Measurements of potential field and/or its derivatives often need to be interpolated onto maps. For very sparse data, this can be problematic. Ground gravity and magnetotelluric surveys are examples of such datasets. Methods which are assisted by recognizing that the measured quantity satisfies Laplace’s equation do better, and have formed the basis for a number of interpolation schemes in the literature. Here we formulate this process as a (possibly damped) inverse problem as generally as possible, and give some simple examples of its performance with gravity data.
Applications of seismic methods in mudrock exploration are limited due to a lack of well developed rock physics models that can relate mudrock matrix and fluid properties to seismic velocities. We propose a first principle based rock physics modeling method for predicting P- and S- wave velocities in mudrocks accounting for silica (quartz), porosity, and free gas. Using sonic and shear-sonic logs we apply the proposed method to estimate silica, porosity, and free gas depth profiles in the Woodford Shale of the Mcneff 2-28 well, Grady County, Anadarko Basin, Oklahoma. Our modeling suggests that silica and free gas decreases (90-50% and 90%-85% respectively) and the porosity increases (10–20%) in the Woodford from the top to its base. The estimated silica and porosity depth profiles are validated by comparison with the photoelectric and the density-porosity logs. X-Ray Diffraction analysis of a rock sample from the Upper Woodford in the Cambell 1-34 well located ~1 km to the southeast additionally supports high silica content in the Upper Woodford. This study strongly suggests that the rock physics method, such as that proposed here, can be potentially used as a guide for relating seismically-derived properties of the subsurface to key parameters such as silica, porosity, and gas saturation; the former two especially important for inferring brittleness and fracability and the latter for the production potential.
The Jubilee discovery offshore Ghana, made by Tullow in 2007, opened up several underexplored basins on the transform margin, and indicated the potential for further discoveries. In 2011, Tullow made the Zaedyus discovery offshore French Guiana, proving up the potential of the conjugate South American margin. Regional structural and stratigraphic analysis of West Africa, alongside studies of the trends and characteristics of successful producing fields from Sierra Leone to Nigeria, is used to define probable play types on the underexplored South American Equatorial conjugate margin. Cretaceous deep marine turbidites represent a key reservoir target, and key risks include source rock migration and reservoir distribution. Use of regional high quality seismic datasets for paleogeographic reconstructions such as these is necessary in order to better constrain the petroleum systems on both the African and South American Equatorial Margins.
The induced polarization (IP) method is an established technique in mining geophysics. One of the major challenges facing mining geophysicists is the ability to differentiate between different lithologies based on geophysical data such as IP data. In this paper, we present a study that focuses on extracting useful information about lithology from the full waveform IP data acquired by Newmont Distributed IP Data Acquisition System (NEWDAS). We first invert the 4D IP data by performing a sequence of 3D inversions. We then propose a new 4D inversion algorithm with time regularization and time weighting function. Once the inversions are done, statistical analysis of these recovered chargeabilities on a regional basis reveals some important differences on how the chargeabilities of different lithologies decay with time, and consequently, helps differentiate between various rock types. We show in this paper that lithology differentiation based on IP data is possible by combining two components: advanced data acquisition system and powerful inversion algorithms.
Successfully acquiring seismoelectric data is extremely difficult, mainly due to the inherently low signal-to-noise (SNR) ratio of the data. This difficulty is exacerbated by the weak sources (usually a sledgehammer) and low channel counts (12 to 24) commonly employed. In this article, we summarise two experiments we conducted to overcome these SNR limitations using a ‘brute-strength’ approach, i.e., employing a large seismic source, a hydraulic vibrator, and relatively high channel counts.
Distinguished from the classical Full Waveform Inversion, the objective functions for migration velocity analysis can be posed in the image domain whose gradient with respect to velocity can be computed in a Reverse Time Migration setting. The focus of this paper is an RTM based gradient computation of this kind. A quasi- Newton method is applied for the inversion. It is shown that both diving waves and reflections make the image focussed in sub-surface offset as the velocity gradually converges to the true velocity by the inversion. Examples in 2d synthetic data and 3d real data demonstrate the robustness of the method proposed.
Using different phase encoding schemes, a composite record comprising of different single-shot data can be acquired at different locations through simultaneous sweep with multiple vibrators. There are many ways to decompose this composite record into independent single-shot records. This paper analyzes the cause that leads to ill-conditioned matrix during data separation, introduces the basic principles of the singular value decomposition(SVD) method and gives a real case of data separation using the method. Other methods such as Gauss-Jordan elimination method can also achieve data separation; however, comparison of the data separation results between the singular value decomposition method and Gauss-Jordan elimination method in this paper shows that compared to the later, the S/N ratio of the single-shot record obtained with the former is much higher. A 90-degree phase shift of the original data is performed using Hilbert transform before data separation operation.
The Quaternary strata in Sanhu area of Qaidam Basin are rich in biogenic gas resources. The reservoirs mainly consist of unconsolidated sandstones with poor cementation and weak diagenesis. The friable sand model, the contact-cement model and the constant-cement model are used to estimate P-wave velocity-porosity relations. The estimated values of these three models are compared to the log data of TN9 well, which shows that a constant-cement fraction model (1% quartz cement) is the most appropriate. By using the constant-cement model, the paper tests various elastic parameters versus water saturation and concludes that the most sensitive top three parameters for identifying gas reservoirs are λρ,λ, and the Poisson ratio. In view of the thin sand-shale interbedding in Sanhu area, the paper analyzes AVO responses versus gas saturation, gas layer thickness and wavelet frequency respectively. The results show that AVO is sensitive for identifying gas/water in reservoirs, but insufficient for the exact prediction of gas saturation.
The present study evaluates the benefits of using PS seismic data to improve the seismic reservoir characterization, as well as, proposes a 3D-3C seismic survey design, which guarantees to record PP and PS wave appropriately. As a result, it was obtained for the target reservoirs that PS seismic data present a shorter wavelength than PP data; therefore, a better vertical seismic resolution can be obtained using PS seismic data. Additionally, it was determined in the study area that some geological interfaces produce stronger PS than PP reflection coefficients, so it could be advantageous to use PS post-stack seismic data to map these interfaces. Furthermore, it was obtained that a key attribute to make an effective seismic lithology discrimination is density. Therefore, due to density can be better estimated using simultaneously PP and PS data, PS seismic represent an important data to improve the seismic lithology discrimination. Finally, it was proposed a 3D-3C seismic survey design that takes in consideration a maximum offset to allow a good PS amplitude response and a suitable bin size for PP and PS image requirements, as well as, smooth fold of coverage for PP data and not zero fold striping PS fold distribution.
In the past, various approximations to the Zoeppritz equations have been derived and used in conventional AVO inversion. These approximations have accuracy limited to small angles, and the number of invertible elastic parameters is limited to two or three (the so-called two- or three-term AVO). Although inapplicable near the critical angle, plane-wave reflection coefficients (RCs) given by Zoeppritz equations are closed form and accurate for models without critical angles. We propose using the “exact” elastic Zoeppritz equations to do AVO inversion for reflections without critical angles. We compare the PP RCs calculated by finite differencing (FD), the Zoeppritz equation, and some classical approximations. For inversion, the Fréchet derivatives can be calculated analytically, and least-squares amplitude fitting is able to invert for the density ratio and three velocity ratios at the reflector. This algorithm is useful for inversion of long-offset PP reflections.