Abstract
In vertical seismic profile (VSP) acquisition, the orientation of the three-component geophones in a borehole is often not aligned due to random rotation of the wireline receiver array. Knowing the geophone orientation is crucial for rotating the waveforms back to the position where the geophones are assumed to be aligned. This procedure is critical in three-component VSP data processing as it provides the correct wavefields for subsequent VSP data imaging and inversion. For zero-offset or near zero-offset VSP measurements, the direct P-wave propagates nearly vertically, and the horizontal geophones receive only a small fraction of the direct P-wave energy. The first-arrival in the horizontal component data is often of low signal-to-noise ratio (SNR). As a result, it is difficult to apply conventional first-arrival–based methods for obtaining accurate geophone orientations. On the other hand, a seismic event comprising individual waveform would achieve maximum correlation if all corresponding geophones are aligned. The geometric characteristics, that is, the slope and continuity of a seismic event in the vector wavefield are the same with those of the modules of the vector wavefield. The latter, also known as the scalar wavefield, can be used for scanning geophone orientations based on waveform correlation. In this study, we propose to use the scalar field to extract the slope of seismic events. The orientation of the individual geophone was calculated with the constraint of the slope. We use both synthetic and field data to demonstrate the effectiveness and applicability of the proposed method. The results show that the new method can provide wavefields of horizontal component VSP data with much higher accuracy and resolution.
Highlights
The orientations of the geophones are hard to control in the borehole, which leads to random rotation of horizontal geophones in the three-component vertical seismic profile (VSP) acquisition
In a VSP acquisition, the polarization direction of first-arrival is parallel to the direction of the shot-to-receiver array
As the objective function in Eq 9, the orientation of the current geophone is determined by the signals of the previous M traces, and it is necessary to ensure that there are enough traces in the shallow layer when calculating the first trace in the deep layer
Summary
The orientations of the geophones are hard to control in the borehole, which leads to random rotation of horizontal geophones in the three-component VSP acquisition. The inconsistent orientation of geophones leads to inconsistent coordinate systems of geophone data and cause poor continuity of the events of the received vector wavefield (DiSiena et al, 1984). This makes the processing and analysis of VSP data difficult. Accurate information about the orientations of Method for Determining Geophone Orientations geophones is critical for transforming the geophone coordinate systems with Z-, X-, and Y-component axes into a consistent acquisition coordinate system with Z-, R- (radial of shotreceiver), and T-component (transverse of shot-receiver) axes (Liu et al, 2003), which is the foundation of subsequent vector wave separation (Lu et al, 2018, 2019). The aforementioned methods rely on accurate picking of P-wave first-arrival The advantages of these methods are fewer data input and fast evaluation. In practice, it is difficult to obtain the orientations of geophones by analyzing the first-arrival signals
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