An acoustic reflection instrument detects background noise and reflections present in a well bore that extends into the earth. The instrument includes a shot break channel with a fixed gain amplifier, a collars channel with a variable gain amplifier and a liquid level channel with a variable gain amplifier. The outputs of all these amplifiers are input to an analog-to-digital converter which selectively provides digital output signals to a central processing unit which in turn processes the received data and prints it on a strip chart with two channels, one for principally showing reflections from collars and the other channel for principally showing the reflection from the liquid surface. Background noise is measured for a predetermined time to set a gain level for the amplitude of the noise to appear on the strip chart record. After the acoustic pulse is detected by the instrument, the gain of the collars channel is adjusted, first with fixed values and then followed by variable values based upon the peak value within a sample of preceding output data samples. However, the maximum gain of the collars channel amplifier is limited to the gain setting determined for measurement of the background noise. The amplification can also be performed in software. The output signal is printed on a strip chart.