Exploration Geophysics - Volume 43, Issue 3, 2012

In reflection seismology, detailed knowledge of how seismic waves propagate in anisotropic media is important for locating reservoirs accurately. The SH-wave possesses a pure mode polarization which does not convert to P- and SV-waves when reflecting from a horizontal interface, and vice versa. The simplicity of the SH-wave thus provides an easy way to view the details of SH-wave propagation in anisotropic media. In this study, we attempt to inspect the theoretical reflection moveouts of SH-waves reflected from transversely isotropic (TI) layers with tilted symmetry axes and to verify the reflection point, which could be shifted away from the common midpoint (CMP), by numerical calculations and physical modelling.

In travel time-offset analyses, the moveout curves of SH-waves reflected from horizontal TI media (TIM) with different tilted angles of symmetry axes are computed by the TI modified hyperbolic equation and Fermat’s principle, respectively. It turns out that both the computed moveout curves are similar and fit well to the observed physical data. The reflection points of SH-waves for a CMP gather computed by Fermat’s principle show that they are close to the CMP for TIM with the vertical and horizontal symmetry axes, but they shift away from the CMP for the other tilted angles of symmetry axes. The shifts of the reflection points of the SH-waves from the CMP were verified by physical modelling.

The reflection moveouts of SH-waves reflected from a transversely isotropic layer with tilted symmetry axis computed by the modified hyperbolic equation and Fermat’s principle fit well to the physical data. Their reflection points will shift away from the common midpoint when the symmetry axes are not vertical and horizontal.

A modified EOM method is developed for PS-wave migration based on the equivalent offset migration (EOM) for P-waves. This gives better imaging quality than the previous EOM methods by reducing errors from the discretisation of equivalent offsets and suppressing noise from the co-location of source-receiver. An equivalent PS-wave velocity is also introduced. Processing real 2-dimensional 3-component seismic data shows that the method can produce a better migrated image than the conventional common conversion point (CCP) method.

A modified EOM method is developed for PS-wave migration based on the equivalent offset migration (EOM) for P-waves and an equivalent PS-wave velocity. A 2D3C seismic case study shows that the method can give better imaging quality than the conventional CCP method by reducing discretisation errors and suppressing noise.

Based on the assumption of the equivalent velocity and offset, the converted wave travel-time equation, which has a double square root due to the asymmetric ray-path of the down-going P-wave and the up-coming S-wave, can be transformed into a single square root equation if the common scatterpoint (CSP) gathers are binned. This method simplifies the equation and decreases the errors of converted wave migration transferred by P-wave velocity error, compared to the equivalent offset method (EOM) migration proposed by Bancroft, Geiger and Foltinek . In this paper, the errors caused by the introduction of equivalent velocity for the PS-wave are analysed in detail. The discrete errors and effects introduced by discretization of the equivalent offset are presented, and finally the conditions for applying CSP gathers for PS-wave processing under the control of reasonable error limits are derived.

Under the assumption of the equivalent velocity, the converted wave can be easily migrated in the time domain based on the equivalent offset method (EOM). In this paper, the analysis of errors caused by the introduction of the equivalent velocity for the PS-wave and the conditions for applying common scatterpoint (CSP) gathers are presented.

We present a new method of amplitude-preserved pre-stack time migration in 3D vertical transversely isotropic (VTI) media. The travel time and amplitude of a wave propagating in VTI media are calculated by using one-way wave operators and the stationary-phase theory. An imaging weight from the deconvolution imaging condition is introduced to produce amplitude-preserved image gathers for hydrocarbon and fluid detection. Velocities and stacking anisotropic parameters are estimated by a semblance scanning method. A time-dependent migration aperture corresponding to the dip of subsurface structure is then obtained to balance imaging quality and imaging noises. Numerical examples demonstrate that our method is accurate, robust and easy to implement.

We present a method of amplitude-preserved pre-stack time migration in 3D VTI media. An imaging weight and the estimation of stacking anisotropic parameters are introduced to produce amplitude-preserved image gathers. The time-dependent migration aperture corresponding to the dip of subsurface structure is used to balance imaging quality and imaging noises.

We have developed a simple method to determine the depth, inclination parameter and amplitude coefficient of a buried structure from a residual magnetic anomaly profile using a new formula representing the magnetic anomaly expressions produced by most geological structures. The method is based on defining the anomaly value at the origin and four characteristic points and their corresponding distances on the anomaly profile. Using all possible combinations of the four characteristic points and their corresponding distances, a procedure is developed for automated determination of the best fit model parameters including the shape (shape factor) of the buried structure from magnetic data.

The method was applied to synthetic data with and without random errors and tested on two field examples from Canada and India. In both cases, the model parameters obtained by the present method, particularly the shape and depth of the buried structures, were found to be in good agreement with the actual parameters. The present method has the capability of avoiding highly noisy data points and enforcing the incorporation of points of the least random errors to enhance the interpretation results.

We present a method to determine the model parameters of a buried structure from a residual magnetic anomaly profile using a new formula representing the magnetic anomaly expressions produced by most geological structures. The method has the capability of avoiding highly noisy data points to enhance the interpretation results.

We present a simple grid/image-based method of data fusion for use with georectified geophysical data. In this study, we fuse aeromagnetic, radiometric and VLF data acquired over the Hemlo gold deposit of Northern Ontario, Canada. The results clearly show a strong and unique anomaly within the area of surficial mineralization.

We present a simple and adaptive method of data fusion using grey-scale grids for general geophysical exploration. The methodology relies upon: (1) understanding the physical property variations that might be associated with the mineral exploration target, and (2) applying appropriate (forward or inverse) grey-scaling to each input dataset so that before addition of the grids the anomalous patterns all express the phenomena of interest in the same sense (i.e. all positive anomalies). If the resulting fused dataset has a Gaussian population distribution then a linear grey-scale is applied to the data within the 95% (2σ) confidence interval; if it is non-Gaussian then the linear grey scale is applied to the entire dataset.

The methodology has been applied to very low frequency (VLF), aeromagnetic and radiometric data measured during the 1980s over the Hemlo disseminated lode-gold deposit. The resulting fused data derived from our methodology produces a coherent region of anomalous geophysical response that is coincident in location and geometry to the surficial extent of the known mineralized zone of the deposit. Integration of multi-sensor response has the added advantage of significantly reducing the number of false-targets. Further, this method also illustrates the continued benefits that can be obtained from re-evaluation of older data.

Dissolution of limestone in a karst area resulted in ground shakes and enhancement of fractures. Gravity and electro-resistivity profiles mapped concealed caves, determined groundwater level, and defined five risk areas by correlating gravity lows with caves and conductive zones with brine water, providing information for future hydrological and geotechnical work.

On October 2008, the large extent of dissolution of limestone in the Irecê Basin resulted in the occurrence of minor ground shakes and the enhancement of fractures along the surface of the ground and on the walls of buildings in the urban and periurban area of the city of Lapão. After preliminary geological and geotechnical studies to analyse the danger of collapse of underground caves, we have carried out gravity and electro-resistivity profiles in order to map the concealed caves and to help the determination of the level of groundwater within them. The interpretation of the results provided useful information both for the immediate safety of the population and for subsequent hydrological and geotechnical work. We have defined five risk areas based on the variations of the Bouguer and of the electrical resistivity data, by correlating the distribution of gravity lows with the presence of caves and the less resistive zones with the presence of brine water underground.