Short Note Hydrophone Calibration at Very Low Frequencies

Abstract : We propose a new method to study the response of a hydrophone at very low frequencies. In our method, the hydrophone is placed in a calibration chamber filled with water and, by instantaneously changing the water height, an abrupt pressure increase of about 1000 Pa is produced. The pressure variation mathematically corresponds to an input signal close to a step function. The response is recorded after filtering and digitizing so that we obtain the response of the complete system. We also report on the development of an automatic method to determine the number of poles and zeros and their values that describe the observed response. We apply our method to the RAFOS II hydrophone, used on the Mobile Earthquake Recorder in Marine Areas by Independent Divers (MERMAID) floats. As an illustration of the method, an instrumental response in terms of poles and zeros is used to correct seis-mograms from the 7 April 2014 (M w 4.8) Barcelonnette earthquake, recorded by three MERMAIDs deployed in the Mediterranean Sea, and to express the observed signals in pascals.