Research On Airwave Suppression Technology For Marine Electromagnetic Signal

The ocean not only has a wealth of marine oil、gas and mineral resources, butalso it is a natural place of research of geology. Our country is a maritime country,there are32,000km of coastline, and more than300million square kilometers Coastalwaters. In recent years, due to the huge shipping strategic position and economicvalue, every country pay much attention on ocean.Ocean exploration is an important means to obtain seabed resources. In recentyears, controlled source electromagnetic (Control-Source Electromagnetic referredCSEM) is widely used in marine exploration, this exploration methods has become ahot research for most countries. Experimental results show that marine CSEM methodof detecting geological structures have a good environment exploration results onlyfor the depths greater than500m, but in the case of shallow water, since the airwave’sinterference, it is difficult to detect the target (either low or high impedance targetgoal), but most of our country’s coastal waters have a depth of below500m, evensome place only have a few dozen meters, the CSEM method used in shallow watersalso need further research.In the shallow waters’ experiment, the electromagnetic wave generated by thehorizontal electric dipole excitation source, propagate on coastal water-air interface,so the signal which receiver collected contains both the seabed medium wave signaland airwave signal(air wave does not contain information from undersea formation).Due to the strength of the airwave is much greater than the strength of the usefulsignal, the presence of the airwave limit CSEM method of detection in shallow water.Therefore suppress or remove the airwave to promote the application of CSEMmethod has important significance in the shallow probing. So far, the approach tosuppress airwave have three ways: The first method is the airwave characteristic in time or frequency domain remove the airwave; The second method is the method ofmeasurement without airwave; The third method is inversion and interpretation of theraw data directly. Since every method have their own advantage, so this paper deeplyanalyzed and studied them, and analyzed instantaneous spectral analysis method forthe application in the marine electromagnetic signal in detail and then using theshort-time Fourier transform, Wigner-Ville distribution and Hilbert Huang transformtime-frequency analysis method for the analysis of the three method’s concentrationand accuracy, and the analysis results were compared. Comparative results show that:the degree of concentration can be seen from the analysis results, the three methods,the concentration degree of the Hilbert-Huang transform analysis is the best, W-Vmethod is the second, the last is the STFT; From the accuracy analysis results can beseen: the W-V deformation distribution method smoothed pseudo W-V distribution(SPWVD) is the best, and the accuracy of Hilbert-Huang transform and STFT areboth poor. Ultimately select the Wigner-Ville frequency transform analysis andsimulate the marine electromagnetic signals.According to marine electromagnetic signals in the air and underwater havedifferent propagation speeds, by pseudo W-V smooth distribution can be observedwhen there is no air to wave interference Underground electromagnetic signal period,so as to achieve the purpose of air-wave suppression.