Horizontal Ocean-Bottom-Sensor sediment coupling; Estimation of coupling parameters from seismic data

The presence of a sensor-node in the seabed produces changes in the local wave field, usually referred to as wave field-distortion due to coupling. In challenging ocean bottom environments it is complicated to enhance coupling of the sensor nodes. But the interaction of Ocean-Bottom-Seismometer (OBS) or Ocean-Bottom-Cables (OBC) with the seabed can be estimated. The system response of the sensor-sediment interaction can be modeled as a mass-spring-dashpot transfer-function with two coupling parameters: resonance frequency and damping-factor. The transfer-function is related to the mass and size of the sensor-housing and the physical properties of the sediment. In order to be able to withstand the hydrostatic pressure at the seafloor, the OBS/OBC is a large and heavy system compared to the soft and water-saturated sediment. This can result in a system resonance which will be within the frequency-band of interest. In order to improve the system coupling it is necessary to estimate the coupling-parameters to shift the coupling resonance to a higher frequency and the damping to critical-damping. The reliable replication of seismic waves depends on the interaction of the Ocean-Bottom-Cable (OBC) with the seabed, regardless of the direction in which the wave travels. The interaction is called coupling and is typically better on the in-line sensor-component because of the surface enhancing effect of the cable. Inconsistent coupling of multi-component sensor-nodes causes distortions between the horizontal components and this makes the interpretation of converted wave difficult. Horizontal OBC data are often characterized as "ringy" and have different noise levels between inline and crossline. We will show that these characteristics are expected if coupling to the sediment is poor. Coupling and data quality are generally good for the inline component, except for a higher noise floor caused by cable noise. However, the crossline component often exhibits low-frequency resonance. Also, OBCs are susceptible to rotational modes about the cable axis that produce spurious 'S-waves' resonance on the vertical component. We will present a method to estimate the coupling parameters for both horizontal components independently by using a "feed-back transfer-function" method. The result can be used to optimize the sensor-housing design or to apply an inverse filter in order to extract the coupling transfer-function from the data. The presentation will show that inconsistent coupling of horizontal components can be estimated by using a data-driven approach. The presenting method estimates the two coupling parameter direct from the first arrival wave (first-break) without any affected earth-responses. Neither assumptions like perfect inline coupling have to be made nor will in-situ measurements such as internal shakers be necessary to estimate the coupling parameters.