1-D Modeling Of Low-resistivity Object Detection In Seabed Based On Electromagnetic Method

Seabed is a place rich in oil resources, biological resources and natural gas hydrateresources. There is an increasing demand for detecting the low-resistivity object in theseabed for ocean development and engineering, such as the survey and maintenanceof pipelines in seabed. One of low-resistivity object that threaten the underseaconstruction and human life, is called the Unexploded Ordnance(UXO). These UXOsneed to be detected and removed urgently. The traditional methodologies of detectingUXOs are magnetic and sonar methods. However these methods have somedisadvantages in locating small-size objects under the sediment. Because the UXO’sresistivity is much lower than the sediment of the seabed, marine controlled-sourceelectromagnetic survey(CSEM) is an alternated method for UXO detection, as theresistivity of the seabed could be obtained by observing the secondary fields excitedby the controlled source by use of CSEM.In this thesis, detection methods for UXO and metal objects buried in the sediment areevaluated based on the success of land and marine application of CSEM. Possibilityof low-resistivity objects detection by use of CSEM in frequency domain has beenverified through theoretical study and modeling evaluation. Theoretical foundationsfor designing and application new device for low-resistivity objects in seabed areprovided. The research works are concluded as following.(1) By evaluating present electromagnetic methods, the towed structure by combiningtransmitter and receivers are chosen for the study and1-dimension forwardformulas for horizontal and vertical electric dipole and data interpretation methodare formulated based on electromagnetic theory.(2) Formulas are used for modeling the electromagnetic responses with differentconfigurations. Simulation results of all configurations are compared so that theoptimal solutions are discussed. (3) Through this modeling study, solutions for improve detection ability andsuggestions for a towed system configuration and optimal design are given, whichcan be adopted for the design of practical device in the future.