GEM Systems Magnetism Survey Tutorial Colton Dudley ***GEOPHYSICS IS ABOUT CONSISTENCY*** ***READ THROUGH EVERY PARAGRAPH*** ***REMOVE ALL BELONGINGS THAT MAY POSE A MAGNETIC THREAT TO THE SURVEY (METALS)*** Equipment Both GEM system cases. Each case should have one GEM system control (one with and without straps), two white sensors, and two sets of cables. The mobile case should have a set of shoulder straps as seen in the bottom right fitting shown in Figure 2. Fig. 1: Base GEM system case Fig. 2: Mobile GEM system case Cooler filled with the CHARGED mag battery (should have a military connector). It is best to keep the battery in the cooler at all times. Set of stakes. Metal plate and string to tie down magnetometer sensor pole. Hammer/pick. Electrical tape. This is shown in Figure 3. Fig. 3: Items found within cooler.
Base Setup The base setup will include the Base GEM System case and the cooler. First, take out the pole assembly located behind the foam padding with the case as shown in Figure 4. Fig. 4: Pole assembly behind the pad in the base GEM case Next, assemble the pole together along with the metal plate shown in Figure 5. Locate the pole to a place that is away fro the road, people, or anything that may temper with the pole. Try and also have the pole in the open terrain. The plate should be between part 1 and part 2 of the pole assembly as seen in Figure 6. Fig. 5: Plate assembly with pole. Fig. 6: Plate location on the pole. After the pole has been assembled, the pole needs to be fixed to the ground with the orange stakes and strings as shown in Figure 7. Do not attach the sensor until the pole is assembled and fixed to the ground. Use the stakes and the strings to tie it into the
ground. Make sure the strings are tight and the pole is straight, perpendicular to the ground. Fig. 7: Strings connected to the stakes holding the pole in an upright position. Next, you will be attaching the sensor to the top of the pole as shown in Figure 7. There are two sensors in each box; one is for use and the other is a spare. You will only be using one from each box. Each sensor has a label as shown in Figure 8. Be sure to choose a pair of sensors that are of the same colored label. The numbers will not be similar. Also be sure to attach the sensor from the base to the base pole and attach the sensor from the mobile case to the mobile pole (located later in this tutorial). It is not critical to attach the sensor in a northerly direction, but geophysics is all about consistency. The front end or the end that should point north is shown in Figure 8. Be sure that all threads are tight on the pole and the sensor after assembly. Fig. 8: Location of label and front-end on the sensor.
Within each box are two sets of cables as shown in Figure 9. Like the sensors, there is a cable for use and one cable for spare. Try and use the older of the two cables. The data transfer cable will not be used in the tutorial. Fig. 9: The two sets of cables in each case. Unwrap the cables and connect one of the ends with two attachments to the sensor on top of the base pole as shown in Figure 10. The cable-end that connects to the sensor is the end with two light gray steel attachments. The other end of the cable has a light colored attachment and a raw, dark colored attachment. This end will be attached to the GEM base system later. Fig. 10: The cable with two attachments about to be attached to the sensor. The GEM system is very sensitive to any change in the magnetic field whether that is from the earth, sun, or the surveyors. So be sure to tie down the wire cable to the pole using electrical tape as seen in Figure 11. Also, tie down the cable with the Velcro straps.
Figure 11: Cable taped to the base pole. The GEM control system needs to be connected to the sensor. Take the cable recently taped to the pole and thread it through the hole in the bottom of the cooler as shown in Figure 12. Fig. 12: Cable threaded through a hole in the bottom in the cooler.
The cable needs to be connected to the GEM system; though, there are two channels that the cable can be attached to: a lower and upper channel. By default, the GEM system uses the lower channel. Geophysics is all about consistency. So connect the two cable attachments to the lower channel and connect the battery as well as shown in Figure 13. Do not rush the cable connections. If the cable does not fit, the connection is probably incorrect. The upper channel is most likely used as a spare in case of a lower channel malfunction. How to sync the GEM system to either channel will be covered later. Fig. 13: Cable with two attachments connected to the lower channel and the battery attached to its specific channel. The base GEM control system needs to be programmed. Turn on the GEM base system by pressing the black B button (also has power printed in blue). The GEM screen should look like Figure 14. Fig. 14: The main screen of the GEM control system.
If it is desired to return to the main screen, press 1 and C (both have menu printed in blue). As previously stated, the channel needs to be checked. The channels can be found by pressing C for info. A new screen should appear. Press E to view the channel input for the GEM system. A new screen should appear as shown in Figure 15. Fig. 15: The screen showing the channel input for the GEM system. As stated previously, the GEM system defaults to the lower channel. Please refer to Figure 13 to determine with channel is the lower or upper. Since the system is connected to the lower channel press F for ok. The main screen should appear. Now, setup the survey by pressing A. The next screen should is shown in Figure 16. Fig. 16 Since this is the base station press B for base. The next screen is shown in Figure 17.
Fig. 17 The next screen shows how many more readings the base station can withstand within its memory. The reading should be plenty! Press F for setup. The next screen is shown in Figure 18. Fig. 18 This screen shows the time. This will need to be changed. Press C for change. The next screen is shown in Figure 19. Fig. 19
This screen has already been filled out. The screen should be empty for the surveyor initially. There are a multiple of letters seen in the new screen. The following describes these letters: i. Time is as follows 1. w = day of the week numerically. Monday = 1, Tuesday = 2.Sunday = 7 2. yy = year, ex. 2012 =12 3. mm = month, ex. June = 06 4. dd = day of the month, ex. 7 th = 07 5. hh = hour of the day (military) ex. 3pm = 15, 6am = 06 6. mm = minute of the current hour, ex. 3 rd minute = 03 7. ss = second of the current minute, always set to 00 The example above is Tuesday, July 17, 2012 at 4:19:00pm. Once you finish this by pressing the last zero (0) for the second column, the next screen will appear as shown in Figure 20. Fig. 20 Before you start the clock, it is best to sync the time with your GPS system and the GEM mobile system. Follow the same steps for the mobile system in order to reach the time screen. Press F to start the clock when you are ready to synchronize the systems and your GPS. If you accidently set the clock incorrectly, pressing C can change the time as shown in Figure 18 or you can continue with F for ok. The next screen is shown in Figure 21. Fig. 21
This the file link to the file that will be saved with the GEM system data. The file link,.b, is perfect for the base station. It is also the default for the GEM system. Press F for ok. The next screen is shown in Figure 22. Fig. 22 This next screen is the file type. A text file is the most ideal. Press F for ok. The next screen is shown in Figure 23. Fig. 23 The next screen is the ID number. The ID number is not to relatively important and can be skipped. Press F for ok. The next screen is shown in Figure 24.
Fig. 24 The next screen is the cycle time. The cycle time for the base station needs to be 30 seconds. The zeros before the 3 in this case are neutral. So this screen reads 3 seconds. It needs to be changed to 30 seconds. Press B to increase the cycle time and A to decrease to time. Press F when the time is satisfied. The next screen is shown in Figure 25. Fig. 25 The next screen is the tune initialize. The base station does not need to be tune initialized. In this screen, it needs to be changed. Press C to change to no. Press F for ok. The next screen is shown in Figure 26.
Fig. 26 The next screen is the auto tune screen. The auto tune needs to be on for the base. Press F for ok. The next screen is shown in Figure 27. Fig. 27 The next screen is the tuning screen. The tuning needs to be set to 35 for the base. Press F for ok. The next screen is shown in Figure 28. Fig. 28 The next screen is the AC filter screen. 60 Hz is the ideal frequency for most of our surveys. Press F for ok. The next screen is shown in Figure 29.
Fig. 29 The next screen is the datum screen. The datum screen, for the most part, should be 51,000 nt. This number may need to change depending on location. For most surveys for UNR, 51,000 nt should be ok. Press F for ok. The next screen should look like Figure 30 after some time has passed by; though, these numbers are incorrect. The numbers should be more near 51,000 nt. Fig. 30 If this screen is reached and the readings seem legitimate, the base station is programmed correctly. If not, follow the entire procedure again with the GEM control system. Be sure to put the cooler close to the pole and shut the cooler with all the tools inside. Mobile Setup The mobile GEM system needs to be set up. Take out the pole assembly behind the foam casing within the mobile GEM case as shown in Figure 31.
Fig. 31: Location of the pole within the mobile GEM system case. There is cap that attaches to the end of the brown base of the pole assembly. Be sure to attach this peace to keep the threads clean on the pole. If no cap is present, find something to protect the threads such as electrical tape. The attached cap is shown in Figure 32. Fig, 32: the cap attached to the brown mobile pole assembly piece. After the threads are safe, find the sensor in the box that is relative to the sensor on the base. This is not crucial that they match, but geophysics is about consistency. Screw on the sensor to the top of the pole similarly to the base station pole as shown in Figure 33. It is ok to set down the pole after the sensor is attached, but it is sensitive. Be careful. Also, it is best to strap on the GEM system before connecting the cables to the sensor.
Fig. 33: The similar sensor attached to the mobile pole. Time sync the mobile GEM to the base GEM Also, it is best to strap on the GEM system before connecting the cables to the sensor. Now find the cable within the mobile case and attach it to the sensor like the base station sensor. Similarly, find the older cable because geophysics is about consistency. Tape the cable to the pole like the base station to keep the cable from swaying because geophysics is about consistency. Connect the cable attachments to the lower channel rover just like the base station for consistency as shown in Figure 34. Check that the lower channel is selected for the mobile GEM system just like the base station setup mentioned earlier by pressing 1C for the main menu, C for info, and E for channel.
Fig. 34: The cable attachments connected to the lower channel as well as the shoulder straps connected to the mobile GEM system. The mobile setup is now ready, but the control system needs to be programmed. Follow the same procedure as the base GEM control system; though, the parameters will differ. There are also a multiple of different screens that appear with the mobile setup. These new screens are line, line incr., station, station incr., and EOL incr. Skip all the screens by pressing F for ok. They are not needed. The time needs to be synchronized with the base station. Turn on the mobile GEM control system with B and press A for survey. Press B for base if prompted. Now you will be prompted with the time screen as shown in Figure 18. This time will need to be synced with the base. Here is the procedure. Set the time just like the base, but set the time to a futuristic time so the mobile control system time can be synced with the base control system time. For example, if the base time is say 14:07:56, set the mobile to 14:09:00. The screen shown in Figure 20 will appear when the time is set. Press F for ok when the times are about to meet. The time should be within three seconds of each other. Now since the time is synced the rest of the parameters need to be set. Here are the parameters for the mobile control system: 1. File name -.m 2. File type text 3. ID skip 4. Cycle time 0003 sec. (3 seconds, zeros are neutral before real number) 5. Tune Initialize YES 6. Auto Tune YES 7. Tuning There is no tuning screen for the mobile! 8. AC Filter 60 Hz 9. Datum 51,000 (should be within 1000 nt of readings. If not, adjust the datum.) You should see the screen as shown in Figure 30 after the parameters are set, but there will not be any readings. If the mobile control system shows the screen shown in Figure 30, the mobile control system is ready. The mag survey is now ready to be performed. Be sure to keep the mag mobile sensor in a northerly direction. Be sure to record the following four items on a piece of paper incase the mag data is lost: time, total mag intensity (nt), waypoint name, and UTM coordinate.