Marine Streamer Research Articles

Envelope normalized reflection waveform inversion

AbstractThe reflection waveform inversion has the capability to reconstruct the background velocity model using only the reflection data by employing a migration/demigration process. Utilizing the waveform discrepancy to update the background velocity model, the conventional reflection waveform inversion method heavily relies on the true‐amplitude migration/demigration technique to reproduce the primary amplitude information from the observed reflections. We can reproduce the amplitude of observed reflections by performing least‐squares reverse time migration to estimate the reflectivity in each iteration. However, this strategy is quite time‐consuming. To avoid the need for the true‐amplitude migration/demigration or least‐squares reverse time migration, we develop an amplitude‐independent reflection waveform inversion method that uses an envelope‐normalized objective function. The envelope‐normalized waveform difference can extract the phase residuals accurately as a function of time. Compared with the global energy–normalized misfit, our proposed envelope‐normalized objective function is essentially a phase‐matched measurement. At the same time, due to the amplitude independence of our proposed objective function, the subsequent weak reflections contribute with a similar weight to the total value of the misfit as the strong early reflections do. This makes it possible to recover the deep subsurface velocity. Synthetic data of the Sigsbee model and marine streamer field data applications validate that our amplitude‐independent reflection waveform inversion method can further improve the resolution and accuracy by aligning the reflection events of synthetic and observed data phase to phase without the need to perform true‐amplitude migration/demigration or least‐squares reverse time migration as in conventional reflection waveform inversion.

High-resolution seismic imaging of shallow structure at proposed IODP drilling sites, Kane oceanic core complex, Mid-Atlantic Ridge

Two 500-m-deep holes are proposed to be drilled at the Kane oceanic core complex (OCC), one aimed at a serpentinized peridotite massif and the other at a gabbroic body. Ocean drilling is vital to validate lithological interpretation derived from seismic structures. Utilizing a long marine streamer, seismic imaging can effectively delineate the dominant lithologies within the OCCs. This study applied a suite of techniques, including downward-continued multi-channel seismic data, full waveform inversion, and reverse time migration, to obtain detailed fine-scale shallow structures beneath the Kane OCC. Through the downward continuation method, refracted seismic data at near offsets were utilized, thus doubling the resolution at shallow depths. Compared to previous findings, our results greatly enhance the understanding of shallow subseafloor structures, revealing a more precise morphology of the gabbroic bodies and well-imaged very shallow low velocities caused by seawater percolation in the shallow fissure due to footwall rotation extension. We present reference drilling hole velocities and link them to the likely composition, considering potential alteration processes such as serpentinization of mantle peridotite. Our investigation suggests that the lower crustal melt fluxes of the Kane OCC represent a transitional phase from low to medium accretion, providing valuable insights for future scientific ocean drilling projects in this region.

Innovative application of full-waveform inversion applied to extended wide-azimuth marine streamer seismic data in a complex salt environment

We demonstrate the application of full-waveform inversion (FWI) guided velocity model building to an extended wide-azimuth towed streamer (EWATS) seismic data set in the Gulf of Mexico. Field data are collected over a historically challenging imaging area, colloquially called the “grunge zone” due to the formation of a compressional allosuture emplaced between two colliding salt sheets. These data have a poor subsalt image below the suture with conventional narrow-azimuth data. Additional geologic complexities are observed including high-velocity carbonate carapace near the top of salt and multiple intrasalt sedimentary inclusions. As such, improved seismic imaging is required to plan and execute wells targeting subsalt strata. Significant improvements to the velocity model and subsalt image have been evident with wide-azimuth towed streamer and later EWATS data using conventional top-down velocity model building approaches. Subsequently, high-impact improvements have been made using EWATS data with an FWI velocity model building workflow; this study represents an early successful application of FWI used to update salt body geometries from streamer seismic data, wherein many prior applications were limited to improving sedimentary velocities. Later petrophysical data have verified the new FWI-derived model, which has significantly increased confidence in the structural and stratigraphic interpretation of subsalt reservoir systems below the grunge zone.

Multi-Task Learning for Low-Frequency Extrapolation and Elastic Model Building From Seismic Data

Low-frequency signal content in seismic data as well as a realistic initial model are key ingredients for robust and efficient full-waveform inversions. However, acquiring low-frequency data is challenging in practice for active seismic surveys. Data-driven solutions show promise to extrapolate low-frequency data given a high-frequency counterpart. While being established for synthetic acoustic examples, the application of bandwidth extrapolation to field datasets remains non-trivial. Rather than aiming to reach superior accuracy in bandwidth extrapolation, we propose to jointly reconstruct low-frequency data and a smooth background subsurface model within a multi-task deep learning framework. We automatically balance data, model and trace-wise correlation loss terms in the objective functional and show that this approach improves the extrapolation capability of the network. We also design a pipeline for generating synthetic data suitable for field data applications. Finally, we apply the same trained network to synthetic and real marine streamer datasets and run an elastic full-waveform inversion from the extrapolated dataset.

Pseudoelastic pure P-mode wave equation

We have developed a pseudoelastic wave equation describing pure pressure waves propagating in elastic media. The pure pressure-mode (P-mode) wave equation uses all of the elastic parameters (such as density and the P- and S-wave velocities). It produces the same amplitude variation with offset (AVO) effects as PP-reflections computed by the conventional elastic wave equation. Because the new wave equation is free of S-waves, it does not require finer grids for simulation. This leads to a significant computational speedup when the ratio of pressure to S-wave velocities is large. We test the performance of our method on a simple synthetic model with high-velocity contrasts. The amplitude admitted by the pseudoelastic pure P-mode wave equation is highly consistent with that associated with the conventional elastic wave equation over a large range of incidence angles. We further verify our method’s robustness and accuracy using a more complex and realistic 2D salt model from the SEG Advanced Modeling Program. The ideal AVO behavior and computational advantage make our wave equation a good candidate as a forward simulation engine for performing elastic full-waveform inversion, especially for marine streamer data sets.

Subbasalt Marchenko imaging with offshore Brazil field data

Subbasalt imaging for hydrocarbon exploration faces challenges with the presence of multiple scattering, attenuation, and mode conversion as seismic waves encounter highly heterogeneous and rugose basalt layers. A combination of modern seismic acquisition that can record densely sampled data and advanced imaging techniques make imaging through basalt feasible. Yet, the internal multiples, if not properly handled during seismic processing, can be mapped to reservoir layers by conventional imaging methods, misguiding geologic interpretation. Traditional internal multiple elimination methods suffer from the requirement of picking horizons of multiple generators and/or a top-down adaptive subtraction process. Marchenko imaging provides an alternative solution to directly remove the artifacts due to internal multiples, without the need of horizon picking or subtraction. In this paper, we evaluate a successful application of direct Marchenko imaging for subbasalt demultiple and imaging with an offshore Brazil field data set. The internal multiples in this example are generated from the seabed and basalt layers, causing severe artifacts in conventional seismic images. We have determined that these artifacts are largely suppressed with Marchenko imaging, and we have developed a general workflow for data preprocessing and regularization of marine streamer data sets. We find that horizontally propagating waves also can be reconstructed by the Marchenko method at far offsets.

Deblending of Simultaneous-Source Seismic Data Using Bregman Iterative Shaping

Deblending plays an important role in preparing high-quality seismic data from modern blended simultaneous-source seismic data. The conventional methods become problematic when the random ambient noise becomes extremely strong and the inversion iteratively fits the random noise instead of the signal and blending interference. In this article, an improved method is proposed to separate blended seismic data. We treat the deblending problem as a regularization problem under the frame of compressed sensing (CS), and propose a new nonlinear Bregman iterative shaping (BIS) algorithm to solve the minimization problem. Based on the principle of CS, we transform the simultaneous-source data to the seislet-domain for inversion and separation, and verify the effectiveness and superiority of the proposed method by two theoretical model data and an actual marine streamer blended data. Compared with the traditional iterative shaping (TIS) algorithm, BIS adds data residuals to the observed data (the blended data) in each iteration, so that the error between the calculated data after each iteration and the main source signal in the known blended data is the smallest, and helps to recover the weaker information in the seismic data and obtain higher precision separation results. BIS uses a fixed soft threshold, which speeds up the convergence speed. In addition, BIS can effectively improve the separation accuracy when the given sparse domain threshold is not suitable.

Adaptive de-blending of dithered simultaneous sources

In marine seismic data acquisition, de-blending multiple simultaneous sources is a challenging, undetermined problem and sparsity constraints are often used to derive a stable solution. In this article, we discuss the impact of additional adaptive constraints. In particular, we investigate conformity to a) direct wave modelling, b) ghost modelling and c) variations of signal-to-noise ratio. We find that these additional constraints improve convergence significantly. We demonstrate the performance of the proposed approach on a narrow-azimuth marine streamer survey acquired with triple-source simultaneous shooting and natural dithers. The proposed approach addresses the key de-blending challenges for this dataset which are protecting aliased steeply dipping events and avoiding signal damage.

Sub-Basalt Imaging Reveals Deeper Plays Offshore India

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30279, “Revealing Deeper Plays, Offshore Kutch, India: A Success Story of Sub-Basalt Imaging,” by S.K. Biswal, N.N.B. Naidu, and S. Basu, ONGC, et al., prepared for the 2020 Offshore Technology Conference Asia, originally scheduled to be held in Kuala Lumpur, 2-6 November. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. The Deccan Traps volcanic province of India is considered one of the largest basalt-covered regions in the world but is essentially unexplored because of the limitations of conventional marine streamer P-wave seismic acquisition in imaging structures both intrabasalt and sub-basalt. In the complete paper, the authors demonstrate that, even with legacy marine streamer surveys, an appropriate work flow of combining suitable advanced technologies can help to overcome the long-standing challenges of sub-basalt imaging. The reprocessed data show clear uplift in sub-basalt imaging, and inversion results validate the quality of the new data in relation to the well logs. Introduction The Kutch offshore basin is characterized by the presence of the Deccan Traps, a large igneous province of up to 2000-m-thick basalt lava flows. These lava flows have hindered successful imaging of sub-basalt Mesozoic sediments for hydrocarbon exploration. To date, no single technique has been found to produce considerable improvements in deeper image quality. The solution lies in an appropriate combination of advanced technologies. The project consists of three legacy data sets acquired in 2004, 2010, and 2014 in the shallow-water area (water depth ranges from 25 to 50 m). Two of the surveys were shot in the north/south direction with six streamers having 100-m separation, 25-m shot spacing, 12.5-m receiver spacing, and 6-second record length. The third survey was acquired oblique to these with a similar acquisition geometry; however, it featured sparser 25-m receiver spacing and 8- second record length. These surveys were matched and merged before migration to ensure a seamless image across the surveys in the post-migration domain. A tailored processing work flow improved existing data quality significantly and provided new insights into the sub-basalt geology, thereby opening a new play to exploration and production. Challenges and Work Flow Sub-basalt imaging challenges include transmission losses, scattering, complex wave kinematics, prevalent multiples, interference effects, and variable illumination caused by high and variable acoustic impedance of thick heterogeneous basalt layers. The tertiary sedimentary sequences overlying the Deccan Trap consist predominantly of carbonates, shale, and fine-grained clastic sediments, accompanied by channels and nearly vertical faulting. The geological complexities from the water bottom to the base of the basalt present a substantial geophysical challenge to successful deeper imaging and require an appropriate work flow to mitigate them. Broadband processing, including de-ghosting, can increase the signal-to-noise ratio across the broad range of frequencies in the seismic bandwidth and can enhance the lower frequencies required to achieve enhanced imaging at sub-basalt targets. Demultiple methods can reduce the presence of surface-related and interbed multiples that prohibit reliable interpretation of Mesozoic sediment; imaging methods can focus the recorded data when used in conjunction with an accurate Earth model that captures the velocity complexities of carbonates, shale, basalt, channel, and faults.

Reprocessed time-lapse seismic data provides new reservoir information on the Njord field

Improvements in time-lapse (4D) seismic processing methods provide opportunities to extract additional information from older time-lapse seismic data sets. This new information can help to optimize reservoir production. Four marine streamer time-lapse seismic data sets on the Njord field, offshore mid-Norway, were previously processed in 2007. Reprocessing in 2020 concentrated on better noise and multiple attenuation, improved imaging, and a focused step-by-step workflow to estimate and correct timing variations within and between surveys. The results dramatically improved on the 2007 processing. Gas injection could be tracked in greater detail and previously unidentified 4D signal was interpreted. The new data sets are expected to enhance reservoir understanding and improve future field performance.

Signal Detection and Enhancement for Seismic Crosscorrelation Using the Wavelet-Domain Kalman Filter

Crosscorrelation is a classical signal-processing technique that plays an important role in exploration and earthquake geophysics. Seismic velocity estimation utilizes the crosscorrelation between observed and predicted seismic records in traveltime tomography. The crosscorrelation between two stations represents the Green’s functions retrieved from ambient noises in passive seismic interferometry. It can be used to estimate the subsurface velocity and amplitude information. The calculation of crosscorrelation usually assumes that the input data are stationary; however, the real seismic data are often non-stationary, due to the presence of multiple wave-modes and background noises. The seismic crosscorrelations often have low signal-to-noise ratio and frequently fail to provide correct information for subsequent processing. To address this problem, we develop a comprehensive technique to reduce contamination and improve the quality of crosscorrelation in the wavelet domain. The new procedure includes the forward wavelet transformation of raw records, the crosscorrelation between wavelet coefficients, single-channel image object detection, multi-channel Kalman-filter object tracking, and inverse wavelet transformation to produce the new crosscorrelation gathers. We effectively remove the unwanted components associated with contaminated wave-modes as the proposed detection and tracking algorithm can accurately extract the target wave-mode. We validate the method for three datasets: a marine streamer survey, a borehole survey, and a broadband dataset from seismology stations. We demonstrate that the proposed method can significantly improve the signal-to-noise ratio of the seismic crosscorrelations, considerably enhancing the quality of the data for subsequent advanced crosscorrelation-based seismic processing.

Seismic acquisition and imaging strategies for unlocking subsurface complexities in Malaysian basins

Seismic data acquisition and imaging technologies have made important contributions to hydrocarbon discoveries and enhancing recovery from existing reservoirs in Malaysian basins. PETRONAS has been leveraging these technologies to address the exploration and development challenges encountered in Malaysian basins and other parts of the world and to support its play-based exploration. Some of the key technology applications include imaging below shallow gas and carbonates, imaging of complex geology for exploring deep water and deep plays, and high-resolution imaging of shallow clastic plays. Dual and multimeasurement streamers, multiazimuth and full-azimuth seismic, triple- and penta-source blended acquisition, and two- and four-component seabed seismic technologies have been integrated with high-end processing and imaging technologies such as advanced demultiple techniques, deblending, full-waveform inversion, reverse time migration, and PS-converted wave imaging to address complex subsurface challenges. In this article, we present an overview of the evolution and application of innovative seismic acquisition and imaging technologies in Malaysian basins. Selected case histories demonstrate how these technologies have enabled explorers to unlock subsurface complexities, adding value to exploration and development activities. We share the advancements in 3D seismic survey design, marine streamer acquisition, seabed seismic acquisition, and seismic imaging technologies, and how an integrated approach helped PETRONAS address geologic challenges to enhance its exploration success.

Elastic Least-Squares Imaging in Tilted Transversely Isotropic Media for Multicomponent Land and Pressure Marine Data

Traditional elastic reverse-time migration (RTM) involves P-/S-wave separation for the source and receiver wavefields, followed by applying the zero-lag cross-correlation imaging condition to produce PP and PS images. In anisotropic media, P-/S-wave decomposition requires a higher memory and computational cost than that in isotropic media. In addition, finite acquisition apertures and band-limited source functions result in unsatisfactory resolutions and amplitudes. To mitigate these problems, we present an elastic least-squares imaging method for tilted transversely isotropic media and apply it to land multicomponent and marine pressure data. Unlike traditional RTM, we use the relative perturbations to the product of density and squared axial (compressional/shear) velocities as reflectivity models ($$\Delta \ln{C}_{33}$$ and $$\Delta \ln{C}_{55}$$), and estimate them by solving a linear inverse problem. Numerical experiments illustrate that subsurface reflectors can be well resolved in adjoint images for land multicomponent data, because of the presence of both P- and S-waves in seismograms. Least-squares migration helps to further improve spatial resolution and image amplitudes. Since there are no direct S-waves in marine streamer data, adjoint RTM images of $$\Delta \ln{C}_{55}$$ are mainly resolved with the converted S-waves and are not as good as those in $$\Delta \ln{C}_{33}$$ images. By approximating the Hessian inverse, least-squares migration allows us to take advantage of the weak converted P–S–P-waves and improve the $$\Delta \ln{C}_{55}$$ image quality. Numerical experiments for synthetic and field data demonstrate the feasibility and advantage of the proposed least-squares TTI RTM compared with wave-mode separation-based elastic RTM. In field data experiments, we observe that since there are no strong P–S–P converted waves in streamer pressure records from the marine survey, the reflectors in $$\Delta \ln{C}_{55}$$ image might be mainly imaged from P-waves due to the amplitude versus offset (AVO) effects.

Optical fiber marine seismic exploration system feasibility study

The signal-to-noise ratio (SNR), vertical resolution and fidelity of marine seismic data are closely related to the characteristics of hydrophones, namely the sensitivity and the frequency band range. In this article, we analyzed and processed seismic data collected in 2017 from a towed marine streamer equipped with optical fiber cable and hydrophones. Results from a pre-stack time migration section revealed that the optical fiber seismic data yielded high SNR, vertical resolution and fidelity. For validation purpose, a comparison of these data with those acquired by classic piezoelectric hydrophones along a migration section located in a very close location was carried out. This comparison showed that the SNR, fidelity and vertical resolution from the two means of measurements were comparable.

The Computation of Attributes from Multi-fold Seismic Refraction Data

Seismic attributes are any measure that helps to better visualize or quantify features of interest in seismic data. Refraction attributes can be readily computed from multifold seismic data using the standard algorithms of the generalized reciprocal method (GRM), a common offset refraction (COR) modification of the GRM, and the refraction convolution section (RCS). The COR GRM algorithms are applied prior to the parameterization of the traveltime data into individual layers. The COR GRM presentations facilitate the efficient quality control and interpretation of large sets of multi-fold data. Furthermore, the COR GRM algorithms can be readily applied to single ended refraction data, which are routinely recorded with marine and land streamers. The time model attribute obtained with the COR GRM algorithm is of comparable accuracy to that computed with the standard multi-fold GRM. However, the COR GRM seismic velocity model is a smoothed version of the fully optimized standard GRM model. The refraction attributes computed from the head wave amplitudes include the multi-fold head coefficient, and a COR equivalent with the amplitude product. Although the COR amplitude product for each offset is of comparable accuracy to the multi-fold head coefficient, it also includes a residual geometric spreading component. Another refraction attribute computed from the head wave amplitudes is the geometric spreading coefficient, which is generally assumed to be two in the far field for plane homogeneous refractors. This parameter is consistent over a wide range of offsets, but varies considerably from less than one to up to four along the seismic traverse The attribute derived from the RCS is bandwidth in octaves. It is proposed that refraction attributes can be employed to derive more useful models of the regolith using multivariate geostatistics. It is concluded that, refraction attributes offer the opportunity of extending the refraction method to new applications.

Shot repetition: An alternative seismic blending code in marine acquisition

In blended seismic acquisition, or simultaneous source seismic acquisition, source encoding is essential at the acquisition stage to allow for separation of the blended sources at the processing stage. In land seismic surveys, the vibroseis sources may be encoded with near-orthogonal sweeps for blending. In marine seismic surveys, the sweep type of source encoding is difficult because the main source type in marine seismic exploration is the air-gun array, which has an impulsive character. Another issue in marine streamer seismic data acquisition is that the spatial source sampling is generally coarse. This hinders the deblending performance of algorithms based on the random time delay blending code that inherently requires a dense source sampling because they exploit the signal coherency in the common-receiver domain. We have developed an alternative source code called shot repetition that exploits the impulsive character of the marine seismic source in blending. This source code consists of repeated spikes of ones and can be realized physically by activating a broadband impulsive source more than once at (nearly) the same location. Optimization of the shot-repetition type of blending code was done to improve the deblending performance. As a result of using shot repetition, the deblending process can be carried out in individual shot gathers. Therefore, our method has no need for a regular dense source sampling: It can cope with irregular sparse source sampling; it can help with real-time data quality control. In addition, the use of shot repetition is beneficial for reducing the background noise in the deblended data. We determine the feasibility of our method on numerical examples.

Optimal full-waveform inversion strategy for marine data in azimuthally rotated elastic orthorhombic media

The orthorhombic (ORT) anisotropic description of earth layers can allow the capture of much of the earth’s anisotropic complexity. The inversion for high-resolution azimuthal variation of anisotropy is important for reservoir characterization, among other applications. A high-resolution description of the azimuth of fractures can help us to predict flow preferences. To verify the feasibility of multiparameter full-waveform inversion (FWI) for marine data assuming azimuthally rotated elastic ORT media, we have analyzed the radiation patterns and gradient directions of ORT parameters to the reflection data. First, we express the gradient direction of the ORT parameters considering the azimuthal rotation of the symmetric planes. Then, to support our observations in the gradient direction, the radiation patterns of the partial derivative wavefields from each parameter perturbation are also derived under the rotated elastic ORT assumption. To find an optimal parameterization, we compare three different parameterizations: monoclinic, velocity-based, and hierarchical parameterizations. Then, we suggest an optimal multistage update strategy by analyzing the behavior of the rotation angle as a FWI target. To analyze the trade-off among parameters in different parameterizations, we calculate the gradient direction from a hockey-puck model, in which each parameter is perturbed at the different location on a horizontal layer. The trade-off analysis supports that the hierarchical parameterization provides us with more opportunities to build up subsurface models with less trade-off between parameters and less influence of the azimuthal rotation of ORT anisotropy. The feasibility of the proposed FWI strategy is examined using synthetic marine streamer data from a simple 3D reservoir model with a fractured layer.

Full‐waveform inversion with multisource frequency selection of marine streamer data

ABSTRACTThe theory and practice of multisource full‐waveform inversion of marine supergathers are described with a frequency‐selection strategy. The key enabling property of frequency selection is that it eliminates the crosstalk among sources, thus overcoming the aperture mismatch of marine multisource inversion. Tests on multisource full‐waveform inversion of synthetic marine data and also the Gulf of Mexico data show speedups of 4 × and 8 × , respectively, compared with conventional full‐waveform inversion.

Operational deployment of compressive sensing systems for seismic data acquisition

Compressive sensing (CS) provides a new basis for sampling that can increase sampling efficiency for seismic data acquisition by an order of magnitude. A major challenge for this new technology is to show that theoretical increases in sampling efficiency can be translated to real efficiency gains in the field. Along with efficiency gains, data quality must be preserved in order to gain acceptance of a new acquisition technology. CS designs require solution of large optimization problems that are consistent with compressive sampling theory. We refer to our optimization framework for CS-based acquisition design and processing as compressive seismic imaging (CSI). We illustrate our CSI framework on example projects for ocean-bottom node, narrow-azimuth marine streamer, and land vibroseis acquisition. The ocean-bottom-node project was conducted in the UK North Sea during the difficult winter season. A CSI dual-source design was used to significantly reduce shooting time for this project. The project was completed on time, under budget, and with data quality that exceeded the quality of an overlapping uniformly sampled survey. The narrow-azimuth marine CSI survey project was acquired in offshore Australia for field development purposes. Nonuniform CSI sampling was used to increase sampling efficiency for both sources and cables, resulting in significant improvements in data quality and lateral resolution. The land vibroseis project was conducted on the North Slope of Alaska. In this case, the goal was to acquire a development survey of sufficient size within a short time window. Nonuniform CSI sampling was used to support the use of 10 or more vibrators shooting simultaneously, along with improving sampling efficiency for both sources and receivers. Compared to conventional designs, the CSI survey achieved an order of magnitude improvement in field acquisition efficiency and step-function improvements in data quality. These examples show that theoretical improvements in sampling efficiency from CS can make real and significant impacts on seismic data acquisition and processing.

Survey on Methane Hydrate Detection with Marine Electromagnetic

Methane hydrate (MH) is required to be another vitality asset in light of the fact that a lot of methane gas might be contained in the MH layer. MH has high resistive element, so that the marine EM studies will be a valuable apparatus for recognizing subseafloor MH. We present two contextual investigations of marine electromagnetic overviews off Japan drift to identify MH beneath the ocean bottom. The primary case is done in the Sea of Japan, 2005. Our marine profound towed EM streamer link could picture the subseafloor resistivity dissemination to the profundity of 100m beneath the ocean bottom, and effectively distinguished MH zones as high resistivity. The second case, a marine CSEM try, with a profound towed link and a sea base electromagnetometer (OBEM), is done off the Tokai territory, along the Pacific side of Japan, 2006. A thin and profound MH zone (with thickness of around 30m and profundity of around 200m beneath the ocean bottom) perceived in the borehole is not obviously imaged by our overview. The reason is principally because of variance of the track and bearing of profound towed source dipole. Further and cautious investigation of CSEM information with acoustic route information will enable us to picture the profound MH zone.