This is error is found when two transducers are used one for transmission and other for reception.
The error can be determined by the formula
e = d – √(d2 – x2 / 4)
where, d – recorded depth and
x – spacing between two transducers.
The error is more dangerous in shallow depth and is negligible when one transducer is used because the same transducer is used for transmission and reception of the acoustic waves.