Multilevel method based on improved radar chart to evaluate acoustic frequency spectrum in periodic pipe structure

It is one of the most promising measures to transmit downhole information by acoustic wave through drill string However, the transmission distance and quality of acoustic signals in field conditions are still not satisfied because the acoustic frequency spectrum in drill string is vulnerable to interference. This paper studied acoustic frequency spectrum characteristics in periodic drill string by experiments under different axial tensile stresses. The multilevel method is established to evaluate frequency spectrum through drill string based on improved radar chart. The impact of axial tensile stress on spectrum characteristics is analyzed by the proposed method. The results indicate that the tensile stress has remarkable impact on the acoustic frequency spectrum and passband characteristics. The spectrum characteristics change with axial stress. These changes are mainly reflected in the position, width and attenuation of different passbands. And the extent of changes is distinct to different passbands. Both the evaluation indicators of each passband and spectrum are balance change with increasing axial tensile stress. The spectrum quality is excellent in the range of 17-134 MPa for axial tensile stress in drill string. And when the stress exceeds 152 MPa, methods such as adjusting the carrier frequency and relay installation location are necessary to improve the transmission quality of acoustic signals.