ 10 Hz (881) and 0.25nT @ 0.5 Hz (877). The differences between the two magnetometers are large for some applications and small for others.

For instance, if the application is geological survey where the source body is many hundreds or thousands of feet deep, then either system will measure the earth's magnetic field to make a geological map. If the anomalies are large as would be expected from steel shipwrecks or large pipelines, both systems will faithfully measure and record the anomalies.  However, when the anomalies are small such as in archeological targets or UXO (unexploded ordnance) then the high speed high sensitivity Cesium mag performs much better at similar tow speeds.  However, the proton can also be used for these survey objectives at slower tow speeds (3 to 4 knots)

Both systems employ our MagLogLite logging software. This powerful tool is under continual advancement and upgrade as we find new ways to improve its performance. Recently we improved the anomaly detector portion of the program to allow automatic determination of position of sources of magnetic disturbance. There are two methods of setting up the program to analyze the data in real time or playback mode.  The first is in the magnetometer setup window where the user can set min/max and 8th difference levels for audio and visual alarms. The 8th difference measurement is a tool used in geophysics to discriminate between geological and man-made sources of magnetic distortion. A value of 2 to 4 nT is a good default number as besides discriminating between different sources, it is a good estimate of the noise of the system and an anomaly caused by a ferrous object will often trigger at that level. 

Signal level is also measured continuously to alert the user should there be any degradation in the signal quality due to cable problems or orientation errors.

Another more diagnostic method of determining the anomaly magnitude and position is to use the Trace Configuration dialog box. This part of the MagLog program allows the user to set up criteria for automatic flagging of the position of the FISH when an anomaly exceeds preset limits.

 In the case on the right, we see that the Min/Max Auto Detection box is checked and the parameters are set to trigger if the anomaly is greater than 10nT for a period of 10 seconds. Note that the program will automatically mark the fish position on the chart graph and on the GPS plot as well as put the flags in a flag list for export and later analysis. The positions can be used to steer the boat back to the anomaly for a close in survey or dive operations.

Many people ask us to compare the performance of the 881 and 877 since they are of different technologies and prices.  As mentioned before, they compare favorable for deep large targets but the proton precession system may not see all the small targets that are visible in the cesium data. We have a survey area that was run sequentially by the 881 and 877 systems and the data is displayed here.

Note that while the larger anomalies are clearly seen by both magnetometer systems, the G-881 data shows more and smaller anomalies with greater detail and clarity. This mostly a function of the sample rate of the magnetometer but also due to the inherent higher sensitivity of the Cesium Vapor technology.  Contact Magnetometer Sales for more examples.