Acoustical propagation characteristics of the communication channel of downhole drill string

Using acoustic waves to transmit downhole information through the drill-string transmission channel is a promising approach to meet the modern drilling requirements for the high-speed and large-volume transmission of data. Herein, we established a model to analyze acoustic transmission characteristics and examined acoustic characteristics of different drill-string assemblies by means of transmission matrix calculations. The results showed that a distinctive feature of the drill-string transmission channel lies in the existence of passbands and stopbands, in other words, the drill string, serving as a linear comb filter, is a narrow-frequency multi-band communication channel. The transmission performance of a drill string depends considerably on structure dimensions, physical parameters and signal frequencies. Along with the increase of signal frequency and transmission distance, the attenuation rate received increases obviously. In addition, the transmission performance is also co-influenced by the elastic modulus and density of drill-string materials. As for a periodic drill string, when the length of drill pipes increases, the passband center tends to shift to low frequency and both the passband and the stopband become narrower. When the sectional area of joints decreases, the passband widens while the stopband narrows, and the passband center within a band cycle tends to shift to frequencies of the band center. The closer to the band center the passband center is, the greater the offset is. As for a non-periodic drill string, the increase of differences in structure dimensions and physical parameters will change characteristics of the channel frequency domain of the drill string remarkably and reduce the acoustic transmission performance considerably. The signal to noise ratio is the key factor in designing an acoustic signal transmission system within the drill string. In order to realize the acoustic telemetry, the communication frequency of acoustic transmission should be chosen, to the greatest extent, within the low frequency range.