Linfei Wang

Retrieving drill bit seismic signals using surface seismometers

Seismic while drilling (SWD) is an emerging borehole seismic imaging technique that uses the downhole drill-bit vibrations as seismic source. Without interrupting drilling, SWD technique can make near-real-time images of the rock formations ahead of the bit and optimize drilling operation, with reduction of costs and the risk of drilling. However, the signal to noise ratio (SNR) of surface SWD-data is severely low for the surface acquisition of SWD data. Here, we propose a new method to retrieve the drill-bit signal from the surface data recorded by an array of broadband seismometers. Taking advantages of wavefield analysis, different types of noises are identified and removed from the surface SWD-data, resulting in the significant improvement of SNR. We also optimally synthesize seismic response of the bit source, using a statistical cross-coherence analysis to further improve the SNR and retrieve both the drill-bit direct arrivals and reflections which are then used to establish a reverse vertical seismic profile (RVSP) data set for the continuous drilling depth. The subsurface images derived from these data compare well with the corresponding images of the three-dimension surface seismic survey cross the well.

Shallow water body data processing based on the seismic oceanography

Physical properties of sea water, such as salinity, temperature, density and acoustic velocity, could be demarcated through degradation of energy caused by water absorption, attenuation and other factors. To overcome the challenging difficulties in the quick monitoring of these physical properties, we have explored the high resolution marine seismic survey to instantly characterize them. Based on the unique wavefield propagating in the sea water, we have developed a new approach to suppress the noise caused by the shallow sea water disturbance and obtain useful information for estimating the sea water structure. This approach improves seismic data with high signal-to-noise ratio and resolution. The seismic reflection imaging can map the sea water structure acoustically. Combined with the knowledge of local water body structure profile over years, the instant model for predicting the sea water properties could be built using the seismic data acquired from the specially designed high precision marine seismic acquisition. This model can also be updated with instant observation and the complete data processing system. The present study has the potential value to many applications, such as 3D sea water monitoring, engineering evaluation, geological disaster assessment and environmental assessment.

Phase characteristic analysis of continuous depth air-gun source wavelet

Air guns are important sources for marine seismic exploration. Far-field wavelet of air gun arrays, as a necessary parameter for pre-stack processing and source models, plays an important role during marine seismic data processing and interpretation. When an air gun fires, it generates a series of air bubbles. Similar to onshore seismic exploration, the water forms a plastic fluid near the bubble; the farther the air gun is located from the measurement, the more steady and more accurately represented the wavelet will be. In practice, hydrophones should be placed more than 100 m from the air gun; however, traditional seismic cables cannot meet this requirement. On the other hand, vertical cables provide a viable solution to this problem. This study uses a vertical cable to receive wavelets from 38 air guns and data are collected offshore Southeast Qiong, where the water depth is over 1000 m. In this study, the wavelets measured using this technique coincide very well with the simulated wavelets and can therefore represent the real shape of the wavelets. This experiment fills a technology gap in China.