The Seismic Noise Magnitude And Precision Evaluation On The Terrestrial Gravity Measurement Environment

The precision of gravity measurement not only depends on the instruments capability,but also depends on the noise level of measurement environment.The evaluation of the station environment is related to the quality of observation data and can be used as an quantitative index to classify the stations,site selection and adjustment.A conventional method to evaluate the gravity measurement environment at a station is taking observation data for one year or several months,by reducing the earth tide variation and the atmosphere effect.After the drift fitting,obtained the gravitational residual,and selected the 5 day recordings with the lowest of the root mean square(RMS)as the quiet period to calculate the results.The seismic noise magnitude(SNM)and precision were used as the indices to evaluate the gravity measurement environment.This study selected five stations of Nanshan(GS-15),Wujiahe(gPhone),Wushi(PET),Shiquanhe(PET),and Qiongzhong(PET)for consecutive four years of gravity observation recordings.We have calculated the SNM and precision of the seismic frequency band(200 ~ 600s),respectively.The results showed that the SNM of Qiongzhong station was larger than that of the other four stations,indicated that the tide would affect the SNM and precision;Shiquanhe and Wushi stations used the same type of instrument,but the SNM during four years variation range and level in Shiquanhe station are large than Wushi,proving that the measurement environment of Shiquanhe was not as stable as that of Wushi,and the noise level was large;The SNM and precision of the five stations for consecutive four years have not changed much,which means that the data at different times period have little influence on the results.The conventional method is to select the 5 quietest days from the long-term observation data as the input dataset,but the measurement time of the campaigned gravity survey is short than continuous gravity measurement.This study attempts to calculate the SNM and precision by taking the one quietest day from the short-term observation data,the result can provide a meaning reference for evaluating the environment of the campaigned gravity survey stations.Then,this study selects short-term observation data in three stations: Xiaomiao(GS-15,23 days),Institute of Geophysics(CG-5,46 days)and Baijiatuan(CG-5 and gPhone,10 days).Comparing the results of the five quietest days and the one quietest day,it can be found that the noise level of the metal spring gravity meter is smaller than that of the quartz spring gravity meter.In the short-term measurement,just take the one quietest day we could evaluate the measurement environment and precision.This results are similar to the metal spring gravimeters and quartz spring gravimeters.This study will provide a straightforword and effective approach to evaluate the campaigned gravity survey environment.