Seismic Reflection Imaging With Horizontal DAS Cables
Figure 2 below is a sketch that depicts ray paths from a surface seismic source traveling into the earth, reflecting from interfaces below a horizontal DAS cable, followed by the reflections being recorded on the DAS cable system.
Figure 2. Raypath Sketch. This sketch depicts ray paths from a surface seismic source reflecting from an interface below a DAS cable and then being recorded on a horizontal DAS cable.
The raypath geometry of Figure 2 results in the seismic sources and seismic receivers being at very different elevations. This geometry invalidates the common midpoint assumption that is critical to CDP processing and makes creating a reflection image from data recorded in this geometry far more complex than data recorded with sources and receivers at approximately the same elevation. Processing the data for reflection images in the native geometry described above requires special skills and software in order to be successful in generating velocity fields and migration images. Fortunately, we at Sterling Seismic & Reservoir Services were able to develop a novel method of downward continuing surface seismic sources to the elevation of the DAS cable. In other words we are able to create virtual source points on the DAS cable such that the seismic data was recorded with the seismic sources on or near the DAS cable at depth (See Figure 3).
Figure 3. Downward continued virtual sources on DAS cable. This figure shows the concept of virtual source points being placed on or near the DAS cable at depth even though the actual source points were at the surface of the earth. The value of virtual source points being at the same elevation as the DAS receiver cable is that surface-seismic data processing techniques can be applied to generate reflection images rather than requiring specialized skills and software to generate images with the geometry of surface seismic sources and the DAS receiver cable at depth.
The value of the virtual source geometry is that we can now treat the data like a typical surface seismic dataset with sources and receiver near the same elevation. The data can be processed on any seismic data processing system by anyone skilled in seismic data processing. The number of software tools for processing surface seismic data is enormous and have been proven to be valid for decades. Thus, the results with surface seismic sources and a horizontal DAS cable can be produced in a short period of time using software and imaging concepts that are proven.
Figure 4. Map view of sources and DAS cable for field survey. This North American DAS seismic survey had a DAS cable at a depth of about 9,000 ft, a horizontal DAS cable length of about 10,000 ft, and a surface source interval of 150 ft over approximately 18,000 ft from north to south.
Figure 4 shows a map view sketch of a DAS seismic data acquisition project in North America. The blue dots depict the projection of the horizontal DAS cable to the surface from a depth of 9,000 ft. The length of the DAS cable in the north-south direction is about 10,000 ft. The red dashes roughly show the source locations that span approximately 18,000 ft in the north-south direction at a nominal source interval of about 150 ft. The sweep frequency of the Vibroseis source was 4-120 Hz. The horizontal well was toe-up to the north with a slight dip from north to south.
The first arrival and reflection data was relatively high quality in spite of containing severe interrogator noise (a strong DC shift and additional stripes that cross each shot record when the interrogator unit experienced vibrations from the local environment). The dataset was processed via the downward continuation/virtual source method discussed above and then passed through a fairly standard 2D seismic data processing flow including post stack Finite Difference migration. Figure 5 shows the seismic reflection image that was produced.
Figure 5. Seismic reflection image from horizontal DAS cable. The 2D seismic image produced from surface seismic sources and a horizontal DAS cable is shown here. The trace spacing is 10 ft which is possible due to the DAS receiver spacing of 13 ft and the virtual source technology employed which allows source points to be output at arbitrary locations. The datum elevation for the image is less than 10 ft above the elevation of the toe of the well (toe-up well configuration). The image is in the time-domain. The two-way time of 100 ms is about 650 ft below the datum elevation.
Figure 6 shows the amplitude spectrum of the reflection image in Figure 5. The Vibroseis source sweep spanned 4 to 120 Hz.
Figure 6. Amplitude spectrum of seismic image in Figure 5. The amplitude spectrum of the DAS reflection image has a spectrum that covers the spectrum of the sweep of 4-120 Hz.
The small trace spacing of the DAS system allowed for far higher spatial resolution than is typically produced from surface seismic data. The structural features of the DAS data match the structure observed in the PSTM volume of the surface seismic data while the amplitude spectrum of the surface seismic data has a maximum frequency near 85 Hz rather than the maximum frequency of the DAS data being near 110 Hz.
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