Rie Kamei

Misfit functionals in Laplace-Fourier domain waveform inversion, with application to wide-angle ocean bottom seismograph data

In seismic waveform inversion, non-linearity and non-uniqueness require appropriate strategies. We formulate four types of L2 normed misfit functionals for Laplace-Fourier domain waveform inversion: i) subtraction of complex-valued observed data from complex-valued predicted data (the ‘conventional phase-amplitude’ residual),ii) a ‘conventional phase-only’ residual in which amplitude variations are normalized, iii) a ‘logarithmic phase-amplitude’ residual and finally iv) a ‘logarithmic phase-only’ residual in which the only imaginary part of the logarithmic residual is used. We evaluate these misfit functionals by using a wide-angle field Ocean Bottom Seismograph (OBS) data set with a maximum offset of 55 km. The conventional phase-amplitude approach is restricted in illumination and delineates only shallow velocity structures. In contrast, the other three misfit functionals retrieve detailed velocity structures with clear lithological boundaries down to the deeper part of the model. We also test the performance of additional phase-amplitude inversions starting from the logarithmic phase-only inversion result. The resulting velocity updates are prominent only in the high-wavenumber components, sharpening the lithological boundaries.We argue that the discrepancies in the behaviours of the misfit functionals are primarily caused by the sensitivities of the model gradient to strong amplitude variations in the data. As the observed data amplitudes are dominated by the near-offset traces, the conventional phase-amplitude inversion primarily updates the shallow structures as a result. In contrast, the other three misfit functionals eliminate the strong dependence on amplitude variation naturally and enhance the depth of illumination. We further suggest that the phase-only inversions are sufficient to obtain robust and reliable velocity structures and the amplitude information is of secondary importance in constraining subsurface velocity models.

Acoustic Waveform Tomography of OBS Data In the Nankai Subduction Zone

Acoustic waveform tomography was applied to Ocean Bottom seismograph data acquired in the seismogenic Nankai subduction zone. To reduce artifacts arising from elastic effects, the inversion focused on early arrivals, and was conducted in two steps; i) fitting phase information, and ii) the incorporation of amplitude information. Image sources were used to simulate the source ghost. Our image successfully retrieved detailed structures, including seaward continuation of the mega-splay fault responsible for past earthquakes.

Velocity model estimation by full waveform inversion of time-lapse 4D passive seismic array data

In passive-source monitoring, an accurate velocity model is important to precisely estimate microseismic source locations, and to understand changes in reservoir properties. In this study, we employ frequency-domain full waveform inversion in order to obtain a high-resolution velocity model by exploiting full wavefields. We demonstrate the feasibility of the method for a surface geophone array by inverting for time-lapse 4D velocity changes in a realistic subsurface model. Our method successfully estimates the small velocity changes of a few percent within layers of 10s of meters, even for a single passive seismic source event. The analysis of wavepaths and gradients suggests that keys for the successful inversion are the use of full wavefields (both first and scattered arrivals), the vicinity of velocity changes to the source, and the wide-aperture surface array.

Waveform tomography imaging of deep crustal faults - Application to Nankai subduction zone

In many areas the geological interpretation of seismic images can benefit from quantitative, high resolution velocity images. Waveform Tomography can yield the appropriate images provided the input data contain large offsets and low frequencies, such as those available from a suite of Ocean Bottom Seismographs (OBSs). We provide an illustration in the Nankai subduction zone, where a full interpretation requires both the delineation of detailed crustal fault structures and a quantitative velocity map. Previous prestack migrations have revealed key structures, including a megasplay fault, and two distinctive low velocity zones. However the migration sections are negatively affected by the influence of the sea bottom topographic relief. In this study we applied acoustic Waveform Tomography to the OBS data using the traveltime tomography to provide a suitable starting model; synthetic wavefields generated from the final model reproduced wide-angle reflections from the megasplay fault. Our image clearly delineated major geological features including the megasplay fault, thrusts within the accretionary prisms, and an apparent connectivity of two low velocity zones. By comparing the image with a slice from the 3D migration volume, we confirmed the reliability of our image, and the improvement enabled by the use of wide-angle data with Waveform Tomography.