Early Tools
Early tools had one Tx and one Rx (Fig. 16.4). The body of the tool was made from rubber (low velocity and high attenuation material) to stop waves travelling preferentially down the tool to the Rx. There were two main problems with this tool. (i) The measured travel time was always too long because the time taken for the elastic waves to pass through the mud was included in the measurement. The measured time was A+B+C rather than just B. (ii) The length of the formation through which the elastic wave traveled (B) was not constant because changes to the velocity of the wave depending upon the formation altered the critical refraction angle.
Dual Receiver Tools
