A standard procedure in processing vertical seismic profile (VSP) data is the separation of up- and downgoing wavefields. We show that these wavefields in boreholes can be retrieved using only single-component data, given that a full set of surface reflection data is also available. No medium parameters are required. The method is an application of the Marchenko method and uses a focusing wavefield. It is a wavefield that satisfies certain focusing conditions in a reference medium. We show that the method is applicable to boreholes with any orientation, and no receiver array is required. By this work, we present two contributions. One is that we investigate the effect of using only the traveltime from borehole data to form the focusing wavefield. The second is that we validates standard separation methods (PZ summation and f-k filtering) by retrieving the one-way wavefields from a completely different approach. We use the numerically modelled data from a realistic field velocity model in the North Sea. Three borehole geometries (horizontal, deviated and vertical) are tested. We discuss the practical aspects for field application in the end.

For a robust way of estimating time shifts near horizontal boreholes, we have developed a method for separating the reflection responses above and below a horizontal borehole. Together with the surface reflection data, the method uses the direct arrivals from borehole data in the Marchenko method. The first step is to retrieve the focusing functions and the updown wavefields at the borehole level using an iterative Marchenko scheme. The second step is to solve two linear equations using a least-squares minimizing method for the two desired reflection responses. Then, the time shifts that are directly linked to the changes on either side of the borehole are calculated using a standard crosscorrelation technique. The method is applied with good results to synthetic 2D pressure data from the North Sea. One example uses purely artificial velocity changes (negative above the borehole and positive below), and the other example uses more realistic changes based on well logs. In the 2D case with an adequate survey coverage at the surface, the method is completely data driven. In the 3D case inwhich there is a limited number of horizontal wells, a kinematic correct velocity model is needed, but only for the volume between the surface and the borehole. Possible error factors related to the Marchenko scheme, such as an inaccurate source wavelet, imperfect surface multiples removal, and medium with loss are not included in this study.