Magnetic Survey

A total of 17 magnetic profiles were acquired during the Voltaire cruise, comprising a total length of 1469 km for 162.2 acquisition hours. The acquisition was interrupted at the beginning of profile VOL19 due to cable problems that could not be solved in due time to allow the acquisition of the last profile, VOL20, that was also lost. Details on routines and scripts used to process the data are given in Appendix.

Normally the total field values were stored in the navigation files 1 point every 10 seconds. A few missing lectures were discarded during the decoding of these files (program nav03m.f).

The first processing step is to correct for the International Reference Field (IGRF). To perform this we used the program geonew.c and the file .igrf with the field parameters needed to correct for the year 2000. Information required is latitude, longitude, year, month and day. The program mag01.f prepared the input file for processing by geonew. Then the IGRF field and the anomalous field were computed and appended to the original .mag profile by program mag02.f For quality control we produced plots for the field recorded and the IGRF2000 correction using the script plota-brut.csh.

To perform the diurnal variation correction we used the daily observations at the Real Instituto y Observatorio de la Armada in San Fernando Cadiz, kindly provided trough mail by Dr. Jose Martin Davila, Jefe Seccion Geofisica/Head Geophysical Department. The Observatory coordinates are (WGS-84 GPS): 36º 29'50'' N, 006º 07'10'' W, H= 77.74m. These data was available for processing onboard on a weekly basis up to the 3rd December as a time series 1 point every 60 seconds. To obtain the diurnal variation at the ROA observatory we converted the time series to the magnetic profile format used in Voltaire with program roa01.f. Then we used the same procedure as before (mag01, geonew and mag02) to obtain the diurnal anomaly at ROA. Finally the onboard corrected anomaly was obtained by subtracting the ROA anomaly using program roa02.f.

To evaluate the contribution of the new magnetic data to the knowledge of the area we compared the anomaly obtained to the one predicted from the compilation published by Verhoef et al. (1996). For the profiles acquired latter than the 3rd December a simple shift was applied to make a better correspondence with the Verhoef et al. (1996) compilation. A synthesis of these data is shown in Fig. 12.

In many profiles the Voltaire data confirms the data already compiled and published. However, in some places, like South of the Guadalquivir Bank, at the top of the Gorringe bank and the magnetic quiet zone near the coast, the new profiles show a considerable difference and thus make a significant contribution to the knowledge of the crust magnetic susceptibility variation. It seems that the SW Iberia is bordered by a strong positive magnetic anomaly from South the Guadalquivir Bank to the eastern border of the Tagus Abyssal plain. This may correspond to a major crustal change from thinned continental crust to transitional crust. Voltaire results also show that the top of the Gorringe bank merits a high resolution magnetic compilation and that the weak anomalies near the coast must be better constrained.

figure

Figure 12: comparison between magnetic anomaly obtained during on board acquisition and Verhoef et al. (1996) compilation.