Estimation of the displacements among distant events based on parallel tracking of events in seismic traces under uncertainty

Highlights

• The parallel cross-correlation method can correctly identify similar events in two distant seismic traces.

• Uncertainty in the identification of similar seismic events increases with depth.

• The cross-correlation and standard deviations help address the uncertainty in identification of distant seismic events.

• Correct identification of seismic events can happen in the worst case, when is including geometric constraints.

• The accuracy of events depth estimates depends on the procedures of correlation.

Abstract

The method we propose in this paper seeks to estimate interface displacements among strata related with reflection seismic events, in comparison to the interfaces at other reference points. To do so, we search for reflection events in the reference point of a second seismic trace taken from the same 3D survey and close to a well. However, the nature of the seismic data introduces uncertainty in the results. Therefore, we perform an uncertainty analysis using the standard deviation results from several experiments with cross-correlation of signals. To estimate the displacements of events in depth between two seismic traces, we create a synthetic seismic trace with an empirical wavelet and the sonic log of the well, close to the second seismic trace. Then, we relate the events of the seismic traces to the depth of the sonic log. Finally, we test the method with data from the Namorado Field in Brazil. The results show that the accuracy of the event estimated depth depends on the results of parallel cross-correlation, primarily those from the procedures used in the integration of seismic data with data from the well. The proposed approach can correctly identify several similar events in two seismic traces without requiring all seismic traces between two distant points of interest to correlate strata in the subsurface.

Introduction

A significant proportion of the geophysical prospecting data is obtained from seismic surveys. These data are processed to produce graphical interpretations of the subsurface. A migrated seismic section or volume may reveal geological formations, outlined with reflections. Some of these formations may represent either mineral deposit or petroleum reservoirs (Davies et al., 2004). An experienced interpreter can mark the horizons over the images to determine the boundaries of a stratum that could represent a mineral deposit. Moreover, the automatic methods for tracking horizons are useful to interpreters because they can simplify the work involved and reduce uncertainty in the interpretation of profiles (Totake et al., 2017). Nevertheless, a suitable parameterization of the tracking method and of an increase of the signal-noise rate is necessary (Nicoli et al., 2002; Brown, 2005; Porsani et al., 2010).

There are different horizon tracking methods (Schneider and Backus, 1968; Howard, 1991; Glinsky et al., 2001; Chopra and Marfurt, 2007; Yan et al., 2013), including methods based on cross-correlation, whose metric allows quantification of the similarity between portions based on seismic traces and the displacement among seismic events. Displacement estimation can be useful in estimating the dip angle, thickness and depth of certain structures. The data used in estimation are from 2D or 3D seismic surveys and from synthetic seismic traces at a well (Darling, 2005). Data from vertical seismic profiling (VSP) are used to tune the seismic data and the synthetic seismic traces of a well (Serra, 2008). However, such data is not always available, so in this study we proposes an alternative way of solving this problem.

The cross-correlation of parallel seismic events that we propose in this paper searches for similar events in only two seismic traces at different locations. In order to determine the displacements of the events, the first seismic trace shall be close to a well, and the second trace close to the point where the displacements in depth will be estimated. However, some difficulties and issues emerge before the estimation of displacements can be achieved. For instance, since an event from the second seismic trace could be similar to the standard event, what criteria could be used to relate them? It is also difficult to determine whether an event or events in the first seismic trace share any traces with the second seismic trace, or whether they simply disappear due to destructive interference in the migration process or in a low signal-noise rate (Cooper, 2004). All of these factors introduce uncertainty in the seismic trace data. The tracking methods can overcome some of these difficulties, but the tracking can be complex in a seismic volume with irregular distribution of traces. One way to simplify the identification of events is through a parallel search of events in the two seismic traces, and by including geometric constraints.

Now, how can the identified events be related to the strata in the well log? One way is through the preparation of suitable synthetic seismic traces to compare with real seismic traces (Schlumberger, 1998; Darling, 2005; Serra, 2008). However, there are several factors that make this task uncertain. Among these factors are the unknown frequency and shape of the seismic source wavelet, the well log errors and the changes when processing real seismic traces, among others. To address this issue, we propose an adaptation under uncertainty for the cross-correlation between the seismic trace from a 3D survey and a synthetic seismic trace prepared using the sonic and density logs of a well. Furthermore, we propose procedures for estimating the depth of events with information extracted from the well logs and seismic signals.