NOTE: The information provided in this section only applies to ShapeArrays (SAAs) for which optional magnetometers were ordered.
Checking magnetometer data collected with SAARecorder
Step 1 - Load and View your Magnetometer data
Load your data in SAARecorder and view magnetometer data for all magnetometer devices by clicking on the 'Data and graphs \ Magnetometer data' menu item. This presents a window that allows a quick view of heading information and total magnetic field magnitude for each magnetometer.
Figure 1: Magnetometer Data window.
Step 2 - Review your Magnetometer data
The X-axis heading values should remain relatively fixed over time (within +/- 1 degree) and should also agree fairly well (within +/- 5 degrees) with one another (Figure 1). If the values are outside of these ranges, the following causes are possible:
- There is an extensive twist along the length of the ShapeArray
- The magnetometers are not functioning correctly
- The ShapeArray is moving
- The ShapeArray is dipping less than 45 degrees of vertical
In the absence of any magnetic interference, the magnetic field strength values for each magnetometer should agree with the other magnetometers to within +/- 10% (Figure 1). If magnetic field strength measured by one or more magnetometers differs from that of the other magnetometer(s) by more than 10% then it is likely that there is some magnetic interference near the ShapeArray that is adversely affecting one or more of the magnetometers. It is generally advisable to disable twist correction for magnetometers that are affected by magnetic interference. To disable twist corrections for individual magnetometers, click the 'SAA Setup \ Advanced \ Enable / Disable magnetometer twist correction' menu item and uncheck the magnetometers you do not want to be used for twist corrections (Figure 2).
Figure 2: Enable/Disable Magnetometer Twist Corrections window.
Checking magnetometer data collected with a Data Logger using SAACR_raw2data
Step 1 - Load and Configure your SAA data
Load and configure your SAA data by following Steps 1 through 6 in the SAACR_raw2data How-To Guide. Once these steps have been completed, in the SAACR_raw2data settings window click the Magnetometer button (Figure 3) to view your magnetometer data (Figure 4).
Figure 3: SAACR_raw2data settings window.
Figure 4: The SAACR_magtool window showing the ‘Change in Azimuth vs. Time’ Graph (Top) and ‘Local Magnetic Strength (LMS) vs. Time’ Graph (Bottom)
Step 2 - Review the ‘Change in Azimuth vs. Time Graph’
The top graph in the SAACR_magtool window (Figures 4) shows the change in a magnetometer’s azimuth (y-axis) as a function of time (x-axis) (Figure 5). If the ShapeArray is stable (i.e. not being rotated or moved at all, and none of the magnetometers are affected by magnetic interference) then the change in azimuth plots for each magnetometer should remain quite close to 0 degrees. Significant changes from 0 degrees (i.e. more than +/- 1 degrees) would indicate that either:
- The ShapeArray was rotated OR
- There was some magnetic interference acting upon the magnetometer(s)
Significant differences between magnetometer azimuth readings will be used by the SAACR_raw2data software to apply the necessary twist corrections to your data.
Figure 5: 'Change in Azimuth vs. Time' Graph - note that the top magnetometer is disabled
Step 3 - Review the ‘Local Magnetic Strength (LMS) vs Time’ Graph
The bottom graph in the SAACR_magtool window (Figure 4) shows the change in a magnetometer’s Local Magnetic Strength (y-axis) as a function of time (x-axis) (Figure 6). The value of these traces is calibrated to be 1.0 for the Earth's magnetic field as measured at our factory located in Fredericton, NB, Canada, but will in general be slightly different depending on the strength of the Earth's field at the ShapeArray’s current location. Variable LMS values can be a result of:
- Differences in magnetic fields related to global position. For example, in many parts of South America, the average strength of the Earth's magnetic field is only about half that of the magnetic field strength in Eastern Canada
- Concentration of ferrous material (casing, vehicles, structures, etc.) in close proximity to the ShapeArray
- Geological formations with a magnetic component
If the total magnitude measured by one or more magnetometers differs from that of the other magnetometer(s) by more than 10% then it is likely that there is some magnetic interference near the ShapeArray that is adversely affecting one or more of the magnetometers. It is generally advisable to turn off any magnetometers that are affected by magnetic interference.
Figure 6: 'Local Magnetic Strength (LMS) vs. Time' Graph - note that the top magnetometer is disabled
To turn off a magnetometer, deselect its checkbox from the Enabled Mags section near the top of the SAA# tab, as shown in Figure 7.
Figure 7: Only magnetometers whose checkbox is selected are enabled - note mag 1 (Top) in this example is disabled
Step 4 - Complete the Magnetometer Configuration
Complete the magnetometer configuration steps as described in the Using the SAACR_raw2data Magtool Guide.