A metrologic method of anomaly field amplitude bottom reduction in undersampled geomagnetic marine surveys

We show the results obtained by means of a seabed reduction technique on the intensity of geomagnetic anomaly fields applied to a synthetic case and then to the real case of a geomagnetic survey of eastern Ligurian Sea (Italy). The eastern Ligurian Sea has very intense short waves anomaly fields and a sea bed that varies greatly in depth. As a result the geomagnetic space signal is characterized by a very large spectral content; in these conditions it is not possible to obtain a full sampled marine survey and vertical continuation analytic procedures and standard numerical bottom reduction based on a single vertical incremental parameter, whichever is applicable, fails to give accurate results. The present technique, which has been fine-tuned over 4 years of experimentation in environmental researchs, aims to provide a simple and efficient means to reduce the distortion of geomagnetic anomalies field caused by the variation of distance between survey plane and magnetic outcrop source position. The compensation procedure is based on evaluation, by comparison of two measurements carried out at different altitudes, of the mean vertical increment typical of each anomaly field principal frequency component bands. The component anomaly fields are then corrected by application of the corresponding vertical increments and lastly, the anomaly geomagnetic field reduced to the sea-bed is computed as Inverse Fourier Transform of a spectrum built as synthesis of the component anomaly fields' spectra. The results obtained have shown a notable increase in definition of anomaly field intensity without the production of appreciable distortions or false geomagnetic echoes. This revised version was published online in November 2006 with corrections to the Cover Date.     

Quantitative evaluation of the time-line reduction performance in high definition marine magnetic surveys

The effectiveness of the time reduction procedure is the critical problem for the informative contents evaluation in the high definition (HD) geomagnetic marine surveys. The use of magnetogradiometers is a good solution for the time-reduction in the regional offshore studies but often, for technical reasons, this is not a practicable method in the very detailed coastal, canal or harbour surveys where the use of coastal base stations may be preferable. On the other hand the uncertainty of the full transferability of the coastal magnetograms to large areas of marine surveys can disrupt the time-reduced data sets by a geomagnetic residual time component. The phenomenon is related to the distance from the coastal observatory and to the homogeneity of the local magnetic characteristics of the crust. The maximum applicability distance of the time-line correction (TL) is qualitatively evaluated and is shown to be inconsistent with geomagnetic marine surveys in high definition. In this work we show a quantitative method to evaluate the stability of the coastal observatory magnetograms over the nearby marine area, together with the numerical degree of precision of the correction. The method is based on a double survey of the same profile (timer track: TT) at two different times. The surveys produce two different row data sets where the difference is related only to the geomagnetic time variations. Using the coastal observatory magnetograms we time-reduce the two data sets: if the coastal observatory magnetograms are fully coherent in the whole survey area the difference between the two reduced data sets will be zero. However, if the time variations measured in the observatory are inadequate in amplitude or phase to model the corresponding time variations in the surveyed profile (TT), the discrepancy between the time reduced data sets will not be zero. Similar TTs starting with various course from the base station permit the surveyed area to be split into sectors with variable degree of time coherence and to assign a degree of precision to the time-reduced survey.