Evaluation of seismic acquisition methods at the Uppsala University test site at Marsta

Description

The Marsta test site is located about 10 km north of Uppsala and provides a nearby location for both hydrological and geophysical studies. Nine boreholes were drilled in the area in 2012. Eight were drilled through the near surface glacial sediments into the top of the bedrock to depths of few meters to 15 meters. One borehole was drilled about 100 meters into the bedrock to a total depth of 118 meters. This borehole has been logged with several geophysical methods. Several past seismic surveys have also been performed in the area, providing a baseline for future experiments. The groundwater level is continuously monitored at the site. This allows studies on the influence of the groundwater level on geophysical measurements. The Geophysics Program acquired an interrogator for measuring vibrations along fiber optic cables in the beginning of 2023, a method know as Distributed Acoustic Sensing (DAS). In order to compare how the measured vibrations using DAS differ from conventional geophones an experiment was conducted in July 2023. A 500 m long fiber optic cable was buried in a 30 cm deep snaking trench with geophones co-located. The 300 geophones were planted in holes about 20 cm deep. A sledgehammer was then used to generate seismic waves that were recorded both on the DAS system and the geophones. Preliminary results indicate that the geophones are superior for recording seismic waves compared to the DAS system. However, the DAS system has the advantage that is it permanently installed so it is simple to repeat the measurements. This is important for monitoring purposes, since it is time consuming to plant geophones and there is always uncertainty if the coupling to the ground is the same from survey to survey. Plans are to repeat the DAS measurements using a similar seismic source and to test recording on a fiber optic cable in the deeper borehole. Recording downhole allows the velocity of the sediments and the rocks to be directly measured in the vertical direction. This is important for depth conversion of surface seismic data. The fiber optic cable is also sensitive tube waves in the borehole. Tube waves tend to be generated and reflected where there are fracture zones or permeable rock, allowing identification of such zones on the recorded data. This may be important for better understanding groundwater movement in the bedrock. Measurements using the DAS system in two deep (to 800m) boreholes on Gotland show that tube waves are the main propagating wave along the borehole when the fiber optic cable is hanging loose. There are also clear reflections of the tube waves at zones where there is expected to be higher permeability. If the main objective of the DAS measurements is to image structure using P- or S-waves then it is generally necessary to cement in the fiber optic cable in the borehole. This is more costly and may require a dedicated borehole, but for long term monitoring this may be cost efficient. Plans are to drill a dedicated borehole at Marsta deeper into the bedrock and cement in a cable. Marsta can then be used as a testing ground for DAS measurements with a buried surface cable, a cemented cable and an open borehole for temporary cable deployments.