Table of contents

Using the qP-wave processing parameters and standard interpretation parameters, we derive a new weak-contrast approximation for qPqP- and qPqSV-reflection coefficients in a VTI medium. Being tested on few numerical models, the proposed approximation results in better accuracy compared with the standard weak-contrast approximation.

In this study, we apply a derivative-free optimization algorithm to estimate porosity and permeability from time-lapse seismic data and production data from a real reservoir (Norne field). In some circumstances, obtaining gradient information (exact and/or approximate) can be problematic e.g. derivatives are not available from a commercial simulator, or results are needed within a very short time frame. Derivative-free optimization approaches can be very time consuming because they often require many simulations. Typically, one iteration roughly needs as many simulations as the number of optimization variables. In this work, we propose two ways to significantly increase the efficiency of an optimization methodology in model inversion problems. First, by principal component analysis we decrease the number of optimization variables while keeping geostatistical consistency, and second, noticing that some optimization methods are very amenable to being parallelized, we apply them within a distributed computing framework. If we combine all this, the model inversion approach can be robust, fairly efficient and very simple to implement. In this paper, we apply the methodology to two cases: a semi-synthetic model with noisy data, and a case based entirely on field data. The results show that the derivative-free approach presented is robust against noise in the data.

Spectral evaluation of global geopotential models (GGMs) is necessary to recognize the behaviour of gravity signal and its error recorded in spherical harmonic coefficients and associated standard deviations. Results put forward in this wise explain the whole contribution of gravity data in different kinds that represent various sections of the gravity spectrum. This method is more informative than accuracy assessment methods, which use external data such as GPS-levelling. Comparative spectral evaluation for more than one model can be performed both in global and local sense using many spectral tools. The number of GGMs has grown with the increasing number of data collected by the dedicated satellite gravity missions, CHAMP, GRACE and GOCE. This fact makes it necessary to measure the differences between models and to monitor the improvements in the gravity field recovery. In this paper, some of the satellite-only and combined models are examined in different scales, globally and regionally, in order to observe the advances in the modelling of GGMs and their strengths at various expansion degrees for geodetic and geophysical applications. The validation of the published errors of model coefficients is a part of this evaluation. All spectral tools explicitly reveal the superiority of the GRACE-based models when compared against the models that comprise the conventional satellite tracking data. The disagreement between models is large in local/regional areas if data sets are different, as seen from the example of the Turkish territory.

In the Tarim basin in northwestern China, due to the existence of a thick, shallow, low-velocity layer in a desert area, it is very difficult to acquire high-quality seismic data. The main problems are bad shot and receiving condition, which cause a very low signal-to-noise ratio and low-resolution, undulant hypsography, which makes statics complicated, and surface thick sand stratum, which induces absorption and attenuation of the seismic wave. In order to overcome these difficulties and acquire high-quality seismic data, we adopt the following six methods. (1) Refining investigation of the surface structure for the perfect surface structure model, which serves for choosing better shot and receiver points. (2) Exploding under the water table to guarantee enough shot energy, and embedding the geophone to get a better receiving condition. (3) Selecting dynamite with a bigger diameter to form a better point explosion. (4) Surveying the noise waves to design the best acquisition geometry. (5) Calculating better static correction by refraction tomography constrained by normal surface structure investigation. (6) Compensating the absorbed energy of the seismic wave to gain a high-resolution signal. After applying these methods, we significantly improve the signal-to-noise ratio and the resolution of the seismic data in the main prospecting targets, which enable us to make correct geological interpretation.

A large volume airgun array is effective in generating seismic waves, which is extensively used in large volume bodies of water such as oceans, lakes and reservoirs. So far, the application of large volume airguns is subject to the distribution of large volume bodies of water. This paper reports an attempt to utilize large volume airguns in a small body of water as a seismic source for seismotectonic studies. We carried out a field experiment in Mapaoquan pond, Fangshan district, Beijing, during the period 25–30 May 2009. Bolt LL1500 airguns, each with volumes of 2000 in³, the largest commercial airguns available today, were used in this experiment. We tested the excitation of the airgun array with one or two guns. The airgun array was placed 7–11 m below the water's surface. The near- and far-field seismic motions induced by the airgun source were recorded by a 100 km long seismic profile composed of 16 portable seismometers and a 100 m long strong motion seismograph profile, respectively. The following conclusions can be drawn from this experiment. First, it is feasible to excite large volume airguns in a small volume body of water. Second, seismic signals from a single shot of one airgun can be recognized at the offset up to 15 km. Taking advantage of high source repeatability, we stacked records from 128 shots to enhance the signal-to-noise ratio, and direct P-waves can be easily identified at the offset ∼50 km in stacked records. Third, no detectable damage to fish or near-field constructions was caused by the airgun shots. Those results suggest that large volume airguns excited in small bodies of water can be used as a routinely operated seismic source for mid-scale (tens of kilometres) subsurface explorations and monitoring under various running conditions.

Calibration of PP- and PS-wave reflection events is a crucial step in multicomponent seismic data inversion and quantitative analysis. This paper presents a workflow of calibration including an optimized estimation of the P- to S-wave velocity ratio and wavelet preservation after a PS-wave trace transformation to the PP-wave two-way time. The optimized velocity ratio is obtained from the spectral analysis of correlation coefficients versus perturbations of a time-variant velocity-ratio function. This analysis may also be conducted in a target-oriented fashion which involves not only searching for the additive perturbation but also the gradient of the local velocity ratio, to overcome possible ambiguities in the correlation spectrum. However, when a PS-wave trace is transformed from the PS time to the PP time, the PS reflection wavelets along the trace are compressed. This wavelet distortion needs be removed to preserve the original PS-wave frequency content, before further inversion processing. Wavelet compression means stretching in the frequency spectrum, and such spectral stretching is time variant depending upon the local P- to S-wave velocity ratio. Thus, the restoration is implemented on the time–frequency spectrum in the Gabor transform domain. Thereafter, wavelet-preserved PS-wave reflections, presented in PP time, may be used in a PP- and PS-wave joint inversion.

To position arbitrary outdoor measurements, the most common methods are the use of global positioning system (GPS) technology or distance tracking from reference locations. The system to be introduced here consists of a combination of an optical distance tracking system and a civil, autonomous GPS receiver. Through the application of a recursive filter, both components support and correct each other mutually. The system is intended as an autonomous, real-time and low-cost alternative to expensive differential GPS solutions in geophysical field surveying. The combination approach features a very good relative and decent absolute positioning precision. First measurements using the two-coil geo-electromagnetics instrument Geonics EM31 in combination with the developed positioning system show promising results.

Underwater abnormal object detection is a current need for the navigation security of autonomous underwater vehicles (AUVs). In this paper, an automated gravity gradient tensor inversion algorithm is proposed for the purpose of passive underwater object detection. Full-tensor gravity gradient anomalies induced by an object in the partial area can be measured with the technique of gravity gradiometry on an AUV. Then the automated algorithm utilizes the anomalies, using the inverse method to estimate the mass and barycentre location of the arbitrary-shaped object. A few tests on simple synthetic models will be illustrated, in order to evaluate the feasibility and accuracy of the new algorithm. Moreover, the method is applied to a complicated model of an abnormal object with gradiometer and AUV noise, and interference from a neighbouring illusive smaller object. In all cases tested, the estimated mass and barycentre location parameters are found to be in good agreement with the actual values.

This paper presents a nonlinear stability analysis of a double-diffusive convection in a magnetized ferrofluid layer confined between stress-free boundaries using a thermal non-equilibrium model by the energy method. A Darcy–Brinkman model is used for the momentum equation and a two-field model is used for the energy equation, each representing the solid and fluid phases separately. The mathematical emphasis is on how to control the nonlinear terms caused by magnetic body and inertia forces. The effects of the interface heat transfer coefficient , magnetic parameter (M₃), Darcy–Brinkman number and porosity-modified conductivity ratio (γ') on the onset of ferroconvection in the presence of solute (S') have been analysed. For all the cases studied, it is found that solute gradient enhances the stability of the system. A comparison with linear instability theory shows that there is a difference between the critical Rayleigh numbers and thus indicates the possibility of the existence of a subcritical instability region for ferrofluids. However, for non-ferrofluids stability and instability boundaries coincide.

It is important to discover the deformation characteristics of the Tianshan mountain range for a better understanding of the geodynamics of the Tianshan orogenic belt. Constrained by the GPS-derived velocity vectors of crustal movement, the current velocity field, stress field and strain rate in the Tianshan mountains have been retrieved from a three-dimensional numerical model presented in this paper by using the finite-element code ANSYS, on the basis of geological structures, tectonic regimes, active fault belts and seismic velocity structures of the crust and upper mantle. The results suggest that: (1) the general direction of crustal movement is NNE, and yet gradually turns to NE from west to east; (2) the regional stress field is characterized by near N–S tectonic compression, resulting in crustal shortening in the near N–S direction as well; and (3) the shortening strain rate is ∼10⁻⁸ a⁻¹ and decreases gradually from west to east. Our results support the opinion that the crustal deformation of the Tianshan mountain range is controlled by the clockwise rotation of the Tarim basin.

A theoretical analysis of the effect of a logging tool on kinematic and dynamic parameters of elastic waves generated in a fluid-filled borehole by an impulse acoustic source is presented. To calculate synthetic microseismograms we use the semianalytical real axis integration method. The calculations performed have shown that the logging tool parameters have a significant influence on the amplitudes of all wave packets. The comparison of synthetic microseismograms with borehole waveforms shows good agreement.

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