Exploration Geophysics - Volume 46, Issue 3, 2015

Carbon dioxide injection monitoring in offshore environments is a promising future application of the marine controlled-source electromagnetic (mCSEM) method. To investigate whether the mCSEM method can be used to quantitatively monitor variations in the distribution of the injected CO2, we developed a mCSEM inversion scheme and conducted numerical analyses. Furthermore, to demonstrate the monitoring capability of the mCSEM method in challenging environments, we used a deep brine aquifer model in shallow sea as an injection target. The mCSEM responses of the injected CO2 in the deep brine aquifer were severely decayed and heavily masked by the air wave due to the proximity of the free space. Therefore, the accurate computation of small mCSEM responses due to the injected CO2 and the proper incorporation into the inversion process are critically important for the mCSEM method to be successful. Additionally, in monitoring situations, some useful a priori information is usually available (e.g. well logs and seismic sections), and the proper implementation of this to our inversion framework is crucial to ensure reliable estimation of the distribution of the injected CO2 plume. In this study, we developed an efficient 2.5D mCSEM inversion algorithm based on an accurate forward modelling algorithm and the judicious incorporation of a priori information into our inversion scheme. The inversion scheme was tested with simplified and realistic CO2 injection models and successfully recovered the resistivity distributions of the injected CO2, although it still required the presence of a considerable amount of the injected CO2. Based on these inversion experiments, we demonstrated that the mCSEM method is capable of quantitatively monitoring variations in the distribution of injected CO2 in offshore environments.

We developed an efficient 2.5D mCSEM inversion algorithm based on an accurate forward modelling algorithm and the judicious incorporation of a priori information into our inversion scheme. We demonstrated the successful recovery of resistivity distributions of the injected CO2 from the deep brine aquifer model in the shallow sea.

A gradient projection method for seismic interpolation based on the tight frame property of curvelet transform is proposed. Some smooth L1 norm functions were analysed, and the Huber function was chosen to replace the L1 norm. The tight frame property of the curvelet transform is utilised to improve the computational efficiency.

Seismic interpolation, as an efficient strategy of providing reliable wavefields, belongs to large-scale computing problems. The rapid increase of data volume in high dimensional interpolation requires highly efficient methods to relieve computational burden. Most methods adopt the L1 norm as a sparsity constraint of solutions in some transformed domain; however, the L1 norm is non-differentiable and gradient-type methods cannot be applied directly. On the other hand, methods for unconstrained L1 norm optimisation always depend on the regularisation parameter which needs to be chosen carefully. In this paper, a fast gradient projection method for the smooth L1 problem is proposed based on the tight frame property of the curvelet transform that can overcome these shortcomings. Some smooth L1 norm functions are discussed and their properties are analysed, then the Huber function is chosen to replace the L1 norm. The novelty of the proposed method is that the tight frame property of the curvelet transform is utilised to improve the computational efficiency. Numerical experiments on synthetic and real data demonstrate the validity of the proposed method which can be used in large-scale computing.

The problem of estimating thin layered model parameters by amplitude variation with offset (AVO) inversion of prestack seismic data has been studied. An algorithm for solving the prestack inverse AVO problem in the case of multilayered media has been derived. Numerical examples show the accuracy and efficiency of the method.

The problem of estimating thin layered model parameters by amplitude variation with offset (AVO) inversion has been studied. The motivation was resolving of the thin layers in inverted prestack seismic data as it contains more information on elastic properties of the subsurface than poststack seismic data.

In this paper, an algorithm for solving the prestack inverse AVO problem in the case of multilayered media is derived. This algorithm is based on iterative corrections to the parameters of the initial model which tend to minimise the misfits between observed and synthetic seismograms. The synthetic seismograms are calculated using the reflection–transmission (RT)-matrices method, assuming a plane-wave with respect to the source position.

A regularised Gauss-type algorithm for the inversion of prestack seismic data has been used. A differential seismogram computation algorithm to characterise the sensitivity of the seismic signal to the variations of a model parameter was used. The derived solution of the inverse problem is constructed in the time domain. This gives a slight advantage because it allows for visual control of the solution process. One can monitor the amplitude reduction of the data residual (difference between observed and synthetic seismograms) during the iteration process. Numerical examples show the accuracy and efficiency of the method.

A procedure for estimation of Rayleigh wave phase velocities from microtremor observations, using an array with a reference station, is investigated in this study. Simultaneous observation of microtremors at a reference station and at a strong motion observation array in the Kanto Basin, Japan, was carried out. We first calculated cross correlations between records at the reference station and those at stations in the array using a seismic interferometric processing method on a 4300-h data series. After identifying dispersive Rayleigh waves from results of multiple filtering analysis of the cross correlations, semblance analysis of the cross correlations for different segments was carried out to estimate phase velocities for fundamental and higher-mode Rayleigh waves. The phase velocities from the proposed method are more appropriate than those from conventional methods at long periods as they avoid contamination by higher mode Rayleigh waves. The fundamental Rayleigh wave phase velocities were inverted to an S-wave velocity profile for deep sedimentary layers.

We also examined the variations in the phase velocity with decreasing data duration. The phase velocities at periods less than 3 s from 6-h records are similar to those from 4300-h records, suggesting that our method is possibly applicable in microtremor exploration.

An interferometric procedure for Rayleigh wave phase velocities from cross correlations of microtremors in an array with a reference station is investigated in this study. After identifying dispersive Rayleigh waves, semblance analysis of the cross correlations was carried out to estimate phase velocities for fundamental and higher-mode Rayleigh waves.

An edge-detection technique for the enhancement of potential field data, which is based on the tilt angle of the first order vertical derivative of the total horizontal gradient, is presented. The new filter clearly enhances the edges of superimposed sources sharply and generates more subtle detail.

We present an edge-detection technique for the enhancement of potential field data, which is based on the tilt angle of the first order vertical derivative of the total horizontal gradient. The technique can be performed using three steps, as follows: first, we calculate the total horizontal gradient of the potential fields, which is stable and effective in determining the horizontal locations; second, we calculate the first order vertical derivative of the total horizontal gradient to increase the vertical-resolution on the basis of the determined the horizontal locations; finally, we display the tilt angle of the first order vertical derivative of the total horizontal gradient tending to balance the amplitude responses from both shallow and deep sources. This technique is designed to reflect the complex distributions of multiple sources with different depths and extents. The effectiveness of our method is demonstrated by synthetic data. The results indicate that the new filter generates more subtle detail for superimposed sources, compared with other edge detection filters. The method is also applied to field surveyed data from the Saskatoon area of Canada, and the results are helpful for qualitative interpretation.

Although spectral depth determination has played a role in magnetic interpretation for over four decades, automating the procedure has been inhibited by the need for manual intervention. This paper introduces the concept of a slope spectrum of an equivalent layer, to be used in an automated depth interpretation algorithm suitable for application to very large datasets such as the complete Northern Territory aeromagnetic grid.

In order to trace the extensive basalts across the Northern Territory, profiles of spectral depths have been obtained at 5 km intervals across the NT stitched grid of total magnetic intensity (TMI). Each profile is a graph from 0 to 1000 m of the probability of a magnetic layer occurring at each depth. Automating the collection of the 50 000 profiles required the development of a formula that relates slopes along the power spectrum to depths to an equivalent magnetic layer. Model slabs were populated with a large number of randomly located dipoles and their power spectra correlated with modelled depth to provide the formula. Depth profiles are too noisy to be used singly, but when a series of depth profiles are lined up side-by-side as a transect, significant magnetic layers can be traced for large distances. Transects frequently show a second layer. The formula is quite general in its derivation and would apply to any mid-latitude area where significant magnetic bodies can be modelled as extensive layers.

Because the method requires a radial power spectrum, it fails to provide signal at depths much shallower than the flight line spacing. The method is convenient for a fast first pass at depth estimation, but its horizontal resolution is rather coarse and errors can be quite large.

By assuming that anomalies in a TMI grid arise from thin magnetic layers, spectral depth estimates to one or more equivalent magnetic layers may be obtained repeatedly across large areas. A general formula is derived for the purpose and demonstrated on the TMI grid of the Northern Territory.

An airborne gravity survey was successfully conducted over the Dongying, Gudao and Gudong oilfields of Shengli oil province, eastern China. These survey areas cover onshore and offshore regions of the south-west Bohai Sea. The data were processed using the potential field transformation approach. The derived Bouguer gravity data correlate well with features such as known faults, swells and sags identified by earlier seismic survey and drilling data. The depth to the Cenozoic basement in the study area, including the Dongying, Gudao, and Gudong oilfields, was calculated by means of gravity inversion constrained by seismic and drilling data. The differences between the depths to the Cenozoic basement calculated from gravity anomaly and those determined by the earlier seismic and drilling data are less than 5%.

An airborne gravity survey was successfully conducted over the onshore and offshore regions of the south-west Bohai Sea. The derived Bouguer gravity data and inversed results correlate well with features such as known faults, swells, sags, and oilfields identified by earlier seismic survey and drilling data.