Comparative induction studies with geomagnetic observatory data in three epochs
The data base are geomagnetic hourly means in the three components of the Earths magnetic field, compiled in global and regional networks of geomagnetic observatories. Various methods are applied to derive from these data EM responses in the period range of daily variations. Their derivation goes to the limits of todays instrumental capabilities. Therefore emphasis will be on the reliability of response estimations, limited by statistical and systematic errors. We present results from data during the IGY (1957/59), the IQSY (1964/65) and during six months in 1982/83.The methods to be applied are the global potential method, leading to response estimates for the Earth as a whole,the Z:H and Z:D method, yielding local C-responses from observations at a single site, and two horizontal gradient methods. They also yield local C-responses, but involve observations in global or regional networks.These local responses assume laterally uniform Earth models in some surroundings of the sounding site.Therefore attempts have been made to combine the gradient method with geomagnetic depth sounding towards local deviations from a layered structure.The comparative study has given confidence in EM sounding results from geomagnetic data. Stochastic errors appear to be correctly estimated, and there are no indications for systematic errors. They could arise, when fields in networks of sites are expanded into series as in the potential and the horizontal gradient method. The resulting coefficients for spherical harmonics and polynomials have an unavoidable dependence on the number and distribution of observing sites. Since the network configurations have changed considerably during the studied epochs, this cause for systematic errors appears to be without critical influence. We have not found any significant improvements in data quality between the IGY and the 1982/83 epoch 25 years later. But definite progress has been made in the documentation. From the 6.5 million hourly values,compiled for the IGY, about 15 000 were clearly erroneous and had to be corrected manually, while the data of the 1982/83 epoch were nearly without faulty values, their problems lay else-where. Hence, the editing of the time series remains the most time-consuming part of the analysis. Disappointing is the undiminished large number of data gaps in all three epochs.
Potential method Z:H/D methods horizontal gradient methods stochastic/systematic errors
Ulrich Schmucker
Planckstrasse 19 D-37073 3 Goettingen, Germany
国际会议
The 19th International Workshop on Electromagnetic Induction in the Earth(第十九届国际地球电磁感应学术研讨会)
北京
英文
879-884
2008-10-23（万方平台首次上网日期，不代表论文的发表时间）