Theoretical Calculation Of Airborne Transient Electromagnetic Field For Loop Source On Layered Earth

Time-domain airborne electromagnetic method (ATEM) used mainly helicopter-borne and fixed-wing detection in two ways. When exploration is implemented by adopting the fixed-wing airborne electromagnetic measurement system , the transmitting loop is affected by the aircraft-wingspan, so it's length is generally dozens of meters. The transmitting loop's length of the helicopter-borne airborne electromagnetic measurement system is generally between a few meters and dozens of meters. When the equivalent radius r of transmitting loop and the receiving coil's height h will not accord with the relation r/h<<1, the response of electromagnetic fields that is produced under transmitting loop impulse will not be equal to the magnetic dipole field approximately. Therefor, study of multilayered earth'time-domain airborne electromagnetic response of the loop source is necessary.Based on existing aerial and ground electromagnetic method , this dissertation is combined with the scientific research fact of the 863 planned major projects" Airborne geophysical prospecting techniques system"'s topic "Time domain helicopter-borne airborne electromagnetic survey theory research and system design ". The main research work and achievements are as follows:1. Study deeply the basic theory of electromagnetic fields and combine the characteristics of airborne electromagnetic method, refer to the expression of ground layered media on the surface magnetic dipole field which A.A.Kaufman derived and the electromagnetic response of ground rectangular loop source which M.Poddar calculated, frequency-domain airborne electromagnetic field's expression is derived for Loop Source on Multilayered.2. Field expressions contain double integral. There is inside integral the oscillatory Bessel function which can't be solved by the general method in the inside integral. D.guptasarma and B.Singh gived a digital filter method which is used to solve the inside integral problem, and the result was calculated precisely. The outer integral is solved by Simpson integral method whose calculation's accuracy is adjustable. The frequency-domain airborne electromagnetic response is converted to time-domain airborne electromagnetic response by Guptasarma arithmetic. Then, the arithmetic is proved to be feasible and correct by making contrastive research between electromagnetic response on the ground and airborne electromagnetic response using several groups of the earth model.3. Characteristics of the time-domain airborne electromagnetic response for loop source are studied. Under the same condition of the electromagnetic response is studied by using two-layer and three-layer typical earth model. Compare to ground electromagnetic response, the early of airborne electromagnetic response decays quickly relatively. The reason is that it can be considered that a layer with high resistance is added to the original model layered over the earth, whose thickness is the fly height, and resistivity is infinite. And the higher the fly height is, the more the early electromagnetic response is affected. Therefore, the fly height should be low as soon as possible .Late reduction regulation of airborne electromagnetic response is the same as the ground electromagnetic response, their curves are superposition ultimately.4. Electromagnetic response under the same earth model is studied when fly height is different. Results calculated show that the changes of fly height affect directly the electromagnetic response's value of points measured.5. Noise which results from dithering of helicopter can bring interference to normal measurement on airborne measurement.6. The relationship among transmitting current and size of transmitting loop and electromagnetic response is discussed. Under the same earth model, the amplitude of electromagnetic response increases linearly with current increasing. A contrastive research is made on electromagnetic response on the ground and airborne electromagnetic response with change of transmitting loop'size. The early change of the ground electromagnetic response is more complex. The electromagnetic response increases with increasing of transmitting loop'size and response attenuates more quickly. The electromagnetic response reduces with transmitting loop'size increasing and response attenuates slowly when loop'size increases at some value. Airborne electromagnetic response increases linearly with the increasing transmitting loop'size when transmitting loop'size is less than 100m. The early of electromagnetic response reduce hardly when transmitting loop'size is more than100m.7. Simple and easy to operate to ATEM for the loop source on layered earth graphical user interface is designed with the graphical user interface tool of MATLAB through the establishment of model, interface design and preparation of callback function.Theoretical calculation of airborne transient electromagnetic field for loop source on layered earth is completed in this paper. Characteristics research of the time-domain airborne electromagnetic response for loop source can provide theoretical basis for the field data's inversion and the design basis for time-domain airborne electromagnetic measurement system.