|SUB-BASALT IMAGING THROUGH SIMULTANEOUS JOINT INVERSION OF SEISMIC AND MMT DATA IN THE NORWEGIAN SEA|
1Schlumberger Cambridge Research, 2WesternGeco
The term “joint inversion” is commonly used in the oil and gas industry to indicate a wide range of technologies and workflows that aim to integrate different measurements for geophysical exploration. Li and Oldenburg (1996) used borehole and surface magnetic data to invert for susceptibility; Dell’Aversana (2001) integrated seismic and electromagnetic data for structural imaging; Hu et al., (2009) describe a general method where one domain acts as a constraint for the others, and De Stefano and Colombo (2007) inverted linked data within a single cost function. This approach is called simultaneous joint inversion (SJI), given that the workflow integrates the measurements in the inversion phase, and it is not simply an alternating sequence of single measurement inversions. In this paper we present SJI for sub-basalt imaging.
After an initial mathematical explanation, we show an application on real data acquired in the Møre basin, offshore Norway.
The SJI workflow presented in this paper combines marine magnetotelluric (MMT) and seismic measurements, using a single objective function which is defined and minimized (De Stefano and Colombo, 2007), in contrast to an approach in which multiple objective functions are inverted in separate domains.
The kernel of the objective function is built from three different elements: residuals from different domains, single domain constraints (internal constraints for regularization) and inter-domain constraints (external constraints, i.e., linking equations across measurements). From this point of view, the role of SJI is to combine the residuals, collate the constraints for single domain models, and set the constraints between the models of the different domains. The algorithm inverts for all models providing updates for the different domains. The SJI workflow joins together only the inversion phases of the different marine measurements involved. From the inversion point of view, SJI is fully described by the single objective in Equation 1. In this context, the implementation of the inversion process proposed here is a linear minimization of an L2 norm cost function through a standard LSQR solver. We used a second order regularization without preconditioning.
The magnetotelluric (MT) method is a geophysical exploration tool where natural electromagnetic fields are measured to investigate the electrical conductivity structure of the subsurface. The natural sources of the MT field in a marine (MMT) environment are the current systems in the magnetosphere created by solar activity. Data are commonly acquired with an array of seabed receivers recording two horizontal components of the electric (E) and magnetic (H) fields.
We used 2D seismic data, collected along line GMNR-102 by Geco-Prakla (now WesternGeco) in 1993. For this study, the 1993 raw seismic data underwent prestack reprocessing and were re-migrated in depth (PSDM) using the SJI approach for the velocity model building. The 1993 prestack time migration (PSTM) is used as a benchmark and displayed for comparison. The SJI PSDM seismic processing went through slightly different processing steps, given the new seismic technologies currently available for data conditioning (i.e., advanced multiple removals). The MMT data were collected along the same line as GMNR-102 in the summer of 2008 (Figure 1) using receivers with variable spacing and processed using a robust remote reference algorithm.
|THE GEOLOGICAL ENVIRONMENT|
The Møre basin lies in the Norwegian Sea, between the uplifted mainland and the Cretaceous Trøndelag Platform to the south-east and the Møre Marginal High covered by Eocene lavas to the north-west. Flood basalts were extruded from uplifted seaward fissure vents as continental break-up and seafloor spreading initiated under the influence of the Icelandic Mantle Plume, ending around 55 Ma. The outer flows form seaward dipping reflector sequences while, landward, the lavas flowed into the pre-existing Mesozoic basin covering a thick Cretaceous and later basin fill. The basin formed during a long period of extension and rifting that thinned the igneous crust beneath the basin centre but when continental separation occurred in Early Eocene times, the rift was seaward of the present basin axis leaving the thicker crust of the Møre Marginal High, itself augmented by flood basalts and, probably, deep crustal intrusion or underplating as has been observed at the continent-ocean transition (COT) east of the Faroes (White et al., 2008).
The Møre basin is bounded to the north-east and offset from the neighbouring Vøring basin by the Jan Mayen Fracture Zone, extending into continental crust as the Jan Mayen Lineament (Eldholm et al., 2002). To the north-west, the basin is bounded by the Faroes-Shetland Escarpment where the top basalt shows significant relief at the former lava delta and marks a pronounced seaward thickening of the basalt pile (Figure 2, left). The lavas thin south-east, towards the centre of the basin where sills and low-angle dykes have intruded the pre-Eocene sediments. While Cretaceous sediments are known to be thick, Lower Cretaceous control is poor and uncertainty exists as to their total thickness as well as the location of base basalt in many areas.
The target for the survey acquired along line GMNR-94-102 was to image below the thick basalt package which is supposed to characterize the area under investigation (Figure 2, right) and to determine whether prebasalt sediments are present.
Solving for complex velocity models is the first goal of the SJI workflow in which seismic data are integrated with MMT data. MMT is an inductive method capable of detecting the resistivity contrast at the base of basalt and it also provides a key benefit for velocity model building within and below basalt (Figure 3, right).
Marine application of SJI confirms the capability of defining a velocity model properly constrained by EM data. Figure 2-(a) shows a comparison between the PSTM converted to depth and the SJI PSDM in a centre area of the line, where the basaltic formation gets thicker.
The seismic data have been migrated prestack using the SJI velocity model described in Figure 3 (right). We used here a common-offset Kirchhoff depth migration and obtained an improved image of reflections below top basalt. Although the PSTM and SJI PSDM are not strictly comparable, it is evident that the SJI images have revealed more coherent events below the top basalt (Figure 2-(b)). Furthermore, the SJI workflow has resulted in a consistent model of the acoustic and electrical properties of the stacked basalt flows.
The final resistivity model resembles the seismic interpretation in terms of geometry (Figure 3, left). It shows a resistivity decrease below the thick resistive basalt layer and this conductive feature can be associated with the presence of sediments. The resistivity distribution, which would have been inevitably smeared by the use of a smooth regularized single-domain inversion and would have suffered from the intrinsic low resolution of MMT, benefits from the high resolution of seismic data.
We present an application of simultaneous joint inversion between seismic and MMT data over the Møre basin offshore Norway, providing an improved image for the interpretation of the sub-basalt formations when compared with the previous PSTM image. SJI improved both convergence speed and seismic migration quality; the robustness is provided by the different domains working as mutual constraints with the benefit of the cross-gradient computations, encouraging the two models to have similar shapes (basalt formation is both very fast and very resistive compared to the sediments).
The intrinsic non-uniqueness of the EM inverse problem has been reduced, improving the resolution and accuracy of non-seismic data domains. The resulting resistivity model benefits from the geometrical constraints introduced by the seismic domain, including the shallow part where the lack of high-frequency MMT data limits the information on the model space. The base basalt-sediment interface is easily interpretable from the resistivity model and it is not smeared out as often happens with smooth, regularized, single-domain inversion.
The SJI project started in January 2007, supported initially by Geosystem S.r.l. and then by WesternGeco. Randall Mackie is thanked for providing the magnetotelluric inversion algorithm and Michele De Stefano as main developer of the 2D SJI program. Marco Mantovani is thanked for his input in the generation of the workflow and for the seismic information for SJI, and Massimo Clementi provided vital support for data conditioning. Don Watts, Stephen Hallinan and Wolfgang Soyer provided their know-how and useful suggestions on the applications for the non-seismic side.
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