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.
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.
Because of its location, the magnitude 5.2 Green Canyon earthquake of 10 February, 2006 has been of considerable interest to the oil industry. Unfortunately, because the nearby stations were all located to the North (onshore), its location and depth were only poorly constrained by the traditional worldwide seismic network. Fortunately, the Atlantis 3D survey happened to be ongoing at the time of the earthquake and recorded it on over 500 4C ocean-bottom nodes. The Atlantis array was closer to the event than any station on shore, and recorded the event from the South. These data thus provide an opportunity to dramatically refine our knowledge of the earthquake's location. We preserved 2.5 hours of data spanning the time of the earthquake and analyzed it for signals, both man-made and natural. Although there are a large number of overlapping signals in the data, they can be clearly separated by their distinct arrival times, temporal frequencies, and phase velocities across the 10km x 6km array. At frequencies of 5Hz and above repetitive air-gun signals from 4 ongoing seismic surveys dominate the data. These account for all the surveys shooting in the Northern Gulf of Mexico at the time. At frequencies of 1Hz and below there is a nearly continuous background noise coming primarily from the South and East that rumbles along for the entire 2.5 hours, with a phase velocity across the array of about 2000 m/s. This is most likely ocean wave-generated noise. At frequencies below 2Hz the earthquake signal dominates the data from 04:14:34 UT to about 04:20. It begins with several discrete arrivals from the NNW over a span of about 20 seconds, which are then followed by a drawn-out coda from the NW lasting over 8 minutes. At 04:22:13 a second event arrives from the SE, which is in turn followed by its own coda from the SSE, which lasts about 3 minutes. (It's not yet clear what this second event might be.) At about 04:27 a 3rd arrival, a weak, diffuse event from the ESE, fades in and lasts about 3 minutes. These data have been provided to earthquake experts and we expect to have a revised location for the event by the time of the OTC meeting in May. Preliminary indications are that the earthquake was well to the North of the originally published position.
Making use of the Atlantis OBS-node array The geometry of the active nodes at the time of the earthquake is shown in Figure 1. We have a large, dense array (at least at low frequencies!) in this dataset, which allowed us to recycle a program called "Radar" for analyzing the signals in the data. "Radar" was originally written for examining surface-wave noise in land seismic data. It performs planewave stacks across the array and produces a graphical result showing the time, phase velocity, and azimuth of arriving energy. Figure 2 shows that "Radar" was able to correctly determine the azimuths of all the active air-gun surveys in the Northern Gulf of Mexico shooting at the time of the dataset.