Le Wan

Large-scale 3D inversion of marine magnetotelluric data: Case study from the Gemini prospect, Gulf of Mexico

Three-dimensional magnetotelluric MT inversion is an emerging technique for offshore hydrocarbon exploration. We have developed a new approach to the 3D inversion of MT data, based on the integral equation method. The Tikhonov regularization and physical constraint have been used to obtain a stable and reasonable solution of the inverse problem.Themethodisimplementedinafullyparallelcomputer code. We have applied the developed method and software for the inversion of marine MT data collected by the Scripps Institution of Oceanography SIO in the Gemini prospect, Gulf of Mexico. The inversion domain was discretized into 1.6 million cells. It took nine hours to complete 51 iterations on the 832-processor cluster with a final misfit between the observed and predicted data of 6.2%.The inversion results reveal a resistive salt structure, which is confirmed by a comparison with the seismic data. These inversion results demonstrate that resistive geoelectrical structures like salt domes can be mapped with reasonable accuracyusingthe3DinversionofmarineMTdata.

Regularization Analysis of Three-dimensional Magnetotelluric Inversion

Inversion of MT data is an inherently nonunique and unstable problem due to the ill-posedness of the electromagnetic inverse problem. A variety of models may fit the data very well. To overcome this illposed nature of the inverse problem, we use Tikhonov’s regularization in which the ill-posed problem is replaced by a family of well-posed problems. We also analyze the behavior of the Tikhonov regularization parameter to find out its optimal value for a typical model of a hydrocarbon reservoir in a marine environment. We have compared two regularization techniques: rigorous and adaptive regularizations. The results of this numerical study demonstrate that adaptive regularization provides practically the same inverse image as the rigorous regularization, while reducing the computational time dramatically.

Focusing Inversion of Marine Full-tensor Gradiometry Data In Offshore Geophysical Exploration

In this paper we demonstrate that marine gravity gradiometry data can be effectively used for offshore hydrocarbon exploration. We apply a three-dimensional (3D) focusing inversion method to interpretation of the marine full-tensor gradient (FTG) data collected in the Barents Sea. We conduct inversion of the different individual components and a joint inversion of several FTG components. The numerical results demonstrate that the joint inversion helps to produce a consistent 3D model of the anomalous density distribution in the area of the FTG survey. The results of 3D inversion of FTG data show a possibility for resolving the complex geological structures of salt diapir formations using gravity gradiometry data.

Rigorous 3D inversion of marine magnetotelluric data in the area with complex bathymetry

Summary Three-dimensional (3D) magnetotelluric (MT) inversion is an emerging technique for offshore hydrocarbon (HC) exploration. In this paper we introduce a new approach to 3D inversion of MT data for offshore HC exploration based on the integral equation method. The method is implemented in a fully parallel computer code. We have applied the developed method and software for the inversion of marine MT data collected by the Scripps Institution of Oceanography (SIO) in the Gemini Prospect, Gulf of Mexico. The inversion domain was discretized into 1.7 M cells. It took 9 hours to complete 51 iterations on the 832 processor cluster with a final misfit between the observed and predicted data of 6.2%. The inversion results reveal a resistive salt structure which is confirmed by a comparison with the seismic data. These inversion results demonstrate that we can map resistive geoelectrical structures like salt domes or HC reservoirs with reasonable accuracy using 3D inversion of marine MT data.

Rapid seabed imaging by frequency domain electromagnetic migration

This paper discusses an application of electromagnetic (EM) migration for sea-bottom EM imaging. We apply to marine EM data the method of frequency domain EM migration developed by Zhdanov et al. (1996) for fast imaging of land EM data. We study a synthetic survey with electric receivers measuring the natural telluric electric field at the sea bottom over a 3-D geoelectrical model of a sea-bottom petroleum reservoir. We also demonstrate that the EM migration can be applied for fast imaging of marine controlled-source electromagnetic data. The results of this modeling show that the migration method can be effectively used for fast sea-bottom imaging of resistive reservoir structures.

Three‐dimensional marine magnetotellurics for petroleum exploration

In this paper we investigate the application of marine magnetotellurics for petroleum exploration. The resistivity of petroleum reservoirs and of salt structures is usually on an order greater than those of surrounding sea-bottom sediments. That is why these structures can be considered easily detectable targets for marine MT methods. We consider the schematic model of an offshore Angola sea-bottom petroleum reservoir. We examine two models. One model contains a reservoir only, another represents a more realistic model of a petroleum reservoir in the presence of a salt dome structure. We invert the synthetic data computed for the reservoir model and the model with the salt dome using the CEMI 3-D MT inversion code QAINV3D. The inversion results show that even in the case of complex sea-bottom geological structures, where the reservoir response is strongly distorted by the salt dome effect, the inversion generates a clear image of the reservoir. These results demonstrate that the marine magnetotellurics can be a powerful method for offshore petroleum exploration.

3D Potential Field Migration For Rapid Imaging of Gravity Gradiometry Data - A Case Study From Broken Hill, Australia, With Comparison to 3D Regularized Inversion

The geological interpretation of gravity gradiometry data is a very challenging problem. With the exception of the vertical gradient, maps of the different gravity gradients are usually very complicated and cannot be directly correlated with geological structures. We introduce the concept of 3D potential field migration and demonstrate how it can be applied for rapid imaging of entire gravity gradiometry surveys. The method is based on a direct integral transformation of the observed gravity gradients into a 3D density model which can be used for interpretation, or as an a priori model for subsequent 3D regularized inversion. For regional-scale surveys, we show how migration runs on the order of minutes compared to hours for 3D inversion. We present a case study is for the 3D migration of a FALCON® airborne gravity gradiometry (AGG) survey from Broken Hill, Australia, and compare our results to 3D regularized inversion and previously mapped geology.

Frequency Domain 3-D Electromagnetic Migration And Imaging of Sea-bottom Geoelectrical Structures

In this paper we develop a fast 3-D electromagnetic (EM) migration method for marine geophysical exploration. The developed migration algorithm is based on downward extrapolation of the observed EM field using a special form of finite-difference equation for the migration field. It allows us to migrate within the sea-bottom formations the EM signals observed by the sea-bottom receivers. The migration field is subsequently transformed in the resistivity image of the sea-bottom geoelectrical structures. This technique is in an order faster than the conventional inversion. It can be used for fast imaging of the marine magnetotelluric (MT) and controlled-source electromagnetic (CSEM) data in off-shore hydrocarbon (HC) exploration.

New development in 3‐D marine MT modeling and inversion for off‐shore petroleum exploration

This paper is focused on the development of threedimensional (3-D) interpretation techniques for a sea-bottom magnetotelluric (MT) method for off-shore petroleum exploration. The results of our study demonstrate that, with MT data, the sea-bottom petroleum reservoir could be mapped with reasonable accuracy. We also present a case history of 3-D inversion of sea-bottom MT data collected by Scripps Institution of Oceanography in Gemini Prospect, Gulf of Mexico.

Feasibility study of gravity gradiometry monitoring of CO2 sequestration in deep reservoirs using surface and borehole data

Geophysical monitoring of carbon dioxide (CO2) injections in a deep reservoir has become an important component of carbon capture and storage (CCS) projects. Until recently, the seismic method was the dominant technique used for reservoir monitoring. However, the cost of seismic surveys makes this method prohibitive in monitoring sequestration projects where there is not a direct profit. Moreover, some environments present challenges for seismic acquisition as in urban areas. In this paper we present a feasibility study of permanent gravity gradiometry monitoring of CO2 sequestration in a deep reservoir using a novel approach involving both borehole and surface measurements. The interpretation is based on joint iterative migration imaging of the surface and borehole data. The advantage of this method is that the surface data provide a good estimate of the horizontal extent of the injection zone, while the borehole data control the depth of the target, which increases the sensitivity and resolution of the method. We illustrate the effectiveness of the gravity gradiometry method by computer simulating CO2 injection monitoring in the Kevin Dome sequestration site in Montana, USA.