Study of internal solitary wave packets in the northeastern South China Sea based on seismic oceanography and remote sensing

Numerous Internal Solitary Waves (ISWs) have been observed and reported around the Dongsha Atoll in the northeastern South China Sea. The propagation and evolution processes of ISWs in this area remain debated. Here, a marine seismic exploration line carried out near the Dongsha Atoll in July 2009 is reprocessed by an advanced method of seismic oceanography. After seismic data processing, an internal solitary wave packet containing 8 well-shaped first-mode depression ISWs is identified in the water below depth 50 m. On the same day of seismic exploration, the Moderate-resolution Imaging Spectroradiometer (MODIS) image captured the same ISW packet. After processing of this MODIS image, the first 5 of the ISWs packet were clearly identified. Two methods are used to calculate ISWs phase speeds. According to the first method, the ISWs apparent phase speeds are estimated by pre-stack migration profiles using the Common Offset Gather (COG) , then the apparent phase speeds are adjusted in terms of ISWs propagation direction on the MODIS image. While the second method can directly obtain the first 5 ISWs phase speeds by locations and times revealed by seismic data and the MODIS image. The results of these two methods consistent well accordingly. Three waveform characteristic parameters, i. e. amplitude, apparent full width of the trough at half amplitude and apparent wave length, can be directly revealed by the seismic oceanography image. The internal solitary wave packet maximum amplitude is 117 m, maximum apparent full width of the trough at half amplitude is 1020 m, and maximum apparent wave interval is 4100 m. Because of the Doppler-like effect and the angle between ISWs propagation and direction of ship travel, horizontal information provided by the MODIS image and phase speeds calculated by the second method should be considered to correct the last two parameters. The ratio of the first ISW wave maximum full width of the trough at half amplitude to the full width half maximum (estimated by remote sensing) is 1. 75, which is close to the theoretical ratio 1. 763; the true wave intervals are consistent with the wave intervals estimated by the MODIS image. Finally, MODIS images and tide data gathered before and after seismic observation permit to infer the ISW pocket is Type-b. The results show that the combination of seismic oceanography and remote sensing can help better characterize ISWs.