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Seismic-while-drilling (SWD) by drill-bit source has been successfully used in the past few decades and is proven using variable configurations in onshore applications. The method creates a reverse vertical seismic profile (RVSP) data set from surface sensors deployed as arrays in the proximity of the monitored wells. The typical application makes use of rig-pilot reference (pilot) sensors at the top of the drill string and also downhole. This approach provides while-drilling checkshots as well as multioffset RVSP for 2D and 3D imaging around the well and prediction ahead of the bit. For logistical (sensor deployment) and cost (rig time related to technical installation) reasons, the conventional drill-bit SWD application is typically much easier onshore than offshore. We have developed a novel approach that uses a network of passive-monitoring sea-bottom nodes predeployed for microseismic monitoring to simultaneously and effectively record offshore SWD data. We study the results of a pilot test in which we passively monitored the drilling of an appraisal well at the Wisting discovery in the Barents Sea with an ocean-bottom cable deployed temporarily around the drilling rig. The continuous passive recording of vibration signals emitted during the drilling of the well provides the SWD data set, which is treated as an RVSP. The study is performed without rig-pilot signal. The results are compared with legacy data and demonstrate the effectiveness of the approach and point to future applications for real-time monitoring of the drilling progress, in terms of geosteering the drill bit and predicting formation properties ahead of the bit by reflection imaging.

Distributed acoustic sensing (DAS) using fiber optic cables is an emerging seismic acquisition technology for the oil and gas industry, geothermal resource exploration, and underground fluid-storage monitoring. This technology offers the advantage of improving seismic acquisition by enabling massive arrays for monitoring of seismic wavefields at reduced cost with respect to conventional methods. In general, it is accepted that this method provides acoustic signals comparable with conventional seismic data, however, without the multicomponent directional information typical of geophones. We have developed a modified data extraction method and found that, as a result of the dense spatial distribution of recording points along the optic cable, DAS can provide two linked wavefield components in the axial direction, even when using a single 1D cable line. These signal pairs consist of dual components that are related to native strain rate (or strain) and particle acceleration (or velocity) fields at a given recording location. These dual signals are easily usable for wavefield separation purposes simply performing a trace-by-trace combination by appropriate scaling coefficient. The analysis performed with borehole data from linear and helically wound cables demonstrates the effectiveness of polarity recovery and dual-wavefield separation. We show real examples in which the data can be combined to provide separation of up- and downgoing wavefields. The ratio of the dual components provides information on local slowness properties in the formation.

In this work, we investigate the wavefields and the broadside response of a helical distributed acoustic sensor cable co-located with a hydrophone streamer in a shallow horizontal borehole. We discuss dual-wavefield separation results for selected optical-cable signals recorded with dense receiver spacing. Moreover, we interpret the phase and amplitude differences observed in the HWC and streamer data using sources at different azimuth. The analysis shows how the frequency response for the DAS signal with respect to the corresponding streamer signal changes with azimuth.

Summary Distributed acoustic sensors (DAS) using fiber opticcables is an emergingacquisitiontechnologyforO&G industry.It offers the advantage of improvingseismic acquisition by massive monitoring of seismic waves at reduced cost .In this paperwe show that,as a result of the dense spatial distribution o f recording points along the optic cable ,DAS can providetwo fieldcomponents in theaxial direction , even when usinga single one-dimensional line .These signalpairsconsist ofdualcomponents,whicharerelated to strainand particle velocity fields at a given recording location.We show withreal exampleswhere the datacan becombined to provideseparation ofup-and down-going wavefields. T he ratioof the dualcomponents provides informationon localslowness properties inthe formation.

In this work we present the results of a seismic while drilling (SWD) application using the drill-bit as a seismic source in geothermal wells. The survey was performed in a well drilled in the Nevada desert with the aim of providing geophysical information while drilling and images of the geological structures around the well. We present a summary description of the drill-bit seismic technology adapted for geothermal purposes, and the SWD results obtained along 2D profiles passing through the well. The good-quality seismic-while-drilling results have been subsequently used to obtain images of the well area in the direction normal to the main fault system, with SWD reverse vertical seismic profiling (RVSP) data migration, using seismic interferometry to extend laterally the coverage of the SWD RVSP images. The migrated seismic interferometry results confirm the main trends of the 2D geological model in the geothermal area.

We analyze the performance of the drill-bit seismic source for near-well investigation purposes. We test two different configurations, crosshole and single-well-imaging (SWI). The crosshole results are used to characterize the high-frequency content and to observe the radiation pattern of the drill bit source. These results are also used to analyze the stability in the signature of the drill-bit signal. We obtain appreciable results up to 350 Hz. The drill-bit data are used for SWI purposes by reference signal analysis in a drilled depth interval of approximately 140 m. The method allows us to detect reflections for prediction of the interfaces to be drilled, and the results are in agreement with other geophysical results and geological evidences.