Takumi Ueda

Fast numerical modeling of multitransmitter electromagnetic data using multigrid quasi-linear approximation

Multitransmitter electromagnetic (EM) surveys are widely used in remote-sensing and geophysical exploration. The interpretation of the multitransmitter geophysical data requires numerous three-dimensional (3-D) modelings of the responses of the receivers for different geoelectrical models of complex geological formations. In this paper, we introduce a fast method for 3-D modeling of EM data, based on a modified version of quasilinear approximation, which uses a multigrid approach. This method significantly speeds up the modeling of multitransmitter-multireceiver surveys. The developed algorithm has been applied for the interpretation of marine controlled-source electromagnetic (MCSEM) data. We have tested our new method using synthetic problems and for the simulation of MCSEM data for a geoelectrical model of a Gemini salt body.

Sharp boundary inversion in crosswell travel-time tomography

The reconstruction of seismic images of the medium from crosswell travel-time data is a typical example of the ill-posed inverse problem. In order to obtain a stable solution and to replace an ill-posed problem by a well-posed one, a stabilizing functional (stabilizer) has to be introduced. The role of this functional is to select the desired stable solution from a class of solutions with specific physical and/or geometrical properties. One of these properties is the existence of sharp boundaries separating rocks with different petrophysical parameters, e.g., oil- and water-saturated reservoirs. In this paper, we develop a new tomographic method based on application of a minimum support stabilizer to the crosswell travel-time inverse problem. This stabilizer makes it possible to produce clear and focused images of geological targets with sharp boundaries. We demonstrate that the minimum support stabilizer allows a correct recovery of not only the shape but also the velocity value of the target. We also point out that this stabilizer provides good results even with a low ray density, when the traditional minimum norm stabilizer fails.

Fast numerical methods for marine controlled-source electromagnetic (EM) survey data based on multigrid quasi-linear approximation and iterative em migration

Abstract In this paper we consider an application of the method of electromagnetic (EM) migration to the interpretation of a typical marine controlled-source (MCSEM) survey consisting of a set of sea-bottom receivers and a moving electrical bipole transmitter. Three-dimensional interpretation of MCSEM data is a very challenging problem because of the enormous number of computations required in the case of the multi-transmitter and multi-receiver data acquisition systems used in these surveys. At the same time, we demonstrate that the MCSEM surveys with their dense system of transmitters and receivers are extremely well suited for application of the migration method. In order to speed up the computation of the migration field, we apply a fast form of integral equation (IE) solution based on the multigrid quasi-linear (MGQL) approximation which we have developed. The principles of migration imaging formulated in this paper are tested on a typical model of a sea-bottom petroleum reservoir.

Fast Numerical Modeling of Marine Controlled-source Electromagnetic Data Using Quasi-linear Approximation

We developed a fast method for 3-D modeling of marine controlled-source electromagnetic (MCSEM) data in seabed logging (SBL) applications. This method is based on a modified version of quasi-linear (QL) approximation, which speeds up significantly modeling of multitransmitter-multireceiver surveys. The developed code has been tested using synthetic problems and for simulation of MCSEM data for a geoelectrical model of a Gemini salt body.

Iterative migration in marine CSEM data interpretation

In this paper we consider an application of the ideas of electromagnetic (EM) migration to the interpretation of a typical marine controlledsource (MCSEM) survey, which consists of a set of sea-bottom receivers and a moving electrical bipole transmitter. The 3-D interpretation of the MCSEM data is a very challenging problem because of the enormous amount of computations required in the case of the multi-transmitter and multi-receiver data acquisition systems used in these surveys. At the same time, we show that the MCSEM surveys with their dense system of transmitters and receivers happen to be extremely well suited for application of the migration method. In order to speed up the computation of the migration field, we apply a fast form of integral equation (IE) solution based on the multigrid quasi-linear (MGQL) approximation. The principles of the migration imaging formulated in this paper are tested on a typical model of a sea-bottom petroleum reservoir.

Three-dimensional Electromagnetic Holographic Imaging In Offshore Petroleum Exploration

Off-shore petroleum exploration nowadays routinely uses the marine controlled-source (MCSEM) survey, which consists of a set of sea-bottom receivers and a moving electrical bipole transmitter. We show that the MCSEM survey with its dense system of transmitters and receivers, is extremely well suited for application of the holography/migration method. The combined EM signal in the receivers forms a broadband EM ”hologram” of the sea-bottom geological target. As in optical and radiowave holography, we can reconstruct the volume image of the geological target by ”illuminating” this EM hologram with the reference signal. The principles of holography/migration imaging formulated in this paper are applied to the interpretation of an MCSEM survey conducted in the Troll West Gas Province (TWGP), offshore Norway.

Joint iterative migration of electric and magnetic field data

Marine controlled-source electromagnetic (MCSEM) surveys become widely used in off-shore petroleum exploration. However, the interpretation of the MCSEM data is a very challenging problem because of the enormous amount of computations required in the case of the multi-transmitter and multi-receiver data acquisition systems used in these surveys. In this paper we demonstrate that this problem can be solved using the method of electromagnetic migration. We extend the basic principles of electric field migration to the case of joint electric and magnetic field interpretation. The joint migration field is produced by back propagation within the conductive sea-bottom sediments of all electric and magnetic signals recorded in the receivers. We demonstrate that the joint migration of the EM field data provides a better quality image of a sea-bottom resistive structure (e.g., a hydrocarbon reservoir) than the results of individual migration for the different (electric or magnetic) field components.

3D electromagnetic holographic imaging in active monitoring of sea-bottom geoelectrical structures

Abstract We consider an application of the ideas of electromagnetic (EM) holography/migration to the interpretation of a typical marine controlled-source (MCSEM) survey, which consists of a set of sea-bottom receivers and a moving electrical bipole transmitter. The 3D interpretation of MCSEM data is a very challenging problem because of the enormous amount of computations required in the case of the multitransmitter and multireceiver data acquisition systems used in these surveys. At the same time, we show that MCSEM surveys with their dense system of transmitters and receivers are extremely well suited for application of the holography/migration method. The combined EM signal in the receivers forms a broadband EM “hologram” of a sea-bottom geological target. As in optical and radiowave holography, we can reconstruct the volume image of the geological target by “illuminating” this EM hologram with a reference signal. The principles of holography/migration imaging formulated in this paper are tested on typical models of a sea-bottom petroleum reservoir. We also apply this new technique to the interpretation of an MCSEM survey conducted in the Troll West Gas Province (TWGP), offshore Norway.

A 3D ground penetrating radar imaging of the heavy rainfall-induced deformation around a river levee: A case study of Ara River, Saitama, Japan

This paper describes a three-dimensional ground penetrating radar (GPR) survey carried out around a levee of the Ara River in Saitama, Japan, where deformation of the ground was observed after heavy rainfall associated with the typhoon of September 2007. The high-density 3D GPR survey was conducted as a series of closely adjacent four directional sets of 2D surveys at an area surrounding vertical cracks on the paved road caused by deformations induced by heavy rain. The survey directions of the 2D surveys were 0, 90, 45, and –45 degrees with respect to the paved road and the intervals between lines were less than 0.5 m. The 3D subsurface structure was accurately imaged by the result of data processing using Kirchhoff-type 3D migration. As a result, locations and vertical continuities of the heavy rainfall induced cracks in the paved road were clearly imaged. This will be a great help in considering the generation mechanisms of the cracks. Moreover, the current risk of a secondary disaster was found to be low, as no air-filled cavities were detected by the 3D GPR survey.

Sub-seafloor resistivity sensing using a vertical electrode configuration

There is growing interest in marine direct current (DC) resistivity methods for sub-seafloor exploration of a broad range of geophysical and geological targets. To address this, we have developed a new marine DC method with a vertical electrode configuration (VEC). Compared to conventional marine DC methods that use a horizontal electrode configuration, the shape and position of our VEC cable can be controlled relatively easily. Therefore, the VEC is suitable for operations in regions of steep bathymetry and for expeditious sub-seafloor resistivity exploration. In this study, we introduce a water-resistant electrode array cable and an onshore multichannel DC measurement system for stable and rapid data acquisition. To evaluate the performance and efficiency of the new system, we conducted field experiments in the shallow water zone at Shimizu Port, Suruga Bay, Japan. In order to quantitatively analyze the VEC-DC data, we adopt a 1-D numerical modeling code that computes the electric potential and apparent resistivity generated by a point and dipole current source used in the VEC-DC measurement. These can be placed at any position with an arbitrary electrode configuration in a multilayered space, including seawater and sub-seafloor layers. We also develop an inversion code for the VEC-DC data based on a simulated annealing (SA) optimization and applied this to the field data. The observed data is of sufficiently good quality to be used for inversion, and the SA result demonstrates that the proposed VEC-DC system is able to estimate the sub-seafloor resistivity structure.