5. Possible reasons

1. The Effect on FGE Fluxgate Magnetometer Caused by the Variation Room Humidity Fuxi Yang Wulumuqi Geomagnetic Observatory, Earthquake Administration of Xinjiang Uygur Autonomous Region Wulumuqi, Xinjiang, 830011 Key words: Humidity, variation room , fluxgate magnetometer 1. Magnetic record affected by Humidity It was found big noise appeared on the vertical component recorded by the DMI Suspended FGE Fluxgate Magnetometer (FGE) of Guiyang Observatory (GYX) from 16pm Nov. 20, 2008. However, the Z component recorded by the GM4 Fluxgate Magnetometer (GM4) did not show any unusual change, although the GM4 is installed in the same variation room with FGE. The observatory stuff installed a sealed glass box outside the FGE sensor, and put the silica-gel desiccant inside the box to reduce the humidity around the sensor. After that, the recording data became normal. Another observatory, Dedu Observatory (DED), also showed the same phenomena with GYX. Fig. 12 The original second profile of WMQ recorded by FGE from 09am to 10am on Mar. 8, 2009 Fig. 11 The F-P minutely profile of WMQ derived from FGE (20090308-20090322) 3. Both noise and base line affected by humidity The FGE sensor was dried at Chengdu Observatory (CDP) on Dec. 26, 2008, big noise disappeared and base line recover to normal level after that. It is the same with both GYX and WMQ, the Z component recorded by the GM4 which installed in the same variation room with FGE did not show any unusual change. Fig.13 the Z minutely profile of CDP derived from FGE (20081223~20090106) Fig.14 the Z minutely profile of CDP derived from GM4 (20081223~20090106) Fig.1 the Z minutely profile of GYX derived from FGE (20081120~20081124) Fig.2 the Z second profile of GYX derived from FGE on Nov. 21, 2008 4. Observatories with FGE affected by humidity and dependence of the affected component on humidity Table 1 appearance of data affected by humidity Fig.3 the Z minutely profile of GYX derived from FGE (20081120~20081204) Fig.4 the Z minutely profile of GYX derived from GM4 (20081120~20081124) Fig. 15 Distribution of observatories with FGE affected by humidity (red dot) Table 2 Dependence of the affected component on humidity Fig.5 the Z minutely profile of DED derived from FGE (20090215~20090216) Fig.6 the Z minutely profile of DED derived from FGE (20090219~20090220) 5. Possible reasons 1. FGE sensor adsorbs water vapors on it so that the centre-of-gravity position changes and FGE system leans, which could cause the change of based line derived from FGE. 2. The impurity outside electric circuits and units mixes with water vapors , and the produced electrolytic solution could cause the big noise. Fig.7 the Z second profile of DED recorded by FGE (20090215~20090216) Fig.8 the Z second profile of DED recorded by FGE (20090219~20090220) 2. Base line affected by Humidity The base line derived from FGE Fluxgate Magnetometer of Wulumuqi Observatory (WMQ) showed big change after heavy rain on Mar. 7, Mar. 14 and Mar. 19, 2009, respectively. It was found that the rainwater had flown into the underground variation room. After the FGE sensor was sealed with a glass box and put the silica-gel desiccant inside it, the base line became normal again. The FGE noise did not show significant change during this period. FGE-GM3 Fig. 16 schematic diagram of tilted suspended FGE Fluxgate Magnetometer • Suggested solutions • 1. Reducing the humidity inside the variation; • 2. Sealing the FGE sensor with a glass box and put the silica-gel desiccant inside it. The silica-gel desiccant which could be discolored by water vapors is recommended. • 3. It is recommended to use above mentioned two methods at the same time, because only to reduce the humidity inside the variation room does not take effect sometimes. • 4. In addition to sensor, other electric units should also be sealed and keep dry. Fig. 10 The difference between the Z minutely value derived from FGE and GM3 Fluxgate Magnetometer (20090301~20090418 ) Fig. 9 The base line of WMQ derived from Z component of FGE (20090101~20090418)