Articles | Volume 7, issue 2
Solid Earth, 7, 685–701, 2016

Special issue: From orogenesis to geoscience in the service of society: the...

Solid Earth, 7, 685–701, 2016

Research article 27 Apr 2016

Research article | 27 Apr 2016

Characterization of a complex near-surface structure using well logging and passive seismic measurements

Beatriz Benjumea, Albert Macau, Anna Gabàs, and Sara Figueras Beatriz Benjumea et al.
  • Institut Cartogràfic i Geològic de Catalunya, Parc de Montjüic, 08038 Barcelona, Spain

Abstract. We combine geophysical well logging and passive seismic measurements to characterize the near-surface geology of an area located in Hontomin, Burgos (Spain). This area has some near-surface challenges for a geophysical study. The irregular topography is characterized by limestone outcrops and unconsolidated sediments areas. Additionally, the near-surface geology includes an upper layer of pure limestones overlying marly limestones and marls (Upper Cretaceous). These materials lie on top of Low Cretaceous siliciclastic sediments (sandstones, clays, gravels). In any case, a layer with reduced velocity is expected. The geophysical data sets used in this study include sonic and gamma-ray logs at two boreholes and passive seismic measurements: three arrays and 224 seismic stations for applying the horizontal-to-vertical amplitude spectra ratio method (H/V). Well-logging data define two significant changes in the P-wave-velocity log within the Upper Cretaceous layer and one more at the Upper to Lower Cretaceous contact. This technique has also been used for refining the geological interpretation. The passive seismic measurements provide a map of sediment thickness with a maximum of around 40 m and shear-wave velocity profiles from the array technique. A comparison between seismic velocity coming from well logging and array measurements defines the resolution limits of the passive seismic techniques and helps it to be interpreted. This study shows how these low-cost techniques can provide useful information about near-surface complexity that could be used for designing a geophysical field survey or for seismic processing steps such as statics or imaging.

Short summary
We study the near surface of a complex study area located in Burgos (Spain) characterized by significant changes in shallow geology. The geophysical techniques include measurements of seismic noise (passive seismic) at different points that provide sediment thickness and seismic velocity of the subsoil. On the other hand, geophysical well data help to interpret the passive seismic results. The product of this methodology can be helpful for planning geophysical field surveys in complex areas.