3D reflection seismic surveying and borehole DAS measurements to image the sedimentary structure in the Sudret area of Gotland, Sweden

Reflection seismic data were acquired in the Sudret area of Gotland in the time window 6 to 13 November, 2023. Objectives of the survey were to obtain images of the subsurface down to the Precambrian basement in the vicinity of two coreholes that had been drilled earlier down to about 800 m. These images would provide a better understanding of the sedimentary strata and local structure near these holes. For these purposes a sparse 3D survey was acquired that covered a c. 300 m by 700 m rectangular area with high fold, including the locations where the boreholes were drilled. A longer c. 2.8 km 2D profile was also acquired adjacent to the 3D survey that ran roughly in the N-S direction. In addition, distributed acoustic sensing (DAS) measurements were performed in the two coreholes. We report here on some results from the 3D survey and from the DAS measurements.

A Bobcat source with a 500 kg weight drop hammer with base plate was used as a source and 410 5Hz nodal units were available for recording. In total, 704 receiver locations were occupied with acquisition along 19 source lines, implying that 294 units had to be moved during the survey and that the source lines had to be shot twice. DAS data were recorded over the depth interval 17 m above sea level to 475 m below sea level in the Nore-1 corehole. In Nore-2 the depth interval was 17 m above sea level to 720 m below sea level. The fiber optic cable was sampled at 2.45 m intervals and data were recorded at a sampling frequency of 4000 Hz. Due to borehole irregularities it was not possible to get the fiber optic cables all the way to the bottom of the coreholes.

Numerous semi-continuous reflection horizons are observed in the c. upper 500 ms after stacking. A particularly strong reflection at 350 ms likely originates from the top of the Ordovician. Cambrian sandstones are also reflective, as well as shallow sandstone layers in the upper 150 ms. Normal moveout (NMO) velocities are relatively constant at about 3500 m/s. However, depth conversion using this velocity places the reflectivity deeper than what is expected from the cores. The DAS data allow the vertically propagating P-wave velocity to be measured at 3100 m/s. Using this velocity for depth conversion provides more reasonable depths to the main horizons. Since the NMO velocities are largely controlled by the horizontal velocity of the rock the difference between these and the DAS velocity can be explained by the rocks in the area having significant anisotropy (about 10%).