Denes Vigh

Developing Earth models with full waveform inversion

Exploration in more geologically complex areas requires new methodologies. In its quest to answer these new challenges, the oil and gas industry has moved from ray-based imaging to finite-difference, wave-equation migration to achieve better subsurface descriptions of target zone and reservoirs. Notable in this progression is the movement from ray-traced Kirchhoff algorithms through one-way wave-equation methods to use the acoustic two-way wave equation.

Comparisons For Waveform Inversion, Time Domain Or Frequency Domain?

The Earth model parameters are essential to hydrocarbon exploration. In particular, the velocity representation of the subsurface permanently engages creative minds to find ways to derive more accurate fields. One of these tools is full waveform inversion. This compute-intensive tool uses acquired seismic data and forward modeling to obtain a velocity field in an iterative manner. Since the mideighties, the geophysical community has been devoting considerable research to waveform inversion. The inversion can be implemented in either the time or frequency domain. In this paper, we investigate the pros and cons for each domain along with some discussion on their ability to be used for 3D surveys.

3D full-waveform inversion at Mariner - a shallow North Sea reservoir

We applied isotropic acoustic 3D full-waveform inversion (FWI) to OBC data from the Mariner field, a shallow heavy-oil field in the North Sea. This resulted in a multiphase workflow that can be adapted to imaging challenges in similar geological settings. FWI improves the resolution of the velocity field compared to the benchmark velocity field from reflection tomography. The background trend of and details introduced in the velocity model correlate well with the geology from seismic data and with well logs down to reservoir level. Resulting depth images show significantly better well tie in the overburden and improved definition of sand bodies at reservoir level.

Full-waveform Inversion Application In Different Geological Settings

After the synthetic data inversion examples, real 3D data sets have been undertaken by the industry for the last three years. As field data are dominated by P-waves, one feasible approach is to use acoustic approximation. Fullwaveform inversion (FWI) determines parameters related to the acoustic wave equation but mostly velocities by minimizing the misfit between the observed data and the model data. It has shown tremendous potential especially in 3D wide-offset acquisitions. This includes wide-azimuth streamer, ocean-bottom surveys and land type of geometry where the advantage of FWI has convinced the oil industry to pay close attention to the technology and apply it in complex geological settings. We demonstrate FWI applicability in different geological environments including marine from the Gulf of Mexico (GOM), the North Sea, and land data examples from desert type of settings for the geology.

3D Prestack Time Domain Full Waveform Inversion

Despite the relatively high computational demand, the early wave equation based prestack depth migration approaches were adequate and valuable, in many geologic situations.Meanwhile, acquisition methods improved, providing enhanced azimuthal sampling for

Acoustic Waveform Inversion Vs. Elastic Data.

Full Waveform inversion is a computer intensive process, especially for 3D seismic data. After a tremendous number of synthetic examples, finally real 3D data sets have been undertaken by the industry. As field data is dominated with P waves, one feasible approach is to use the acoustic approximation. The difficulty we face is using an acoustic propagator can make it difficult to accurately predict the amplitudes of real elastic data. In this paper we show how it is possible to make acoustic inversion work on elastic data. The experiment is carried out by using acoustic timedomain inversion on marine synthetic data created by elastic modeling.

The Contribution of Wide-azimuth Point-receiver Acquisition to the Success of Full-wave Inversion

Low frequencies are critical for the success of full-wave inversion (FWI). With the point-receiver marine and land acquisition system, low frequencies are significantly improved compared with conventional surveys. For marine data, the lowest frequency band that can be used for FWI is 2-3 Hz. For land data, the lowest frequency band that we can use for FWI is 2-4 Hz. Wide-azimuth (WAZ) acquisition facilitates FWI by highly improved data coverage with a higher signal-to-noise (S/N) ratio, better illumination, and longer offset. FWI was successfully carried out with both marine and land data sets from lowto high-frequency bands. The final FWI velocity models were validated by both Kirchhoff depth migration and reverse-time migration (RTM).

Pre-Stack Full Waveform Inversion for Velocity Model Building

An accurate earth model is key to any successful depth imaging effort. Full waveform inversion is an advanced velocity model building process which uses the full two way wave equation. Existing methods use a ray tracing approach to distribute velocity errors, computed from the residual move-out in Image gathers, into the model. In this paper we will briefly introduce the full waveform inversion theory and show some results from synthetic and real data studies.