Low Frequencies With a "dense" OBS Array:The Atlantis Green-Canyon Earthquake Dataset

Dellinger, Joe (BP Advanced Imaging Team) | Ehlers, Jerry (BP Advanced Imaging Team)

OnePetro 

SUMMARY

We extracted 2.5 hours of continuous data from the Atlantis OBSnode array around the time of the 10 February, 2006 magnitude-5.2 Green-Canyon earthquake, for the purpose of characterizing the location, mechanism, and signi_cance of this unusual event. The array, consisting of about 500 active 4C nodes, was several tens of kilometers to the South of the earthquake. Typical earthquakes radiate the bulk of their energy at frequencies much lower than the 10Hz geophones used in the nodes were designed to record, so the dataset also served as a testbed for understanding how standard exploration-seismic geophones might be used at frequencies below 5Hz. At traditional frequencies of 10Hz and above the Atlantis airgun signal completely dominates the data. However, _ltering away frequencies above 2Hz removes the Atlantis airgun signal and reveals a strong series of arrivals from the earthquake. At .earthquake. frequencies of 2Hz and below, the Atlantis array becomes densely sampled in space (i.e., with more than 2 receivers per wavelength), allowing it to be formed into a powerful directional antenna. Beam steering the array reveals many interesting signals in the data. Examining our own airguns, we _nd that .inconsequential. differences in our airgun arrays had unexpected and signi_cant effects. Beam steering resolves a complex sequence of distinct arrivals from the Green-Canyon earthquake spanning 8 minutes of time, and two other mysterious events subsequent to the Green-Canyon earthquake that appear to be temporally related to it, but cannot be traditional aftershocks because they arrive from different azimuths.

INTRODUCTION

The Atlantis survey was a large-scale deep-water 4-component oceanbottom- seismic node survey recorded over the Atlantis _eld in the Green-Canyon block of the U.S. Gulf of Mexico (Clarke et al., 2006; Ross and Beaudoin, 2006). The survey was designed for imaging below salt, with an inter-node spacing of approximately 400 meters. Figure 1 shows the layout of the active array at the time of the earthquake. The array was designed with a node spacing of 400 meters, suitable for recording near-vertical waves propagating upwards from deep re_ectors. At typical seismic frequencies, horizontally propagating waves would be badly aliased at this node spacing. At low frequencies, however, the situation changes. At 1 Hz, a wave propagating with a phase velocity of 1500 m/s has a Nyquist sample interval of 750 meters, and the array becomes densely sampled. The dense sampling allows us to beam form the array into a steerable antenna that can be scanned over phase velocity and azimuth. We do this using the program .Radar., which was originally developed for characterizing surface-wave noise in land data (Regone, 1997). Low frequencies are a topic of considerable current interest (Dragoset and Gabitzsch, 2006). Examining this dataset at low frequencies reveals several interesting signals, both manmade and natural, that would be obliterated by standard processing. We examine two of these here: the signal from our own port and starboard airguns, and the _rst 30 seconds of arrivals from the Green-Canyon earthquake.