Joint inversion of seismic and gravity data

Project description

Motivation (background):

Subsurface characterization, for example for exploration, CO2 storage and geothermal heat extraction is both challenging and costly. Moreover, in practice this characterization also requires actual monitoring of changes in the subsurface because of fluid flow. There are various ways in which these challenges can, at least partly, be met. Seismics provides by far the most important subsurface characterization and monitoring tool. In particular, the highest quality seismic inversion results are obtained using (elastic isotropic) full waveform inversion. However, there is trade-off between the various inversion parameters (vp, vs and density) and it would be useful to reduce this trade-off. It should be possible to do this, at least partly, by joint inversion of seismic and gravity data. Gravity data can be obtained in various ways: on the sea floor, the sea surface and airborne. Each comes with its own advantages and disadvantages in terms of cost and accuracy. The goal of this project is to develop and test joint seismic and gravity inversion methods, with particular attention to the various gravity acquisition methods.

Hypothesis (scientific problem):

There are a number of different ways to acquire gravity data. Airborne and sea surface gravity data typically are acquired on a dense grid but have relatively large uncertainties. Seafloor gravity data, on the other hand, are acquired on a sparse grid but with quite small uncertainties. When doing joint inversion of seismic and gravity data it is important to understand the impact of these different types of gravity data. In particular, there are a number of important questions that need to be addressed: is there an optimal way to jointly invert for time-lapse changes in a reservoir using seismic data and either type of gravity data? What accuracy and resolution can be obtained with airborne and seafloor gravity data? Given, the sparseness of seafloor gravity data: what is the optimal spacing for the seafloor gravity data?

Test (work):

Elastic full waveform inversion of seismic data is very costly. A relatively cheap, and still quite accurate method, uses ray-Born modeling for the inversion. Computer code for this inversion has been developed and tested. This can be used as a starting point for the joint inversion of seismic and gravity data. This joint inversion can be done in a number of different ways and these will first be tested on relatively simple but useful subsurface models, which mimic subsurface fluid flow, using both dense airborne and sparse seafloor gravity data. Particular attention will be paid to the case of stacked reservoirs. Once this has been done, the joint inversion will be applied to a more complicated and realistic model, with particular attention to the questions given in the previous paragraph. This research will be done in close collaboration with Reach Subsea, which has a high quality gravity acquisition system.

Proposed course plan during the master's degree (60 ECTS)

Geov274, 10 sp

Mat212, 10 sp

Geov300, 5 sp

Geov276, 10 sp

AG335, 10 sp

Geov352, 5 sp

Geov75, 10 sp

Prerequisites

Bachelor in geophysics