Joint interpretation of magnetotelluric, seismic and well-log data in Hontomín (Spain)

Abstract

Abstract. Hontomín (N of Spain) hosts the first Spanish CO2 storage pilot plant. The subsurface characterisation of the site included the acquisition of a 3D seismic reflection and a circumscribed 3D magnetotelluric (MT) survey. This paper addresses the combination of the seismic and MT results, together with the available well-log data, in order to achieve a better characterisation of the Hontomín subsurface. We compare the structural model obtained from the interpretation of the seismic data with the geoelectrical model resulting from the MT data. The models corr elate well in the surroundings of the CO2 injection area with the major structural observed related to the presence of faults. The combination of the two methods allowed a more detailed characterisation of the faults, defining their structural and fluid flow characteristics, which is key for the risk assessment of the storage site. Moreover, we use the well-log data of the existing wells to derive resistivity-velocity relationships for the subsurface formations and compute a 3D velocity model of the site using the 3D resistivity model as a reference. The derived velocity model is compared to both the predicted and logged velocity in the injection and monitoring wells, for an overall assessment of the resistivity-velocity relationships computed. Finally, the derived velocity model is compared in the near surface with the velocity model used for the static corrections in the seismic data. The results allowed extracting information about the characteristics of the shallow subsurface, enhancing the presence of clays and water content variations. The good correlation of t he velocity models and well-log data demonstrate the potential of the two methods for characterising the subsurface, in terms of its physical properties (velocity, resistivity) and structural/reservoir characteristics. This work explores the compatibility of the seismic and magnetotelluric methods across scales highlighting the importance of joint interpretation in reservoir characterisation.

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

“…The analysis of seismic noise is a valuable tool for subsurface characterization as shown by many authors such as Aki (1957) or Okada (2003). These techniques are founded on using the energetically dominant part of seismic noise, which consists mainly of surface waves (Bonnefoy-Claudet et al, 2006).…”

“…In any case, subsurface architecture mapping with the seismic reflection method requires a good-quality shallow-velocity structure for static correction performance. Detailed nearsurface models are usually obtained from high-density borehole information or velocity models derived from seismic tomography and/or first arrival inversions (Bridle et al, 2006;Ogaya et al, 2016).…”

Estimating interpretation uncertainty from magnetotelluric inversion

“…Underlying the interpretation of MT data, is the perennial challenge of inversion uncertainty. A variety of approaches to limiting or quantifying inversion uncertainty are available which usually take the form of joint/constrained inversion (Heincke et al, 2006;Zhdanov et al, 2010;Moorkamp et al, 2011;Meqbel and Ritter, 2015;Ogaya et al, 2016) or probabilistic inversion (Brodie and Jiang, 2018;Conway et al, 2018). Less common is detailed consideration of model variability during interpretation.…”