Combined Use of Nearfield Hydrophones and Multiple Migration for Shallow Imaging

Abstract

Abstract Modern acquisition approaches are moving towards ocean bottom node (OBN) designs. By acquiring hydrophone and geophone data, wavefield separation of these data provides access to up-going and down-going wavefields, and powerful techniques such as up/down deconvolution can be employed to attenuate surface related multiples and the source wavelet. In shallow water regions, however, very shallow reflections are not recorded. This paper will illustrate how we can derive an image of the shallow subsurface jointly using near field hydrophone (NFH) data and multiple imaging. Traditionally used for source firing QC, NFH data are gaining popularity for shallow imaging. Typically recorded with a sample interval of 1 ms or less, NFH recordings are higher frequency than OBN data, which are normally limited to 250 Hz. In very shallow water regions, NFHs mounted on the inactive source array may only record beyond-critical arrivals which have limited use for shallow imaging. In this paper, we will demonstrate the use of NFH recordings mounted on the firing array to overcome this problem. However, NFH data on the firing array often suffer difficulties when recovering low frequency reflections due to the extremely strong direct arrival. In addition, processes such as source deghosting and demultiple may be challenging due to the single-fold nature of the data. Another method that is gaining popularity for shallow imaging is multiple migration. Compared to imaging with primaries, multiple imaging provides a larger illumination area with smaller incident angles. We propose to combine the low frequencies from multiple imaging with the high frequencies from NFH imaging to provide a high-resolution image of the shallow section. To illustrate this, we use a shallow water OBN dataset acquired offshore Abu Dhabi in water depths of 3 m to 50 m, where complementary information provided by NFH imaging and multiple migration, and the combination of these images, has resulted in a broadband shallow subsurface section with improved spatial continuity and reduced sidelobes.