Deep onshore reflection seismic imaging of the chalk group strata using a 45 kg accelerated weight-drop and combined recording systems with dense receiver spacing

Abstract

The Chalk Group forms important hydrocarbon reservoirs offshore and water aquifers onshore Denmark. Within a day of fieldwork, a 450 m long reflection seismic profile was acquired onshore in an area in southeast Denmark, where the Chalk Group extends almost to the surface and is approximately 900 m thick. The main objective of the study was to image the complete Chalk Group in high resolution and to study the origin of reflectivity within the different chalk units. A 45 kg accelerated weight-drop source, in combination with dense receiver spacing using microelectromechanical sensors mounted on a streamer and 48 planted geophones, was used for data acquisition. The profile runs subparallel to the cliffs of Stevns, and the recorded signal reaches the base of the Chalk Group at approximately 600 ms. The fully cored 443 m-deep Stevns-1 borehole, which is located at the recorded seismic line, provides excellent control on lithologic and facies changes. Comparison with the borehole data demonstrates that our seismic data set provides a high-resolution image of the internal layering of the Chalk Group. We find that the internal reflection coefficients of the Chalk Group are, in general, small based on wireline-log data. However, the reflected amplitudes are just big enough to be recorded with the receiver setup used, even from the pure chalk beds of the Chalk Group. The reflectivity seen on the high-resolution seismic profile is influenced by occurrences of clay-enriched chalk layers. Flint bands consisting of numerous flint nodules are a characteristic of the uppermost part of the Chalk Group at Stevns. The flint nodules appear to produce significant scattering of the seismic signals, and flint-rich layers appear with diffuse internal reflectivity characteristics. Outcrop-scale mound structures in Danian and Upper Cretaceous outcrops are for the first time seismically resolved.

Seismic imaging using an e-vib — A case study analyzing the signal properties of a seismic vibrator driven by electric linear synchronous motors

“…In response to these challenges, Drijkoningen et al (2006) and Noorlandt et al (2015), proposed the usage of linear synchronous motor (LSM) driven vibrators as an alternative vibroseis seismic source. In this study, the performance and seismic imaging potential of a prototype 1.65 t (6.7 kN) LSM-driven vibrator are evaluated using a merged dataset acquired combining a MEMS-based (micro-electro-mechanical system) seismic landstreamer (Brodic et al, 2015;Malehmir et al, 2015a;Dehghannejad et al, 2017;Kammann et al, 2019) and a spread of 4.5 Hz geophone-based wireless seismic nodes. We analyze the signal bandwidth, penetration depth and vertical resolution of the merged nodal-landstreamer dataset.…”