Disclosed is a method for characterising an object, comprising: emission of ultrasonic waves that enter an object by passing through an external interface, then reflect off an internal interface, then exit the object by passing through the external interface again, the ultrasonic waves comprising first waves having a mode that varies according to a first variation during their propagation in the object, and second waves having a mode that varies according to a second variation during their propagation in the object; reception of echo signals comprising first signals representing echoes of the first waves, and second signals representing echoes of the second waves, construction of a plurality of images showing the object, the plurality of images being constructed from the echo signals, under the assumption that two characteristics of the object are equal to two candidate values, and comprising a first image constructed from the first signals, and a second constructed from the second signals; construction of a composite image from the plurality of images; calculation of a focus metric in the composite image and, depending on the metric, selection or not of the two candidate values as values of the two characteristics. These characteristics can be used, in particular, to estimate a thickness of the object between the two interfaces.