Abstract
The diurnal variation of the magnetic field cannot be predicted or modeled and for that reason, it is monitored during the magnetic surveys, usually by a stationary magnetometer. However, marine surveys have a practical issue with diurnal monitoring, owing to the distance between the survey, stationary magnetometers, and magnetic observatories. This work aims to verify the use of nearby magnetic observatories to estimate the diurnal variation correction in different marine surveys and evaluate its effectiveness. In this study, we selected surveys at the continental shelf near Santos city (Survey 1), continental slope next to the first survey location (Survey 2), continental shelf near Ubatuba city (Survey 3), and Mamanguá ria in the Paraty city (Survey 4), all southeast to the Brazilian coast. The crossing points were implemented to compare the magnetic field values at different times and days at the same measurement point, before and after the correction. Afterwards, we measure the Pearson’s Correlation of the raw data and the diurnal corrected data in all crossing points of each survey which showed an improvement after correction by the value approximating to 1, which indicates a very well correlation. The Ubatuba and Mamanguá surveys allowed comparing the observatory correction results with the base magnetometer results that were rather similar. Our analyses indicate a satisfactory diurnal correction using the observatory data and the crossing points approach, which can be used for every marine magnetometric survey worldwide placed near the coast (< 280 km) that do not have a stationary magnetometer available.
Highlights
The Earth’s magnetic field is formed from the movement of convection currents of the conductive metal fluid in the outer core, which is made up of a Fe–Ni alloy and some lighter elements (Zhang et al 2016)
The data is acquired by a magnetometer in the survey area recording the magnetic field during the entire day
The diurnal variation values at VSS/RJ for the days of Surveys 3 and 4 assumes 23,280 nT as the mean value of the magnetic total field in the year of 2017 (Fig. 6). This value was considered as an average magnetic field, and it was removed from the others to obtain the diurnal anomaly of that day
Summary
The Earth’s magnetic field is formed from the movement of convection currents of the conductive metal fluid in the outer core, which is made up of a Fe–Ni alloy and some lighter elements (Zhang et al 2016). On the Earth’s surface, it resembles a dipole field, and it is continually modified by the external magnetic field that results from the interaction with the solar wind, forming the so-called magnetosphere (Lowrie 2007; Telford et al 1990). The data is acquired by a magnetometer (base station or base magnetometer called BASEMAG) in the survey area recording the magnetic field during the entire day. Another possibility is to reoccupy various stations during the day and use this duplicate data to correct the daily data. It is too expensive for the ship to redo the magnetometric acquired line. Leaving a base magnetometer in the middle of the ocean is not practical and the base station is only used when the survey area is close to the coast and it is possible to leave the base in a safe location avoiding noise and disturbances (Melo et al 2019)
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