Broadband Frequency Response Assessment of Differential Pressure Gauge for Ocean Bottom Seismology Study

For earth quakes happening in the ocean, the signals will be best picked up by ocean bottom seismometers (OBS). The OBS signal consists of two components. The primary part which is of seismology interest is the ground motion driven by earth quakes or tremors. However the amplitude of low frequency seismic signal is also contaminated by the infra-gravity wave (the ocean surface waves induced beating) about the same order as that of the seismic activities. A remedy is to use a differential pressure gauge to measure the hydro-static pressure change in the environment such that the seismic component can be isolated from the combined signal. The schematic of a DPG is shown in Fig. 1. It consists of an oil-filled cylinder partitioned with a rigid plate. The plate is punched through with a syringe needle to allow the static pressure on both side of the partition be in equilibrium. One end of the cylinder is covered with a rubber diaphragm which directly transfers the pressure of the seawater onto the oil. The other end of the cylinder is covered with a rigid end-cap. Therefore, the pressure inside volume between the two rigid plates is not exactly the same as that in the seawater because the syringe needle limits the flow rate. The flow rate depends on the pressure difference, the viscosity of the oil, the inner diameter and the length of the channel. The whole configuration serves as a high-pass filter. Only the relative fast change of the environment pressure can be sensed by the differential gauge mounted on the partition plate. The reading of the sensor reflects the surface elevation with respect to a long term average level. In order to characterize the behavior or to calibrate the response of the DPG, we need to provide minute pressure change similar to that of the infra-gravity wave. The amplitude of the pressure change can be as small as mm water height. Several challenges need to be overcome for such a testing device: (1) the controlled water level moves up and down smoothly and exhibits the characteristics of a harmonic motion. In other words, only one frequency component should be excited in one test round. (2) The amplitude should be adjustable from a couple of centimeters to sub-mm to simulate the actual conditions in the field. In the experiments, we found the change of atmospheric pressure poses a slow AC bias on the simple harmonic signals we generate. To have a better testing condition, the whole calibration should be carried¹ out in an air-tight box to have results with fair quality. In this paper we report the details of the testing device design. The linearity of the amplitude response and the associated phase angles are obtained in the range from tens of seconds to hundreds of seconds. The information can be used to construct the transfer function of the DPG for post-processing of the OBS seismic signals such that the small amplitude events can be detected.