| Airborne electromagnetic method has a good effect in detecting conductive bodies in high-resistance environment,but it is very difficult to find low-resistance anomalous body in ocean exploration because of the low resistance shielding effect of seawater.However,for conductive bodies made of ferromagnetic conducting minerals or ferromagnetic materials,the electromagnetic response will increase with the increase of permeability,which provides favorable conditions for the detection of magnetic and conductive body in seawater by AEM.In order to discuss the feasibility of using timedomain AEM to detect magnetic and conductive body in seawater,this paper carried out airborne electromagnetic response characteristics study of magnetic and conductive body in seawater,and analyzed related factors affecting the response amplitude of magnetic and conductive body in seawater.Firstly,based on the vector finite element algorithm,the three-dimensional forward calculation of time domain AEM was realized,and the convolution algorithm was used to calculate the electromagnetic response of various waveforms;Secondly,a physical model of time-domain airborne electromagnetic detection of magnetic and conductive body in seawater environment was established,and the electromagnetic response of conductive bodies with different magnetic parameters was calculated,and the airborne electromagnetic response law of magnetic and conductive body in seawater was obtained.Thirdly,the effect of launch waveform,launch time,launch magnetic moment and flight height on the detection effect of magnetic and conductive body was analyzed.Based on the actual data of the coastal waters of the East China Sea,the effect of the thickness of sea water layer,sea water temperature and sea water salinity on the detection effect of magnetic and conductive body was studied.Then the size of magnetic anomalies in seawater was changed,and the influence of the size change of magnetic and conductive body on the detection effect was explored.At last,the apparent resistivity contour map of different magnetic and conductive body in seawater was drawn by using the table lookup method,and the resistivity imaging characteristics of different magnetic and conductive body were obtained.Through the above research,the following conclusions are drawn:(1)The increase of the abnormal body's magnetism will increase the total magnetic field strength of the response.(2)When the permeability of conductive body in seawater increases,the multichannel curve shows that in the middle period of measurement after the current is turned off,the multi-channel curve shows an "M" shape distribution,and in the later period the curve shows an inverted "V" shape distribution,indicating that the increase of the magnetism of the abnormal body will increase the response delay and slow the response attenuation.(3)The use of square wave power supply,increasing the magnetic moment of transmission,reducing the height of the transmission,and increasing the pulse width of the transmission waveform will enhance the response of the conductive body,which is conducive to the identification of the magnetic target in seawater.(4)Decrease of the thickness of seawater and increase of seawater resistivity will weaken the low-resistance shielding effect of seawater,thereby improving the detection effect of magnetic and conductive body in seawater.(5)The apparent resistivity profile shows that when the magnetic properties of the conductive body are enhanced,there will be extremely low resistivity areas in the left and right sides and directly below the position of the abnormal body.The imaging characteristics of the abnormal body are conducive to the identification of magnetic and conductive body in seawater.The results of this paper can provide some reference for the setting of airborne electromagnetic working parameters and the identification of magnetic and conductive body in seawater,which has a practical significance. |
PDF Full Text Request