Why bury ocean bottom seismometers?

Theory, testing, and use of ocean bottom seismic sensors show that the fidelity of data, particularly measurements of horizontal motion, can be severely distorted and noisy because of mechanical interactions between the sensor package, the ocean bottom, and the ocean water. This lack of fidelity is common, particularly for instruments with large cross sections in the water. These problems should be minimized by burial of the seismic package below the ocean‐sediment interface. To test this hypothesis, two identical seismic packages were operated on the shallow ocean floor within meters of each other with a real‐time connection to land. The packages were configured in a variety of ways, with both packages buried in the sediment, one buried and one on the bottom, and one on a circular 1‐m‐diameter plate and the other buried or on the ocean floor. The resulting data are compared with data from both packages resting on a stable cement slab in the frequency range from 0.2 Hz to above 20 Hz. Data show convincingly that burial of seismic sensors in soft sediment greatly increases data fidelity over sensors placed on the bottom or on a plate on the bottom. While vertical sensors provide reasonable fidelity in most situations, the horizontal data are consistently reliable only when the sensor packages are buried.