3D controlled-source electromagnetic imaging of gas hydrates: Insights from the Pelotas Basin offshore Brazil

2019 ◽  
Vol 7 (4) ◽  
pp. SH111-SH131 ◽  
Author(s):  
Raghava Tharimela ◽  
Adolpho Augustin ◽  
Marcelo Ketzer ◽  
Jose Cupertino ◽  
Dennis Miller ◽  
...  
Keyword(s):  
Gas Hydrates ◽  
Gas Hydrate ◽  
A Priori ◽  
Inversion Algorithm ◽  
Near Surface ◽  
Bottom Simulating Reflector ◽  
Controlled Source ◽  
Lateral Extent ◽  
Priori Information ◽  
Pelotas Basin

Mapping of natural gas hydrate systems has been performed successfully in the past using the controlled-source electromagnetic (CSEM) method. This method relies on differentiating resistive highly saturated free gas or hydrate-bearing host sediment from a less resistive low-saturated gas or brine-bearing host sediments. Knowledge of the lateral extent and resistivity variations (and hence the saturation variations) within sediments that host hydrates is crucial to be able to accurately quantify the presence of saturated gas hydrates. A 3D CSEM survey (PUCRS14) was acquired in 2014 in the Pelotas Basin offshore Brazil, with hydrate resistivity mapping as the main objective. The survey was acquired within the context of the CONEGAS research project, which investigated the origin and distribution of gas hydrate deposits in the Pelotas Basin. We have inverted the acquired data using a proprietary 3D CSEM anisotropic inversion algorithm. Inversion was purely CSEM data driven, and we did not include any a priori information in the process. Prior to CSEM, interpretation of near-surface geophysical data including 2D seismic, sub-bottom profiler, and multibeam bathymetry data indicated possible presence of gas hydrates within features identified such as faults, chimneys, and seeps leading to pockmarks, along the bottom simulating reflector and within the gas hydrate stability zone. Upon integration of the same with CSEM-derived resistivity volume, the interpretation revealed excellent spatial correlation with many of these features. The interpretation further revealed new features with possible hydrate presence, which were previously overlooked due to a lack of a clear seismic and/or multibeam backscatter signature. In addition, features that were previously mapped as gas hydrate bearing had to be reinterpreted as residual or low-saturated gas/hydrate features, due to the lack of significant resistivity response associated with them. Furthermore, we used the inverted resistivity volume to derive the saturation volume of the subsurface using Archie’s equation.

3D inversion of a scalar radio magnetotelluric field data set

Geophysics ◽  
2003 ◽  
Vol 68 (3) ◽  
pp. 791-802 ◽  
Author(s):  
Gregory A. Newman ◽  
Stephan Recher ◽  
Bülent Tezkan ◽  
Fritz M. Neubauer
Keyword(s):  
Field Data ◽  
A Priori ◽  
3D Analysis ◽  
Borehole Data ◽  
Erroneous Conclusion ◽  
Data Set ◽  
Near Surface ◽  
Lateral Extent ◽  
Priori Information ◽  
2D Data

A radio magnetotelluric (MT) field data set, acquired in scalar mode, over a buried waste site has been successfully analyzed using a 3D MT inversion scheme using nonlinear conjugate gradients. The results of this analysis demonstrate the utility of the scheme where more than 4800 data points collected on multiple measurement profiles have been inverted simultaneously. The resulting image clearly detects the buried waste; when receiver profiles cross pit boundaries, the image maps the lateral extent of the pit. However, the base of the pit is poorly resolved, and depends upon the starting model used to launch the inversion. Hence, critical information on whether contamination is leaching into a resistive gravel bed lining the base of the pit, as well as the deeper geological horizons consisting of brown coal, clay, and tertiary sands, is inconclusive. Nevertheless, by incorporating within the inversion process a priori information of the background media that is host to the waste, sharper images of the base of the pit are obtained, which are in good agreement with borehole data. The 3D analysis applied in this paper overcomes previous limitations in the radio magnetotelluric (RMT) method using 2D data analysis and inversion. With 3D analysis, it is unnecessary to make assumptions regarding geological strike, and near‐surface statics can be accommodated in both source polarizations. Our findings also indicate that 2D MT interpretation can overestimate the pit's depth extent. This may lead to the erroneous conclusion that the geological horizons beneath the pit have been contaminated.

scholarly journals Two-grid full-waveform Rayleigh-wave inversion via a genetic algorithm — Part 1: Method and synthetic examples

Geophysics ◽  
2019 ◽  
Vol 84 (5) ◽  
pp. R805-R814 ◽  
Author(s):  
Zhen Xing ◽  
Alfredo Mazzotti
Keyword(s):  
Genetic Algorithm ◽  
Rayleigh Wave ◽  
A Priori ◽  
Computing Time ◽  
A Priori Information ◽  
Fine Grid ◽  
Near Surface ◽  
Reference Models ◽  
Full Waveform ◽  
Priori Information

When reliable a priori information is not available, it is difficult to correctly predict near-surface S-wave velocity models from Rayleigh waves through existing techniques, especially in the case of complex geology. To tackle this issue, we have developed a new method: two-grid genetic-algorithm Rayleigh-wave full-waveform inversion (FWI). Adopting a two-grid parameterization of the model, the genetic algorithm inverts for unknown velocities and densities at the nodes of a coarse grid, whereas the forward modeling is performed on a fine grid to avoid numerical dispersion. A bilinear interpolation brings the coarse-grid results into the fine-grid models. The coarse inversion grid allows for a significant reduction in the computing time required by the genetic algorithm to converge. With a coarser grid, there are fewer unknowns and less required computing time, at the expense of the model resolution. To further increase efficiency, our inversion code can perform the optimization using an offset-marching strategy and/or a frequency-marching strategy that can make use of different kinds of objective functions and allows for parallel computing. We illustrate the effect of our inversion method using three synthetic examples with rather complex near-surface models. Although no a priori information was used in all three tests, the long-wavelength structures of the reference models were fairly predicted, and satisfactory matches between “observed” and predicted data were achieved. The fair predictions of the reference models suggest that the final models estimated by our genetic-algorithm FWI, which we call macromodels, would be suitable inputs to gradient-based Rayleigh-wave FWI for further refinement. We also explored other issues related to the practical use of the method in different work and explored applications of the method to field data.

Tropospheric NO2 and HCHO derived from dual-scan MAX-DOAS measurements in Uccle (Belgium) and application to S5P/TROPOMI validation

2020 ◽  
Author(s):  
Ermioni Dimitropoulou ◽  
Francois Hendrick ◽  
Martine M. Friedrich ◽  
Gaia Pinardi ◽  
Frederik Tack ◽  
...  
Keyword(s):  
Vertical Profile ◽  
A Priori ◽  
Elevation Angle ◽  
Optical Absorption Spectroscopy ◽  
Azimuthal Direction ◽  
Inversion Algorithm ◽  
Vertical Column ◽  
Near Surface ◽  
Mixing Ratios ◽  
Horizontal Variation

<p>Ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of aerosols, tropospheric nitrogen dioxide (NO<sub>2</sub>) and formaldehyde (HCHO) have been carried out in Uccle, Brussels, during two years (March 2018 – March 2020). The MAX-DOAS instrument has been operating in both UV and visible (Vis) wavelength ranges in a dual-scan configuration consisting of two sub-modes: (1) an elevation scan in a fixed viewing azimuthal direction (the so-called main azimuthal direction) pointing and (2) an azimuthal scan in a fixed low elevation angle (2<sup>o</sup>). By applying a vertical profile inversion algorithm in the main azimuthal direction and an adapted version of the parameterization technique proposed by Sinreich et al. (2013) in the other azimuthal directions, near-surface  concentrations (VMRs) and vertical column densities (VCDs) are retrieved in ten different azimuthal directions.</p><p>The present work focuses on the seasonal horizontal variation of NO<sub>2 </sub>and HCHO around the measurement site. The observations show a clear seasonal cycle of these trace gases. An important application of the dual-scan MAX-DOAS measurements is the validation of satellite missions with high spatial resolution, such as TROPOMI/S5P. Measuring the tropospheric  VCDs in different azimuthal directions is shown to improve the spatial colocation with satellite measurements leading to a better agreement between both datasets. By using  vertical profile information derived from the MAX-DOAS measurements, we show that a persistent systematic underestimation of the TROPOMI  data can be explained by uncertainties in the a-priori NO<sub>2</sub> profile shape in the satellite retrieval. A similar validation study for TROPOMI HCHO is currently under progress and preliminary results will be presented.</p><p><strong>References:</strong></p><p>Sinreich, R., Merten, A., Molina, L., and Volkamer, R.: Parameterizing radiative transfer to convert MAX-DOAS dSCDs into near-surface box-averaged mixing ratios, Atmos. Meas. Tech., 6, 1521–1532, https://doi.org/10.5194/amt-6-1521-2013, 2013.</p>

1D single-site and laterally constrained inversion of multifrequency and multicomponent ground-based electromagnetic induction data — Application to the investigation of a near-surface clayey overburden

Geophysics ◽  
2012 ◽  
Vol 77 (4) ◽  
pp. WB19-WB35 ◽  
Author(s):  
Cyril Schamper ◽  
Fayçal Rejiba ◽  
Roger Guérin
Keyword(s):  
Magnetic Field ◽  
Electromagnetic Induction ◽  
Search Algorithm ◽  
Random Search ◽  
A Priori ◽  
Synthetic Data ◽  
Conductive Layer ◽  
Near Surface ◽  
Priori Information ◽  
Definition Of

Electromagnetic induction (EMI) methods are widely used to determine the distribution of the electrical conductivity and are well adapted to the delimitation of aquifers and clayey layers because the electromagnetic field is strongly perturbed by conductive media. The multicomponent EMI device that was used allowed the three components of the secondary magnetic field (the radial [Formula: see text], the tangential [Formula: see text], and the vertical [Formula: see text]) to be measured at 10 frequencies ranging from 110 to 56 kHz in one single sounding with offsets ranging from 20 to 400 m. In a continuing endeavor to improve the reliability with which the thickness and conductivity are inverted, we focused our research on the use of components other than the vertical magnetic field Hz. Because a separate sensitivity analysis of [Formula: see text] and [Formula: see text] suggests that [Formula: see text] is more sensitive to variations in the thickness of a near-surface conductive layer, we developed an inversion tool able to make single-sounding and laterally constrained 1D interpretation of both components jointly, associated with an adapted random search algorithm for single-sounding processing for which almost no a priori information is available. Considering the complementarity of [Formula: see text] and [Formula: see text] components, inversion tests of clean and noisy synthetic data showed an improvement in the definition of the thickness of a near-surface conductive layer. This inversion code was applied to the karst site of the basin of Fontaine-Sous-Préaux, near Rouen (northwest of France). Comparison with an electrical resistivity tomography tends to confirm the reliability of the interpretation from the EMI data with the developed inversion tool.

scholarly journals Gas Hydrates in Antarctica

2020 ◽  
Author(s):  
Michela Giustiniani ◽  
Umberta Tinivella
Keyword(s):  
Continental Margin ◽  
Gas Hydrates ◽  
Gas Hydrate ◽  
Ross Sea ◽  
Point Of View ◽  
The South ◽  
Bottom Simulating Reflector ◽  
Hydrate Reservoir ◽  
Wilkes Land ◽  
Gas Hydrate Reservoir

Few potential distributing areas of gas hydrates have been recognized in literature in Antarctica: the South Shetland continental margin, the Weddell Sea, the Ross Sea continental margin and the Wilkes Land continental margin. The most studied part of Antarctica from gas hydrate point of view is the South Shetland margin, where an important gas hydrate reservoir was well studied with the main purpose to determine the relationship between hydrate stability and environment effects, including climate change. In fact, the climate signals are particularly amplified in transition zones such as the peri-Antarctic regions, suggesting that the monitoring of hydrate system is desirable in order to detect potential hydrate dissociation as predicted by recent modeling offshore Antarctic Peninsula. The main seismic indicator of the gas hydrate presence, the bottom simulating reflector, was recorded in few parts of Antarctica, but in some cases it was associated to opal A/CT transition. The other areas need further studies and measurements in order to confirm or refuse the gas hydrate presence.

Isostatic constraint for 2D nonlinear gravity inversion on rifted margins

Geophysics ◽  
2019 ◽  
Vol 85 (1) ◽  
pp. G17-G34
Author(s):  
B. Marcela S. Bastos ◽  
Vanderlei C. Oliveira Jr.
Keyword(s):  
Gravity Data ◽  
A Priori ◽  
Petroleum Industry ◽  
Synthetic Data ◽  
Gravity Inversion ◽  
Rifted Margins ◽  
Isostatic Equilibrium ◽  
Priori Information ◽  
Pelotas Basin ◽  
Nonlinear Gravity

We have developed a nonlinear gravity inversion for simultaneously estimating the basement and Moho geometries, as well as the depth of the reference Moho along a profile crossing a passive rifted margin. To obtain stable solutions, we impose smoothness on basement and Moho, force them to be close to previously estimated depths along the profile and also impose local isostatic equilibrium. Different from previous methods, we evaluate the information of local isostatic equilibrium by imposing smoothness on the lithostatic stress exerted at depth. Our method delimits regions that deviate and those that can be considered in local isostatic equilibrium by varying the weight of the isostatic constraint along the profile. It also allows controlling the degree of equilibrium along the profile, so that the interpreter can obtain a set of candidate models that fit the observed data and exhibit different degrees of isostatic equilibrium. Our method also differs from earlier studies because it attempts to use isostasy for exploring (but not necessarily reducing) the inherent ambiguity of gravity methods. Tests with synthetic data illustrate the effect of our isostatic constraint on the estimated basement and Moho reliefs, especially at regions with pronounced crustal thinning, which are typical of passive volcanic margins. Results obtained by inverting satellite data over the Pelotas Basin, a passive volcanic margin in southern Brazil, agree with previous interpretations obtained independently by combining gravity, magnetic, and seismic data available to the petroleum industry. These results indicate that combined with a priori information, simple isostatic assumptions can be very useful for interpreting gravity data on passive rifted margins.

scholarly journals Sensitivity and Resolution of Controlled-Source Electromagnetic Method for Gas Hydrate Stable Zone

Energies ◽  
2021 ◽  
Vol 14 (24) ◽  
pp. 8318
Author(s):  
Zhenwei Guo ◽  
Yunxi Yuan ◽  
Mengyuan Jiang ◽  
Jianxin Liu ◽  
Xianying Wang ◽  
...  
Keyword(s):  
Natural Gas ◽  
Gas Hydrates ◽  
Gas Hydrate ◽  
Synthetic Data ◽  
Electromagnetic Method ◽  
Natural Gas Hydrate ◽  
Controlled Source ◽  
Transverse Resistance ◽  
Marine Csem ◽  
Stable Zone

Natural gas hydrate is one of the most important clean energies and part of carbon cycle, due to the least carbon content. Natural gas hydrates depend on high pressure and low temperatures, located under seabed or permafrost. Small changes in temperature and pressure may lead gas hydrates to separate into water and gas, commonly as methane. As a powerful greenhouse gas, methane is much stronger than carbon dioxide. Therefore, it is necessary to detect the gas hydrates stable zone (GHSZ) before the methane gas escapes from GHSZ. Marine controlled source electromagnetic method (CSEM) is a useful tool to detect gas hydrate in offshore. The results from 3D CSEM method are a resistivity cube to describe the distribution of gas hydrates. In order to study the detectability of CSEM method, we simulate the sensitivity and resolution of marine CSEM synthetic data. By using the sensitivity and resolution, a simple statement may be quickly judged on the existence and occurrence range of the natural gas hydrate. In this paper, we compare the resolution of marine CSEM method with various transverse resistance. This information may help researchers find out whether the GHSZ exists or not.

scholarly journals Identification of gas hydrate on bottom-simulating reflector characteristics with 2D seismic pre-stack time migration of North Bali Waters

2021 ◽  
Vol 944 (1) ◽  
pp. 012004
Author(s):  
I A Sufajar ◽  
H M Manik ◽  
T B Nainggolan ◽  
D Kusnida
Keyword(s):  
Gas Hydrates ◽  
Gas Hydrate ◽  
Oil And Gas ◽  
High Amplitude ◽  
Bottom Simulating Reflector ◽  
Oil And Gas Exploration ◽  
North East ◽  
The North ◽  
Time Migration ◽  
Exploration And Production

Abstract Gas hydrate is a physical compound composed of gas molecules that are formed in a seabed layer characterised by high pressure and low temperature. It is known as one of the alternative non-conventional hydrocarbons besides petroleum and natural gas. One of the identified areas of gas hydrate stability zone is in the North Bali Waters. The North Bali Waters is part of the North East Java Basin, which has oil and gas exploration and production, both conventional and non-conventional. One method of identifying the content of gas hydrates is by looking at the appearance of the Bottom Simulating Reflector (BSR) as shown on the Pre-Stack Time Migrated seismic sections. The detection of gas hydrate zone is determined by the presence of high amplitude, reversed polarity reflection and cross-cut reflection of sedimentary layer. This study aims to determine the existence of a BSR in the waters of North Bali. The procedures for analysing the existence of Bottom Simulating Reflector in this study are pre-processing, processing, and interpretation of 2D marine seismic data. The result shows gas hydrates found with indicated Bottom Simulating Reflector on CDP 35-812 at TWT depth of 1526-1582 ms, characterised by high amplitude-reverse polarity.

2.5D forward and inverse modeling for interpreting low-frequency electromagnetic measurements

Geophysics ◽  
2008 ◽  
Vol 73 (4) ◽  
pp. F165-F177 ◽  
Author(s):  
A. Abubakar ◽  
T. M. Habashy ◽  
V. L. Druskin ◽  
L. Knizhnerman ◽  
D. Alumbaugh
Keyword(s):  
Cost Function ◽  
Regularization Parameter ◽  
A Priori ◽  
Low Frequency ◽  
Inversion Algorithm ◽  
Forward Algorithm ◽  
Linear System Of Equations ◽  
Controlled Source ◽  
Data Inversion ◽  
The Cost

We present 2.5D fast and rigorous forward and inversion algorithms for deep electromagnetic (EM) applications that include crosswell and controlled-source EM measurements. The forward algorithm is based on a finite-difference approach in which a multifrontal LU decomposition algorithm simulates multisource experiments at nearly the cost of simulating one single-source experiment for each frequency of operation. When the size of the linear system of equations is large, the use of this noniterative solver is impractical. Hence, we use the optimal grid technique to limit the number of unknowns in the forward problem. The inversion algorithm employs a regularized Gauss-Newton minimization approach with a multiplicative cost function. By using this multiplicative cost function, we do not need a priori data to determine the so-called regularization parameter in the optimization process, making the algorithm fully automated. The algorithm is equipped with two regularization cost functions that allow us to reconstruct either a smooth or a sharp conductivity image. To increase the robustness of the algorithm, we also constrain the minimization and use a line-search approach to guarantee the reduction of the cost function after each iteration. To demonstrate the pros and cons of the algorithm, we present synthetic and field data inversion results for crosswell and controlled-source EM measurements.

Inversion modeling of gravity with prismatic mass bodies

Geophysics ◽  
1991 ◽  
Vol 56 (9) ◽  
pp. 1365-1376 ◽  
Author(s):  
Tieng‐Chang Lee ◽  
Shawn Biehler
Keyword(s):  
Gravity Data ◽  
A Priori ◽  
Depth Resolution ◽  
Memory Storage ◽  
Inversion Algorithm ◽  
Sampling Window ◽  
Prismatic Bodies ◽  
The Fourier Transform ◽  
Priori Information ◽  
Forward And Inverse Modeling

A combined method for forward and inverse modeling of gravity data is presented. Based on the Fourier transform of Poisson’s equation, the forward modeling is suitable for observation points above, within, and below causative masses with any prescribed density distribution. The inversion is linearized in the spatial domain by superimposing numerous prismatic bodies, each having constant but different density, and fixed geometry. Our inversion algorithm adopts a sampling window to reduce memory storage and computations. Testing, with synthetic and field data, demonstrates that a successful inversion can be obtained from crudely estimated a priori density distributions and uncertainties. Lateral variations in density are well resolved but depth resolution often requires better constrained a priori information. Under various a priori conditions, our modeling indicates that sediment density tends to vary exponentially with depth in the San Jacinto basin, southern California.

Sign in / Sign up

Export Citation Format

Share Document