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Numerical Simulation, Attitude Effect And Correction Of3-D Frequency Domain Helicopter-borne Electromagnetic Method

Posted on:2015-01-21Degree:DoctorType:Dissertation
Country:ChinaCandidate:X X QuFull Text:PDF
GTID:1260330428484014Subject:Earth Exploration and Information Technology
Abstract/Summary:PDF Full Text Request
Airborne electromagnetic method (AEM), as a geophysical exploration method ofinformative, high efficiency and low cost, after recent decades of development, has become acommon exploration method for the census prospecting, groundwater exploration,geological mapping, environmental&engineering, etc.. During the process ofhelicopter-borne electromagnetic survey, the bird’s attitude change can introduce attitudeerror into airborne electromagnetic data.In airborne electromagnetic simulation method, there are mainly including finitedifference method, finite element method and integral equation method. In this paper, wededuced the vector Helmholtz equation for the second order electric field whosediscretization form is achieved using the staggered grid finite difference method. The linearequations are solved using the QMR algorithm in order to get the valves of second electricfield at the grid points and receivers. The magnetic field is calculated by first taking anumerical approximation of the curl of the electric field on the grid surrounding the receiverand then interpolating the result to the point of interest.Most frequency-domain helicopter-borne electromagnetic (HEM) systems currentlycomprise horizontal coplanar (HCP), vertical coaxial (VCX) and vertical coplanar (VCP) coilarrays. Among them the HCP and VCX coil arrays are often used for geologic mapping,and VCP coil array has remarkable advantage in the application of electrical anisotropymedium. In this paper, we use a series of typical3-D models (inc. horizontal layered medium,the vertical plate body, inclined plate body and combined column body models) to calculatethe airborne electromagnetic response of different coils (inc. HCP, VCP and VCX) underdifferent resistivity profile, and analyze their basic characteristics.For horizontal layered medium model, anomalous curves of the real and imaginary partsof the second magnetic field are all nearly horizontal in different transmitting frequency, allfor these three different coils; the abnormal values of imaginary part are higher than those ofreal part respectively, and the abnormal values of the real and imaginary parts increase withthe increase of transmitter frequency; the abnormal curves and numerical value of VCP andVCX coil systems are almost the same. These characteristics reflect the underground mediumis infinite in x and y direction (i.e. horizontal direction). For vertical plate model, the anomaly profile curves of magnetic scattering field of these three coils is symmetrical about the originin all frequencies, and the abnormal values of the real and imaginary parts increase with theincrease of transmitter frequency. For inclined plate model, the crest (or trough) position ofthe magnetic scattering field anomaly profile curve is obviously to move to the left comparedto the vertical plate model, both for the real part and for the imaginary part, in addition, thevalue of crest increases slightly, but the value of trough decreases slightly. The tendency ofinclined plate can be declared with these tips. For a two cube combination model, these threecoil systems have poor capacities of recognizing the two cube-shaped bodies in low ormedium frequency, and barely identify the low resistivity cube on the left. The ability of HCPand VCP coil systems to recognize the high resistivity cube on the right is weak, but strongability for the low resistivity cube on the left, and VCX coil system is able to recognizesimultaneously both the low resistivity cube on the left and the high resistivity one on theright with high frequency.The analysis of helicopter-borne electromagnetic survey data commonly assumes thatthe bird has flown straight and levelly. In actual fact, the bird exhibits some attitude changessuch as roll, pitch and yaw rotation. The roll is caused by the swing of the cable whichconnecting the bird and helicopter; the pitch is caused by the flight speed change; the yaw iscaused by the external lateral wind which forced on the bird. When the bird’s attitude changes,the direction and position of the coil system will alter. The position change mainly affects therelative position of the transmiting and receiving coil, and the direction change mainly affectsthe size of the magnetic dipole moment. In this paper, we deduced the new positioncoordinates of the transmiting and receiving coils, and the expression of magnetic dipolemoment, when their attitude changed. Then, we used three typical models (vertical platemodel, inclined plate model and combined cube model) to calculate the electromagneticresponse of different coils with different attitude changes, and compared with theelectromagnetic response which attitude is unchanged.For different models, while HCP and VCX coils changing in roll or pitch, abnormalcurve is affected in high frequency only, while most anomaly curves in low frequency areonly trace offset in addition to individual offset a relatively lot; But for the VCP coil system,only the anomaly curves in high frequency are affected, ones in low or medium frequencyhas no effect almost; and the VCP coil system in any frequency are not sensitive to pitchchanges.The attitude correction method using a superposed dipole (SD) model only considers thechange of direction but ignore the position variation of the coil systems. In fact, when the angle variety is wider, the influence of the position change of electromagnetic dipole to theelectromagnetic response cannot be easily ignored, thus the approximation using thesuperposed dipole method is bound to have an influence on the correction effect. Based onthe superposed dipole correction method, this paper proposed a new attitude correctionmethod–direction-position (DP) correction method. This method considered the effect ofdirection and position changes on the electromagnetic response of the coil systems. The stepsof DP method are–(1) The uniformity half space models of five different electricalparameters are introduced firstly, and then the ratios, which are the electromagnetic responsebefore to after the attitude change, of the coil systems of these models are calculated;(2) Thetotal attitude changes is divided into two independent process-direction change and positionchange, and the electromagnetic response radios of the coil in a separate direction or positionchange are calculated;(3) Deduced an approximately constant relationship between theelectromagnetic response ratio of the total attitude effect and the product of theelectromagnetic response ratios when the direction effect and position changes separately;(4)Finally, derived the expression of total correction factor.For the rolling HCP and VCP coil systems, the radio of their electromagnetic responseratio of total attitude change to the product of the electromagnetic response ratios when asingle direction and single position changes occurred is roughly equivalent to the secantfunction of the roll angle, also for the pitching HCP and VCX coil systems. However, for therolling VCX and pitching VCP coil systems, the ratio is equal to1. The total attitude changeis associated with both the direction and position changes, total correction factor can bewritten as the product of the direction, position correction factors and correction coefficient.Direction correction factor can be obtained by analyzing the relationship between differentcoil systems, and the position correction factor can be obtained using the curvet fittingmethod.The correction results shows that, no matter which kind of model parameter, coil systemand attitude change chosen in homogeneous half space model, using the DP correctionmethod can get better correction result than using the SD method. The correction results ofthe vertical plate model shows that the DP correction method can get better correction resultthan the SD method for roll change, but the correction results using the two methods aresimilar to each other. The correction result of the inclined plate model is similar to that ofvertical plate model, which suggests that both the SD and DP correction methods were notsensitive to the tendency of plate underground. The correction results of roll change for thecombined cube model show that DP method is much better than SD method for the problems such as this; the SD and DP methods can obtain good correction results for pitch change, andboth of them can play a good role in their specific frequency.
Keywords/Search Tags:Helicopter EM, Staggered grid finite difference method, Numerical simulation, Attitude effect, Attitude correction, Direction-position correction method
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