Acoustic logging while drilling tool to determine bed boundaries

Abstract

A LWD tool is disclosed for detecting the existence of and distance to adjacent bed boundaries. A transmitter assembly generates either a short acoustic pulse or a swept frequency signal that is detected by an associated receiver assembly. The received signal(s) are conditioned and converted to high precision digital signals by an A/D converter. The digitized signals are accumulated and transferred to a digital signal processor via a high speed data bus. The digital signal processor also receives a digital signal representing the transmission signal and compares the transmission signal and the received signals together to enable a downhole microprocessor to derive a time lag for the received signals. The microprocessor can transmit the time lag signal to the surface via a mud pulse for real-time control, or can operate as part of a closed loop drilling system to automatically control inclination of the drilling assembly to stay within, or to enter, a pay zone based upon the time lag associated with the received signals, and the measured speed of sound of the formation. In one embodiment, the receivers in the receiver array are steered to investigate various volume cells in the formation. A time delay is determined based on the volume cell, which is used to align and sum the received signals. The maximum sum value then is used as indicative of a bed boundary in that particular volume call.