The design and simulation of a broadband directional array in a cylindrical waveguide
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Exploration of deep subterranean wells is accomplished using what is known in the oil industry as a “drill string”. A drill string is a set of long hollow steel pipe joined together by large diameter couplings; with a drill bit attached to the end. Modern oil wells reach depths of two to five miles. For over 50 years, the scientific community has been trying to develop an economical and reliable solution to the problem of acoustic telemetry using the drill string as the information carrier. Acoustic telemetry is the process by which intelligence can be transmitted from the bottom of the drilling operations to the surface, using the drill string as an (acoustic) waveguide. The drill string geometry is responsible for the dispersive and filtering properties of the structure. These two properties, make the transmission of information from one end of the drill string to the other very challenging. Two aspects of the ongoing research is the development of a repeater and a termination impedance. An acoustic repeater could be used to rebroadcast signals that have attenuated along the drill string, due to inherent attenuating mechanisms in the data carrier, and its interactions with the harsh environment. Current repeaters lack directionality and transmit information in both directions; clearly an undesirable feature. The terminating impedance concept is an “acoustic black body”. The terminating impedance absorbs all of the incoming acoustic energy at the top of the drill string, once it has been analyzed for decision making. Cancelling the echoes from the top of the structure may allow faster data telemetry, as the reflected energy does not interfere with incoming information. The focus of this dissertation in on the design and simulation of both the repeater and the terminating impedance. This work is unique in that it addresses a method to rebroadcast and cancel broadband signals, using an active, feed-forward adaptive algorithm, coupled with a properly spaced and phased array of sources (piezoelectric transducers). Both the repeater and terminating impedance are capable of reproducing complex transient wave forms. The terminating impedance and repeater designed are directional, robust to frequency content, drill string length and geometry.