However, this is a fairly ad-hoc procedure, and a more recent approach to inversion is called model-based inversion (Russell and Hampson, 1991). The effect of the bandlimited wavelet is to remove the low frequency component of the reflectivity, meaning that it cannot be recovered by the recursive inversion procedure of equation (3).Īfter proper processing and scaling of the seismic data, an intuitive approach to recovering the low frequency component is to simply extract this component from well log data and add it back to the seismic. The most severe problem is that the recorded seismic trace is not the reflectivity given in equation (2) but rather the convolutional model given in equation (1). However, as also recognized by Lindseth, there are a number of problems with this procedure. Lindseth (1979) showed that if we assume that the recorded seismic signal is as given in equation (2), we can invert this equation to recover the P-impedance using the recursive equation given byīy applying equation (3) to a seismic trace we can effectively transform, or invert, the seismic reflection data to P-impedance. Where r Pi is the zero-offset P-wave reflection coefficient at the i th interface of a stack of N layers and Z Pi =ρ iV Pi is the ith ρ-impedance of the i th layer, where ρ is density, V P is P-wave velocity and * denotes convolution. The reflectivity, in turn, is related to the acoustic impedance of the earth by Where s t is the seismic trace, w t is the seismic wavelet and r t is the reflectivity. The seismic traces in the stacked seismic section shown in Figure 2 can be modelled as the convolution of the earth’s reflectivity and a bandlimited seismic wavelet, which can be written However, let us first look at how we can “invert” the section shown in Figure 2. Unfortunately, “bright-spots” were also ambiguous with respect to identifying fluid anomalies, which lead to the development of the AVO (amplitude variations with offset) technique, to be discussed later. A typical “bright-spot” is highlighted by the rectangle in Figure 1, and is associated with the gas sand. The amplitude anomalies on a seismic section became known as “bright-spots” and were considered to correlate well with gas sands.
A seismic section from Alberta, in which the picked even between a time of 600 and 650 ms represents a seismic structure and the rectangle highlights an amplitude anomaly, or “bright spot”, associated with the gas sand. This information could be correlated with porosity changes, lithology changes, or even fluid changes within the subsurface of the earth. By the 1970s, geophysicists had started to realize that information was contained in the amplitudes of the seismic reflections themselves. But this structural interpretation is ambiguous when it comes to identifying gas sands. The inserted curve is the P -wave sonic log. The high on the picked structure corresponds to the gas sand. Such a structure can be seen in Figure 2, which shows a seismic line recorded over a gas-charged sand. The seismic reflection method was developed in the first quarter of the twentieth century and was used initially as a tool for identifying structures, such as anticlines, which could act as trapping mechanisms for hydrocarbon reservoirs. The effect of gas saturation on P and S-wave velocity. This talk will present both a history of seismic inversion and an overview of the inversion techniques themselves. In this figure, it can be noted that the P-wave velocity drops dramatically when gas is introduced into the reservoir whereas the S-wave velocity is largely unaffected by the introduction of the gas. The reason for this can be seen in Figure 1, which plots the P and S-wave velocities as a function of gas saturation.
Software hampson russell full#
To perform a less ambiguous interpretation of our inversion results, we must perform full elastic inversion, in which we estimate P-impedance, S-impedance (the product of density and S-wave velocity) and density. However, these predictions were somewhat ambiguous since P-impedance is sensitive to lithology, fluid and porosity effects, and it is difficult to separate the influence of each effect. Early inversion techniques transformed the seismic data into Pimpedance (the product of density and P-wave velocity), from which we were able to make predictions about lithology and porosity. Seismic inversion is a technique that has been in use by geophysicists for over forty years.