SEISMIC CHARACTERIZATION OF SUBMARINE GAS-HYDRATE DEPOSITS IN THE WESTERN BLACK SEA BY ACOUSTIC FULL-WAVEFORM INVERSION OF OBS DATA

Evidence for gas-hydrate occurrence in the Western Black Sea is found from seismic measurements revealing bottom-simulating reflectors (BSRs) of varying distinctness. From an ocean-bottom seismic data set low-resolution traveltime-tomography models of P-wave velocity vP are constructed. They serve as input for acoustic full-waveform inversion (FWI) which we apply to derive high-resolution parameter models aiding the interpretation of the seismic data for potential hydrate and gas deposits. Synthetic tests show the applicability of the FWI approach to robustly reconstruct vP models with a typical hydrate and gas signature. Models of S-wave velocity vS containing a hydrate signature can only be reconstructed when the parameter distribution of vS is already well known. When we add noise to the modelled data to simulate field data conditions, it prevents the reconstruction of vS completely, justifying the application of an acoustic approach. We invert for vP models from fielddata of two parallel profiles of 14 km length with a distance of 1 km. Results show a characteristic velocity trend for hydrate and gas occurrence at BSR depth in the first of the analyzed profiles. We find no indications for gas accumulations below the BSR on the second profile and only weak indications for hydrate. These differences in vP signature are consistent with reflectivity behavior of the migrated seismic streamer data of both profiles where a zone of high reflectivity amplitudes is coincident with the potential gas zone derived from the FWI result. Calculating saturation estimates for the potential hydrate and gas zones yields values of up to 30 % and 1.2 %, respectively. INTRODUCTION Gas-hydrate deposits store huge amounts of natural gas, and can be found at all continental margins and in permafrost regions. Their stability is controlled by temperature and pressure conditions, and the availability of gas and water. When uprising gas enters the stability zone, hydrate forms and restricts the pathway for further gas migration (Kvenvolden, 1993). Hydrate and gas have distinctive elastic properties, and can therefore be detected with seismic imaging methods. A hydrated sediment layer is typically indicated by an elevated P-wave velocity vP compared to hydrate-free sediment, and is bound by a sharp vP decrease below, where the hydrate-stability zone ends. The identification of possible gas-hydrate sites is therefore achieved by the detection of bottom-simulating reflectors (BSRs), which result from this characteristic vP behavior. Because the BSR depth depends on the stability conditions it is not necessarily coincident with a lithological boundary. It is mostly of high amplitude and of opposite polarity compared to the seafloor reflection. The acquisition of seismic reflection data is therefore the simplest method to identify possible gas-hydrate sites. To quantify the volume of available hydrate, detailed parameter-distribution models need to be derived. To obtain reliable seismic velocity information of the subsurface, long-offset data are required. Therefore, in most surveys ocean-bottom seismic (OBS) measurements are performed to provide input for veloc94 Annual WIT report 2018 ity analysis. Conventionally, refracted-wave first arrivals are used for traveltime tomography, yielding vP models which represent a long-wavelength trend of the subsurface parameter distribution. Detailed structures in the range of tens of meters, necessary to map potential gas-hydrate deposits, cannot be resolved using traveltime-based methods. To improve the resolution of subsurface models, full-waveform inversion (FWI) is applied. The concept of acoustic FWI was introduced by Tarantola (1984), and an elastic formulation by Mora (1988). In theory, multiparameter images of the subsurface can be obtained in great detail (e.g. Sears et al., 2008), provided that simulated and measured traveltimes match within half a wavelength for a starting parameter model (Virieux and Operto, 2009). First FWI studies aiming at gas-hydrate characterization were performed with an acoustic approximation in 1D (Singh et al., 1993; Pecher et al., 1996; Korenaga et al., 1997). They were able to derive vP signatures with elevated velocity by up to 300 m/s above and a velocity drop of 600 m/s at BSR depth. A 2D acoustic FWI approach was applied to multichannel seismic (MCS) data by Delescluse et al. (2011), associating increased velocities with a gas-hydrate zone and low-velocity zones with the occurrence of gas. They conclude that the acoustic approximation is valid also for far-offset data, provided vS is varying smoothly with depth. Kim et al. (2013) show results of 2D elastic FWI, where they observe increased Pand S-wave velocities as well as a reduced poisson ratio in a zone related to hydrate occurrence. An underlying layer of decreased velocity and higher poisson ratio is interpreted as a gas zone. Jaiswal et al. (2012) perform visco-acoustic 2D FWI and relate increased velocities as well as reduced attenuation to the presence of gas hydrate, and reduced velocities and increased attenuation to gas occurrence. In contrast to these results, laboratory measurements indicate increased attenuation in hydrated sediments (Guerin and Goldberg, 2002). While all FWI studies mentioned in the previous paragraph involve the use of MCS data, we demonstrate that local gas and hydrate distribution in marine sediments can also be explored by the inversion of OBS data. Although only few stations are available, the approach is advantageous in terms of computing time, as we can apply the reciprocity principle. OBS locations serve as virtual source positions, and the actual shot locations as virtual receiver positions. Despite the low coverage it is still possible to identify potential gas-hydrate accumulations. In this paper, we first describe the applied FWI approach, and then introduce the measurement area and the field data. Synthetic studies give an insight into the resolution capability of acoustic and elastic FWI for the available geometry and data quality. We show results of acoustic field-data inversion for two profiles and compare two different time-windowing approaches, one with all events and one with muted input data disregarding the direct waves and primary reflections. Finally, we link recovered parameter distribution to potential hydrate and gas concentration. FWI APPROACH To study the subsurface parameters along the recorded seismic profiles we apply a 2D time-domain fullwaveform inversion approach (Köhn, 2011). For a chosen starting model, synthetic seismograms are computed by the finite-difference (FD) method (Virieux, 1986). For each shot an initial misfit E is calculated between synthetic seismograms s and corresponding field data d. A standard L2-norm is used for the misfit definition, according to