Joint Solution for Improving Subsalt Imaging Using 3D Gravity and Seismic Data
Authors: Ni Yudong , Yang Zhanjun , Zhang Jianjun , Yang Boya , Li Yangsheng , Wang Jingfu 
Published: March 2015

Summary
Pre-stack depth migration (PSDM) is one of the most important technologies for accurate subsalt structure imaging. However, in the case of huge salt domes and low signal-to-noise ratios (SNR) of subsalt data, the key to accurate imaging of subsalt structure using PSDM is to establish an accurate velocity model. This paper presents a joint solution of 3D gravity and 3D seismic data. The SNR of seismic data above the several huge salt domes in the area is high, while the SNR below the salt domes is relatively low. Since the target is below the salt domes, the PSDM data can't meet the needs of subsalt structure interpretation. We therefore carried out 3D gravity exploration to solve subsalt structural imaging accuracy problems jointly using 3D gravity data and 3D seismic data. Using the inversion result of 3D gravity formation separation, the cause of clutter in seismic reflections on the interior of salt domes is illustrated and the initial velocity model provided by 3D seismic data is revised in the study area. Through this revised initial velocity model, a new pre-stack depth migrated section with improved subsalt reflection SNR as well as better subsalt structure imaging is achieved. Along with the work flow of joint solution of gravity and seismic data, this paper also gives prerequisites for formation separation technique.

Introduction
Interval velocity modelling plays a key role in imaging subsalt structure using PSDM technology. Generally, the interval velocity modeling covers two parts: the first is to establish an initial interval velocity model using pre-stack time-migrated seismic data. After PSTM and horizon calibration in the time domain of seismic data, the RMS velocity field as well as the structural model in time domain can be acquired. Once these data are determined, a constrained inversion as well as time-to-depth conversion can be applied to the RMS velocity field to acquire the initial interval velocity model in depth domain. The second part involves using the already established initial interval velocity model in depth domain to apply PSDM to the test line and evaluate the migrated section geophysically and geologically. This is an important step to modify the initial velocity model in depth domain to reasonable solution. The second step is a complex iterative processes, through which the final interval velocity model in depth domain for PSDM processing in the whole work area can be obtained. Obviously, the higher the SNR of seismic data, the finer the layer calibration in the time domain, and the more accurate the established initial interval velocity model. Here, the layer calibration in time domain isn't the chronostratigraphic calibration of interfaces but the interval velocity variation calibration of interfaces, taking corresponding well logging data and underground geological features as constraints for inversion of the RMS velocity field. One of the criterions to judge the reasonability of initial or inverted interval velocity model is the leveled reflection events in the CRP gather. Therefore, we can't judge whether the interval velocity model is reasonable or not if the SNR in the CRP gather is too low. Thus, the SNR of seismic data is an important factor in determining the precision of interval velocity model.