Marine Controlled-Source Research Articles

Finite-Element Scheme With Total-Field for 3-D Marine Controlled-Source Electromagnetic Modeling With Bathymetry and Multisources

Finite-Element Scheme With Total-Field for 3-D Marine Controlled-Source Electromagnetic Modeling With Bathymetry and Multisources

New Concept Ocean-Bottom Multiphysics (OBMP) Nodes for Reservoir Monitoring

Marine-controlled source electromagnetics (CSEM) have been extensively applied to various exploration scenarios worldwide. However, its perceived value and cost relative to seismic and the scarcity of realistic case studies have limited the industry’s interest in time-lapse reservoir-monitoring (4D) applications. A feasible way to make demand for CSEM for 4D-monitoring programs would be to increase the value of information and reduce survey costs by performing joint operations where seismic and CSEM data are acquired during the same survey and at equivalent spatial densities. To this end, we propose a new multiphysics ocean-bottom nodes (OBN) concept and show the industry that CSEM can be a cost efficient and effective integrators to 4D seismic projects. To this end, we conducted a feasibility study demonstrating that horizontal magnetic field components have the required sensitivities and can be used instead of horizontal electric field components in mapping the 3D resistivity distribution and 4D fluid change responses in a given reservoir. This makes engineering a new OBN class simpler and cheaper, as various miniaturized magnetic field sensors are available off-the-shelf or readily working along with packaging and coupling solutions.

Identification of induced polarization of submarine hydrocarbons in marine controllable source electromagnetic exploration

The identification of hydrocarbons buried on the seafloor is highly dependent on geophysical exploration capabilities. Seismic exploration has been an important tool in providing information on submarine stratigraphy before offshore drilling, but it is a challenge to identify the nature and saturation of the fluid in the structure by seismic exploration. Of all the physical properties, electrical parameters are the most sensitive to the fluids in the reservoir and would be able to be combined with seismic data to improve the identification of hydrocarbons at depth. However, the marine controlled-source electromagnetic method usually only considers the effect of electromagnetic induction and ignores the induced polarization (IP) effects. The IP effects can occur in the stratum where the reservoir is located due to a variety of factors, so considering the IP effects will make the modeling more reasonable and thus give more accurate results when interpreting and processing the data. We have used the integral equation method for modeling, adopted the scattering and superposition methods to calculate the dyadic Green’s function required in the study, replaced the real resistivity with a complex resistivity that takes into account the IP effects, investigated the response patterns of different ion polarization models, and analyzed the influence patterns of various model parameters. These investigations will provide important contributions to the study of submarine hydrocarbon detection. The field data also show the amplitude, and phase response results of polarizability show that it gradually increases from the offset.

Three-Dimensional Modeling of Marine Controlled Source Electromagnetic Using a New High-Order Finite Element Method With an Absorption Boundary Condition

The marine controlled source electromagnetic method (MCSEM) is widely used in marine oil, gas exploration and deep structures investigation because of its low cost and high efficiency. In this paper, a high-precision forward algorithm for 3D modeling for MCSEM is proposed. Unstructured meshes are used to discretize the model domain, and local mesh refinement techniques are applied, which is conducive to simulating complex terrains and targets. The application of electric dipole discrete technology can load wire excitation sources with arbitrarily complex spatial shapes, which can simulate more realistic electromagnetic field distribution of marine controllable sources electromagnetic in actual exploration. Absorption boundary conditions based on real number and exponential stretching techniques are introduced to improve the numerical solution accuracy. To improve the efficiency of solving, the open-source massively parallel solver (MUMPS) based on the multifrontal algorithm is used to solve the finite element equations. Finally, some typical geoelectric models are designed to verify the correctness and effectiveness of the proposed algorithm. The calculation results show that higher order finite elements bring about higher accuracy. In addition, the loading of absorption boundary conditions is simple and effective than conventional Dirichlet boundary conditions.

3D MCSEM modeling based on domain-decomposition finite-element method

Domain decomposition approach is effective in converting large modeling problems into many small ones, so that the parallel computation can be easily facilitated and the memory consumption can be largely reduced. In this paper, we developed a domain-decomposition method for 3D time-domain marine controlled-source (MCSEM) modeling based on Dual-Primal Finite-Element Tearing and Interconnecting technique (FETI-DP). We use tetrahedral grids to do the spatial discretization first and then partition the mesh into subdomains. In each subdomain, the finite-element equations are independently constructed using the first-order vector shape functions and boundary conditions. To build the interconnections among these subdomains, we establish the interface equation for the Lagrange multipliers that are used as the boundary conditions for the subdomains. After solving the equations, the electric field for all subdomains can be obtained from the Lagrange multipliers. For the time discretization, we use unconditionally stable backward Euler scheme. We validate our algorithm by comparing with a semi-analytical solutions for a uniform half-space model under the ocean. Numerical experiments show that compared to the conventional finite-element method, our domain-decomposed method can save large amount of memory, render it possible to run 3D MCSEM modeling in small computational equipment.

Complex Frequency-Shifted Perfectly Matched Layers for 2.5D Frequency-Domain Marine Controlled-Source EM Field Simulations

Complex Frequency-Shifted Perfectly Matched Layers for 2.5D Frequency-Domain Marine Controlled-Source EM Field Simulations

Improved Detectivity for Detecting Gas Hydrates Using the Weighted Differential Fields of the Marine Controlled-Source Electromagnetic Data

Gas hydrate is seen as a kind of new energy resources, yet it may also be one of the main greenhouse gases as its dissociation may release methane into the atmosphere. Furthermore, a severe hazard to offshore infrastructures may also be introduced by extensive gas hydrate dissociation associated with the stability of the geological structures after gas production. Therefore, it is essential to investigate the gas hydrate as well as its environmental impacts before drilling and extracting it. The geophysical seismic reflection data is usually used for exploring the gas hydrate. The gas hydrate can be effectively identified by the bottom simulating reflectors (BSRs) on seismic reflection data. However, the BSR is only for identifying the bottom boundary and it is difficult to estimate its space distribution and saturation within the hydrate stability zone. The marine controlled-source electromagnetic (CSEM) data is suitable for detecting the gas hydrate as the resistivity of the seafloor increases significantly in the presence of gas hydrate or free gas. In this study, a weighted differential-field method is applied to improve the detectivity for identifying the gas hydrate. Numerical tests show that the difference of the EM fields can effectively suppress the airwaves in shallow waters. Therefore, the detectivity given by the field ratio between the models with and without the gas hydrate target is enhanced.

Marine-controlled source electromagnetic sounding across measurement scales: applications in hydrocarbons and beyond

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The latest development of Marine controllable source electromagnetic transmitter

Marine Controlled-Source Electromagnetic (MCSEM) methods have been recognized as an important and effective tool in accurate analysis of shallow electrically resistive structures such as oil, gas and gas hydrate. However, the MCSEM performance in the deep ocean is strongly affected by the hardware design, especially the transmitter system. We have developed a series of deep-towed MCSEM transmitter systems. The design of the transmitter system meets serious challenges such as long-distance and high-voltage transmission, water sealing under high-pressure, the real-time monitoring of seafloor status information 4 km below the sea-surface. In this paper, we present the latest research and development of the MCSEM transmitter systems by presenting key technologies in hardware development and implementation details. The deep tow transmitter system consists of three parts: the deck unit, the deep tow cable and the deep tow unit, which can transmit the high frequency and high power to the seawater, maintaining a high-speed data communication with control center which in the ship’s lab. We conducted several experiments in the South China Sea from August of 2010, to November of 2019. The results showed that the development of the transmitter is successful and it can be used for oil, gas and gas hydrate exploration as an effective source.

Small power ZVS circuits for the marine‐controlled source electromagnetic transmitter

Marine-controlled source electromagnetic transmitter is an important equipment for the investigation of marine geology and exploration of oil, gas, and mineral resources in recent years. There are some problems from the direct current–direct current (DC–DC) converter of high power marine transmitter, the load is light, when the output current is relatively small, the lagging-leg switching cannot easily achieve the zero voltage switching (ZVS), and the conversion efficiency of the heat converter requires improvement. This paper proposes a small power ZVS circuits for the marine-controlled source electromagnetic transmitter (SPZVSC-MCSET). The resonant inductance and blocking capacitor are appropriately used in the current-doubler-rectifier converter, when in a wide load range the current-doubler-rectifier converter can achieve ZVS. The Saber simulation and 100 W electromagnetic transmitter are used to verify the results.

Prediction of hydrocarbon depth for seabed logging (SBL) application using Gaussian process

Seabed Logging (SBL) is an application of marine Controlled-Source Electromagnetic (CSEM) technique to characterize hydrocarbon-filled layers underneath the seabed remotely in deep water regions. This technique maps structure of subsurface electrical resistivity in the offshore environment. Basically, exploration of offshore hydrocarbon is based on the contrast of electrical resistivity between hydrocarbon reservoir and its surrounding sea sediments. Modelling offshore hydrocarbon is a core analysis and time consuming task. Current numerical modelling techniques used in SBL application involve meshes and complicated mathematical equations. Thus, a simple supervised learning method which is Gaussian Process (GP) is proposed to process synthetic SBL data which are generated through Computer Simulation Technology (CST) software to predict the depth of hydrocarbon. This statistical model is able to provide additional hydrocarbon information by utilizing the prior information. 1-dimensional (1-D) forward GP models have successfully been developed to predict the presence of hydrocarbon and as the continuation work, 2-dimensional (2-D) forward GP model is then developed to be used as the standard profile in order to predict the depth of hydrocarbon. This shall give indication that GP can be used as the methodology to predict the depth of hydrocarbon reservoir underneath the seabed.

3-D Adaptive Finite-Element Modeling of Marine Controlled-Source Electromagnetics with Seafloor Topography Based on Secondary Potentials

We present a solution of using adaptive nodal finite-element (FE) method to solve the marine controlled-source electromagnetic (CSEM) problem for 3-D earth models in the frequency domain. The forward problem is solved based on the secondary Coulomb-gauged electromagnetic (EM) potentials. We implement an adaptive mesh refinement algorithm according to an a posteriori error estimator based on a gradient-recovery operator of the secondary EM potentials. To increase the quality of the mesh at the receiver locations, the elements containing the receiver locations are constrained by assigning them a maximum volume for the initial mesh. An unstructured tetrahedral mesh used in our approach can provide an accurate description of complex structures such as dipping layers and rough topography that are not accurately fitted using structured meshes. We first validate the adaptive FE approach and demonstrate the convergence of the adaptive grid refinement procedure using a 1-D layered model. The canonical disc model example illustrates the capability of the adaptive FE approach for 3-D CSEM modeling. The bathymetry model shows that the algorithm is well suited to deal with complex seafloor topography, which needs to be simulated exactly to avoid the misinterpretation of marine CSEM data sets.

The deep-tow marine controlled-source electromagnetic transmitter system for gas hydrate exploration

The Marine Controlled-Source Electromagnetic (MCSEM) method has been recognized as an important and effective tool to detect electrically resistive structures, such as oil, gas, and gas hydrate. The MCSEM performance is strongly influenced by the transmitter system design. We have developed a deep-tow MCSEM transmitter system. In this paper, some new technical details will be present. A 10,000 m optical–electrical composite cable is used to support high power transmission and fast data transfer; a new clock unit is designed to keep the synchronization between transmitter and receivers, and mark the time stamp into the transmission current full waveform; a data link is established to monitor the real-time altitude of the tail unit; an online insulation measuring instrument is adopted to monitor current leakage from high voltage transformer; a neutrally buoyant dipole antenna of copper cable and flexible electrodes are created to transmit the large power current into seawater; a new design method for the transmitter, which is called “real-time control technology of hardware parallelism”, is described to achieve inverting and recording high-power current waveform, controlling functions, and collecting auxiliary information. We use a gas hydrate exploration test to verify the performance of the transmitter system, focusing on more technical details, rather than applications. The test shows that the transmitter can be used for gas hydrate exploration as an effective source.

Seafloor bathymetry in deep and shallow water marine CSEM responses of Nigerian Niger Delta oil field: Effects and corrections

Topography distortions in bathymetrically acquired marine Controlled-Source Electromagnetic (mCSEM) responses are capable of misleading interpretation to the presence or absence of the target if not corrected for. For this reason, the effects and correction of bathymetry distortions on the deep and shallow seafloor mCSEM responses of the Niger Delta Oil province were examined in this paper. Marine CSEM response of the Niger Delta geological structure was modelled by using a 2.5D adaptive finite element forward modelling code. In both the deep water and shallow water cases, the bathymetry distortions in the electric field amplitude and phase were found to get smaller with increasing Tx–Rx offsets and contain short-wavelength components in the amplitude curves which persist at all Tx–Rx offsets. In the deep water, topographic effects on the reservoir signatures are not significant, but as water depth reduces, bathymetric distortions become more significant as a result of the airwave effects, masking the target signatures. The correction technique produces a good agreement between the flat-seafloor reservoir model and its equivalent bathymetric model in deep water at 0.25Hz, while in shallow water, the corrected response only shows good agreement at shorter offsets but becomes complicated at longer offsets due to airwave effects. Transmission frequency was extended above and below 0.25Hz in the frequency spectrum and the correction method applied. The bathymetry correction at higher frequency (1.75Hz) is not effective in removing the topographic effects in either deep or shallow water. At 0.05Hz for both seafloor scenarios, we obtained the best corrected amplitude profiles, removing completely the distortions from both topographic undulation and airwave effects in the shallow water model. Overall, the work shows that the correction technique is effective in reducing bathymetric effects in deep water at medium frequency and in both deep and shallow waters at a low frequency of 0.05Hz.

가스 하이드레이트 부존양상 도출을 위한 해양 전자탐사 자료의 겉보기 비저항 계산

해양 전자탐사의 겉보기 비저항은 해수층으로 인해 지표탐사와 그 정의가 달라지게 되며, 이를 적절히 계산할 수 있는 알고리듬의 개발은 해양 전자탐사의 출발점이 될 수 있다. 이를 위해, 1차원 층서 가스 하이드레이트 수치모형과 해수층과 그 하부의 반 무한매질로 이루어진 수치모형에서 계산한 전자기적 반응을 비교분석하였다. 겉보기 비저항을 계산하기 위해서는 실수와 허수 성분보다는 진폭과 위상을 사용하는 것이 더 적합하였으며 해양 전자탐사 반응의 민감도를 정량적으로 분석하여, 근거리 영역에서는 위상이 원거리 영역에서는 진폭 성분이 더 안정적인 결과를 주는 것을 알았다. 또한 위상과 진폭의 선택기준으로써 유도상수의 값을 제안하였다. 이러한 분석을 토대로 격자 탐색법(grid search)을 사용하여 겉보기 비저항을 계산하는 수치알고리듬을 개발하였다. 개발된 알고리듬을 이용하여 1차원 층서 가스 하이드레이트 수치모형의 다양한 변수를 변화시켜가며 겉보기 비저항을 계산해봄으로써 알고리듬의 타당성을 검증하였다. 마지막으로, 계산한 겉보기 비저항 값을 이용한 가스 하이드레이트 부존양상 정보의 도출가능성을 살펴보았다. 동해 울릉분지의 가스 하이드레이트 부존양상을 모사한 2차원 가스 하이드레이트 수치모형에서 계산된 자료의 겉치레 단면도는 가스 하이드레이트 부존양상 정보 추출이 가능함을 보여주었다. The sea layer in marine Controlled-Source Electromagnetic (mCSEM) survey changes the conventional definition of apparent resistivity which is used in the land CSEM survey. Thus, the development of a new algorithm, which computes apparent resistivity for mCSEM survey, can be an initiative of mCSEM data interpretation. First, we compared and analyzed electromagnetic responses of the 1D stratified gas hydrate model and the half-space model below the sea layer. Amplitude and phase components showed proper results for computing apparent resistivity than real and imaginary components. Next, the amplitude component is more sensitive to the subsurface resistivity than the phase component in far offset range and vice versa. We suggested the induction number as a selection criteria of amplitude or phase component to calculate apparent resistivity. Based on our study, we have developed a numerical algorithm, which computes appropriate apparent resistivity corresponding to measured mCSEM data using grid search method. In addition, we verified the validity of the developed algorithm by applying it to the stratified gas hydrate models with various model parameters. Finally, by constructing apparent resistivity pseudo-section from the mCSEM responses with 2D numerical models simulating gas hydrate deposits in the Ulleung Basin, we confirmed that the apparent resistivity can provide the information on the geometric distribution of the gas hydrate deposit.

Marine controlled-source electromagnetic interferometry

In marine Controlled-Source Electromagnetics, a boat tows an electric source, whose signal is travelling on various paths to the receiver stations at the ocean bottom. Unfortunately, the signal does not only travel via the subsurface to the receivers, but also directly through the water and via the air-water interface. Signals travelling on the latter two travelpaths do not contain any information about the subsurface. On the contrary, they cover a possible response from a subsurface reservoir. Therefore, one aims to suppress the signal travelling along those paths. Interferometry by multidimensional deconvolution replaces the overburden by a homogeneous halfspace suppressing any interactions with the air-water interface. Furthermore, the direct field is removed and the source is redatumed to a receiver position. Since interferometry by multidimensional deconvolution is a data-driven method, no information about the ocean or the subsurface is needed, except the material parameters at the receiver level. This thesis investigates the benefits and limitations of interferometry by multidimensional deconvolution applied to marine Controlled-Source Electromagnetic data.

2.5D inversion of CSEM data in a vertically anisotropic earth

The marine Controlled-Source Electromagnetic (CSEM) method is a low frequency (diffusive) electromagnetic subsurface imaging technique aimed at mapping the electric resistivity of the earth by measuring the response to a source dipole emitting an electromagnetic field in a marine environment. Although assuming isotropy for the inversion is the most straightforward approach, in many situations horizontal layering of the earth strata and grain alignment within earth materials creates electric anisotropy. Ignoring this during interpretation may create artifacts in the inversion results. Accounting for this effect therefore requires adequate forward modelling and inversion procedures. We present here an inversion algorithm for vertically anisotropic media based on finite element modelling, the use of Fréchet derivatives, and different types of regularisation. Comparisons between isotropic and anisotropic inversion results are given for the characterisation of an anisotropic earth from data measured in line with the source dipole for both synthetic and real data examples.

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

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.

A Magnetic Gradiometric Method as an Adjunct for Marine CSEM

The marine Controlled-Source Electromagnetic (CSEM) method has proven itself an invaluable adjunct to the seismic method in petroleum exploration in the last few years. The problem is that it is not easily adapted to the Australian scene because most of Australia's petroleum deposits are in relatively shallow waters where CSEM does not work very well. Depending on the target, depths up to 500 m can be considered shallow water for CSEM. CSEM surveys in shallow water can have problems with the airwave dominating the received CSEM signal and also with electromagnetic noise induced by ocean waves. Modelling studies suggest that it is feasible to measure CSEM magnetic field gradients as a supplement to E and B field measurements and use this extra information to remove environmental noise (wave effects and magnetotelluric noise), airwave and direct wave components, and other unwanted contributions to the measured E and B fields. This can improve the signal/noise ratio of the CSEM measurements, thereby producing more reliable interpretations, and extend the useful range of the method, which can reduce survey costs. We have been developing a magnetic gradiometer that can be used for marine CSEM surveys. This poster will present the results from preliminary field trials conducted off Sydney's coast, in water depths up to 62 m, which show that the OceanMAG gradiometer was able to measure the magnetic field induced by ocean waves. Approaches for using magnetic gradient measurements as an adjunct to marine CSEM data will also be described.