Leveling HEM and aeromagnetic data using differential polynomial fitting Majid Beiki 1 , Mehrdad Bastani 1 , and Laust B. Pedersen 1 ABSTRACT We introduce a new technique to level aerogeophysical data. Our approach is applicable to flight-line data without any need for tie-line measurements. The technique is based on polynomial fitting of data points in 1D and 2D sliding win- dows. A polynomial is fitted to data points in a 2D circular window that contains at least three flight lines. Then the same procedure is done inside a 1D window placed at the center of the 2D window. The leveling error is the difference between 1D and 2D polynomial fitted data at the center of the win- dows. To demonstrate the reliability of the method, it was tested on a synthetic aeromagnetic data set contaminated by some linear artifacts. Using the differential polynomial fit- ting method, we can remove the linear artifacts from the data. The method then was applied to two real airborne data sets collected in Iran. The leveling errors are removed effectively from the aeromagnetic data using the differential polynomial fitting. In the case of helicopter-towed electromagnetic HEM data, the polynomial fitting method is used to level the measured real in-phase and imaginary quadrature components, as well as the calculated apparent resistivity. The HEM data are sensitive to height variations, so we intro- duce an average-height scaling method to reduce the height effect before leveling in-phase and quadrature components. The method also is effective in recovering some of the attenu- ated anomalies. After scaling, the differential polynomial fit- ting method was applied to the data and effectively removed the remaining line-to-line artifacts. INTRODUCTION Aerogeophysical data often suffer from inconsistencies between adjacent lines called leveling errors. Sources of leveling errors are different in aeromagnetic and airborne electromagnetic AEM data. With increasing altitude, the amplitude of helicopter-towed electro- magnetic HEM responses decreases. The HEM systems work with active sensors. Pairs of transmitter and receiver coils with different frequencies are installed in a bird. A primary magnetic field is trans- mitted, and primary and secondary magnetic fields are recorded by the receiver coils. Inside the sensor, a bucking coil is installed to buck out the primary field. These coils have two configurations, co- axial and coplanar. The change in the response to different frequen- cies provides information on conductivity corresponding to differ- ent skin depths. Each configuration is sensitive to geometrical varia- tions in ground conductivity. For example, in the case of coplanar coils directly over a vertical thin conductor, i.e., a vertical thin dike, the primary magnetic field is vertical. It passes parallel to the vertical conductor and has minimum coupling. But for coaxial coils, the gen- erated magnetic field is directed horizontally below the transmitter coil, so the induced eddy currents are maximum in the vertical con- ductor. The primary field in the coplanar configuration generates maximum coupling to horizontal layers. Valleau 2000 provides an excellent overview on processing and interpretation of HEM data. The dependency of the recorded secondary field on the resistivity of the subsurface and altitude variations is strongly nonlinear Siemon, 2009. Figure 1a-c shows a real example of recorded altitude, in- phase, quadrature, and aeromagnetic data, respectively. The 900-Hz HEM data shown in Figure 1b were collected using the DIGHEM system in Bazman, Iran. This example illustrates clearly that HEM data are correlated with altitude variations. However this relation- ship is not so strong for aeromagnetic data. Consequently, Huang 2008 categorizes HEM data as altitude sensitive and aeromagnetic data as altitude insensitive. Temperature variations are another source of HEM leveling errors Huang and Fraser, 1999; Siemon, 2009 affecting the secondary field measurements. Temperature variations can change coil separa- tion and also can influence electronic systems. For aeromagnetic measurements, some tie-lines are flown per- pendicular to flight lines. Tie-line spacing is normally 3–10 times the flight-line spacing. The recorded magnetic field on tie-lines and flight lines differ by the so-called mis-ties. It is safe to assume that Manuscript received by the Editor 6 March 2009; revised manuscript received 14 August 2009; published online 27 January 2010. 1 Uppsala University, Department of Earth Sciences, Geophysics, Uppsala, Sweden. E-mail: Majid.Beiki@geo.uu.se; Mehrdad.Bastani@geo.uu.se; Laust.Pedersen@geo.uu.se. © 2010 Society of Exploration Geophysicists. All rights reserved. GEOPHYSICS, VOL. 75, NO. 1 JANUARY-FEBRUARY 2010; P. L13–L23, 14 FIGS., 1 TABLE. 10.1190/1.3279792 L13 Downloaded 28 Jan 2010 to 130.238.140.35. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/