New detection method for XBT depth error and relationship between the depth error and coefficients in the depth-time equation

New CTD-XBT (T-7 probe) comparison data are analyzed, which provide additional evidence of XBT depth error and support previous results (Hanawa and Yoritaka, 1987; Hanawa and Yoshikawa, 1991). The depth difference between the corrected and uncorrected data is about 26 m at 750 m. In the present study, new data processing procedures by which the depth errors are automatically detected, are developed and adopted. In the new method, first, temperature gradients (TG) of XBT and CTD profiles are calculated. Then, 20 m segment of the XBT-TG profile which should fit to the CTD-TG profile of 20 m segment to be referred to is searched in the XBT-TG profile. Actually, this is achieved by shifting the XBT-TG profile of 20 m segment so as to minimize the area surrounded by both TG profiles. The shifted depth of XBT-TG profile for CTD-TG profile can be regarded as the XBT depth error. This processing is repeated at intervals of 5 m from 10 m to 790 m of CTD-TG profile. The relationship between the scatter of the quadratic depth-time equation coefficients and the depth error is also discussed. It is shown that when the two coefficients have a certain relationship, the depth differences between the plural depth-time equations are small, even if the two coefficients of those equations have apparently very different values.