Short Note. Effects of the equatorial electrojet on aeromagnetic data acquisition

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1 GEOPHYSICS, VOL. 65, NO. 2 (MARCH-APRIL 2000); P , 5 FIGS. Short Note Effects of the equatorial electrojet on aeromagnetic data acquisition Augustinho Rigoti,Antonio L. Padilha,F.H.Chamalaun, and Nalin B. Trivedi INTRODUCTION In recent years, considerable advances have taken place in aeromagnetic surveying. These improvements involved data acquisition (instruments and survey design), processing, and interpretation. In addition to improved spatial resolution, the high-resolution aeromagnetics, as applied to oil exploration, attempts to resolve very low amplitude (1 nt or even subnanotesla) magnetic features (Paterson and Reeves, 1985). These features are caused by weak intra-sedimentary magnetic sources of magnetite and pyrrhotite, which could have been formed as a result of hydrocarbon seepage (e.g., Reynolds et al., 1990, 1991). For such small spatial variations to be meaningful, it is required that similar temporal and spatial variations due to external sources be corrected accordingly. As the objective of aeromagnetics is to map the static, time invariant magnetic responses of structures and lithologies, any other rapidly varying temporal or spatially varying magnetic field constitutes unwanted noise. Base station subtraction, tieline leveling, and microleveling have been proposed to correct for the time varying external magnetic signal. Microleveling (Minty, 1991) is simply numerical filtering, whereas the other two processes involve the assumption of a certain spatial uniformity of the magnetic variations (MV), which is only partially justifiable (Reeves, 1993). However, it has been regular practice to remove diurnal variation by using tie-line/flight-line leveling schemes and, even when a direct subtraction of a base station is made, a line leveling scheme is used to correct for remaining errors. It is well accepted that linear, or low order, variation in time along the line can be very adequately corrected for through various line leveling schemes (e.g., Yarger et al., 1978). In the equatorial region, there is a concern by some that standard line leveling procedures do not adequately correct for the strong nonuniformity of the equatorial electrojet (EEJ) magnetic field which originates from day-time ionospheric eastwest flowing electric currents. On the surface, its main effect is the amplification of the northward component of the MV within a zone of 5 on either side of the magnetic equator. Based on observations, the present exercise shows the nature of the magnetic variation that is not corrected by the subtraction of a single base station in a hypothetical aeromagnetic survey carried out at the equatorial zone. By subtracting a base station, we remove the large-scale global MV that is approximately uniform even at the equatorial region, and we are left essentially with the spatially nonuniform EEJ field that is not corrected for by the removal of the base station. Our intention is to separate as much as possible the effects of the EEJ field from the other rapidly varying magnetic fields. To accomplish that, a magnetically very quiet day with high amplitude EEJ and an early morning counter EEJ was chosen. The highamplitude EEJ was chosen to evaluate the effect of EEJ in an extreme situation. THE INHOMOGENEITY OF THE DIURNAL VARIATION The available data came from an array of 29 vector magnetometers operated in north-northeast Brazil from November 1990 until March The area extends 3 of latitude on either side of the dip equator, and covers 4 of longitude, centered at 314 E(Figure 1). The array data were used primarily to determine the characteristics of the EEJ and to derive the Earth s conductivity structure by means of geomagnetic induction (Rigoti, 1994). Geologically, a large part of the array is over the Parnaíba Basin, which is mainly Paleozoic although a large area is covered by a small thickness of Mesozoic sediments. The maximum thickness of sediments just exceeds 3000 m, of which the bottom 2500 m are Paleozoic. Manuscript received by the Editor August 20, 1997; revised manuscript received September 23, Universidade Federal do Paraná, Departamento de Geologia, C.P , Curitiba, PR, Brazil. rigoti@setuva.geologia.ufpr.br. Instituto Nacional de Pesquisas Espaciai, C.P. 515, São José dos Campos, SP, Brazil. padilha@dge.inpe.br; trivedi@dge.inpe.br. Flinders University of South Australia, School of Earth Sciences, G.P.O. Box 2100, Adelaide, South Australia, 5001, Australia. c 2000 Society of Exploration Geophysicists. All rights reserved. 553

2 554 Rigoti et al. The quiet-day diurnal variation, whose nonuniformity is analyzed here, comprises the global Sq (equivalent ionospheric current systems associated with the solar quiet geomagnetic daily variations) and the EEJ, but for the distances under consideration, the Sq can be considered uniform, so that the spatial variation is essentially due to the EEJ. The method of analysis consisted of: 1) Subtracting the midnight level from the MV records of each station. The midnight level was taken as the average value of one hour around the preceding midnight. 2) Choosing one station to be used as base, whose MV was subtracted from the magnetic variation recorded at the other stations. 3) Mapping the residual, which the single base station could not eliminate, due to the nonuniformity of the EEJ. FIG. 1. The study area with (a) the array location and (b) detail of sites on the geological background. The quiet day of January 7, 1991 was chosen for the demonstration of the EEJ effect; station RIA (see Figure 1) was taken as the base station. Figure 2 shows examples of the profiles recorded at the base station in comparison to a close station (BAL) and a more distant station (ARA). The first presents a smaller residual MV, because BAL is at approximately the same latitude as RIA. In this case, most of the residual MV comes from the X (north-south magnetic component) contribution to the total magnetic intensity F. The second, representing a larger difference in latitude (ARA is close to the magnetic equator), presents a larger residual, originating from a combined contribution of the three components to F. It is interesting to note that in comparison to the large residuals in each of the components X and Z (vertical), the residual in F is not so pronounced. As seen in the magnetograms, this happens because the excess in Z observed at RIA is to some extent balanced by the excess in X at ARA, as F 2 = X 2 + Y 2 + Z 2. This observation suggests that the effect of the inhomogeneity of the EEJ on the total field recorded in the aeromagnetic surveys will not be as critical as might be assumed if one considered only the variation of the single X or Z component. However, the residual F shown in Figures 2 presents amplitudes far too large to be neglected. Examples of the spatial distribution of the residual F at different times during the quiet day of January 7, 1991 are shown in Figure 3. The part of the array represented in these maps can be seen in Figure 4. The morphology of the residual F undergoes continuous change, and the amplitude of the variation closely follows the daily variation, peaking at about noon. From 10:00 hours until about 15:30 hours local time, the part of the magnetic field for which a single base station does not correct exceeds 30 nt. The contours in Figure 3, especially around 12:00 hours, show that an induction anomaly in the southeastern part of the array (discussed in Rigoti, 1994) also produces a local enhancement of the residual F. Plots of residual F for slightly more disturbed days showed higher values of residual F associated with the geomagnetic induction anomaly around the center of the Parnaíba Basin. However, the demonstration here is restricted to what would happen on a very quiet day, in which there would certainly be no reason for the data acquisition to be halted. In practice, to prevent the contamination of the aeromagnetic data by magnetic storms, the data acquisition is halted when a sequence of three or more 10-nT nonlinear events are

3 EEJ Effects on Aeromagnetics 555 recorded in 10 minutes. These are short duration events when compared to the EEJ variations. FLIGHT SIMULATION The spatial distribution of the part of the MV for which a single base station does not correct (Figure 3) varies with time. A more realistic picture of the residual MV is thus obtained along imaginary flight lines. Figure 4 shows the two lines (L1 and L2) along which the imaginary flights are conducted. Stations were projected onto this line with distances in kilometers. The speed of the aircraft was taken as 250 km/hour, and the height was taken as the ground surface level. FIG. 2. Residual MV observed for (a) small and (b) large differences in latitude between a given station and the base.

4 556 Rigoti et al. Figure 5 shows the profiles of the residual MV for each station L1 (upper left) and L2 (upper right) as a function of time. The vertical dashed lines on each profile mark the instant and value of the residual MV intercepted by the aircraft which flies from BAL to STE in flight L1 at 10:00 hours (lower left of the figure) and from STE to SJP in flight L2 at 11:00 hours (lower right of the figure). Profiles of residual F (in nanotesla) obtained from the imaginary flights are shown at the bottom of Figure 5, plotted as a function of distance (in kilometers). Profile L2 shows that the FIG. 3. Instant shots of the residual F magnetic variation on January 7, The contour interval is 2 nt.

5 EEJ Effects on Aeromagnetics 557 are under the influence of the EEJ, the assessment of the impact of such concentrated ionospheric currents on the aeromagnetic measurements is of special interest. The mapping of the EEJ by an array of magnetometers (Rigoti, 1994) allowed us to estimate its impact on aeromagnetics. The example of uncorrected variation after subtraction of a base station shows the variation of 70 nt over the distance of 250 km, or 0.3 nt/km. Standard tie-line leveling packages are able to remove spatial and temporal variations such as that observed in this experiment. The long-wavelength diurnal variations as estimated and removed using industry-standard flight-line/tie-line intersection missmatches can have gradients as large as 1 nt/km. This experiment therefore concludes that the effect of the EEJ is not detrimental to standard aeromagnetic surveying. ACKNOWLEDGMENTS FIG. 4. Flight lines L1 and L2. residual MV can be as large as 70 nt over the distance of 250 km. Several different analyses were conducted by changing the position of the base station and direction of the flights. These variations caused the maxima and minima of the residual F to move around, modifying the geometry of the profiles, but the maximum gradient did not exceed 0.3 nt/km. Considering that standard available tie-line/flight-line leveling schemes are able to correct for gradients up to 1 nt/km, it seems that the wavelength spatial variation and the long-period temporal variation of the EEJ do not constitute a problem for the aeromagnetic data acquisition. CONCLUSIONS The reliability of the interpretation made on the highresolution aeromagnetic data depends on the accuracy with which the variation of the external field is removed from the recorded total magnetic intensity. As large areas in the world The field operation of the array was initiated under the FAPESP project number 90/1024-7, and the present work is continued as part of CNPq (proc /94-9) and FAPESP (proc. 98/ ) funded projects. Both FAPESP and CNPq are thanked for the financial support. One of the authors (A.R.) is also thankful to the CNPq for the award of a Ph.D. scholarship. The comments and suggestions of H. L. Yarger, R. Smith, and F. de Ridder are gratefully acknowledged. REFERENCES Minty, B. R. S., 1991, Simple micro-levelling for aeromagnetic data: Explor. Geophys., 22, Paterson, N. R., and Reeves, C. V., 1985, Applications of gravity and magnetic surveys: The state-of-the-art in 1985: Geophysics, 50, Reeves, C. V., 1993, Limitations imposed by geomagnetic variations on high quality aeromagnetic surveys: Explor. Geophys., 24, Reynolds, R. L., Fishman, N. S., and Hudson, M. R., 1991, Sources of aeromagnetic anomalies over Cement oil field (Oklahoma), Simpson oil field (Alaska), and the Wyoming-Idaho-Utah thrust belt: Geophysics, 56, Reynolds, R. L., Webring, M., Grauch, V. J. S., and Tuttle, M., 1990, Magnetic forward models of Cement oil field, Oklahoma, based on magnetic, geochemical, and petrologic constraints: Geophysics, 55, Rigoti, A., 1994, Geomagnetic array study of the EEJ in NE Brazil: Ph.D. thesis, Flinders Univ. of South Australia. Yarger, H. L., Robertson, R. R., and Wentland, R. L., 1978, Diurnal drift removal from aeromagnetic data using least-squares: Geophysics, 43,

6 558 Rigoti et al. FIG. 5. Temporal variation of residual F and resultant profiles for the flights L1 and L2. See text for description.

Geomagnetic Field Variations from some Equatorial Electrojet Stations

Geomagnetic Field Variations from some Equatorial Electrojet Stations I.A. Adimula 1, A.B. Rabiu 2, Y. Yumoto 3, the MAGDAS Group 3 1 Department of Physics, University of Ilorin, Ilorin, Nigeria 2 Department