A value of a property of a fluid-filled wellbore system can be detected. This can be accomplished by recording data including at least one of pressure and rate of flow at one or more locations in the wellbore system, and then estimating the value of the property by employing a model for predicting at least one of pressure and rate of flow dependent upon characteristics and locations of disturbances to pressure and flow in the wellbore system, in order to determine a best prediction of some attribute of the recorded data.