Multi-geophysics exploration. Summary

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1 Sub-basalt imaging with broadband magnetotellurics in NW Saudi Arabia Daniele Colombo*, Tim Keho, Emad Janoubi, Saudi Aramco Wolfgang Soyer, WesternGeco Summary A magnetotelluric (MT) survey together with ancillary gravity and Time-Domain EM (TDEM) data acquisition was carried out in a region in NW Saudi Arabia characterized by shallow basaltic layers. The extensive volcanic activity occurred in multiple phases during the late Devonian, Cretaceous and Neogene. A potentially prospective graben structure in the area is partially covered by basalts which severely degrade the quality of the seismic data. The MT inversion results and the interpretation of the multi-geophysics dataset reveal important structural elements such as deep-seated faults controlling the graben evolution and allow characterization of the complex basalt cover. Integration of the different geophysical datasets is ongoing via simultaneous joint inversion, followed by reprocessing of the seismic data. Introduction Exploratory drilling and coring in the NW of Saudi Arabia has revealed two distinct igneous rock suites, late Devonian (360 Ma) and late Cretaceous (76-82 Ma). The Devonian sills are significant intrusive events due to extensional and compressional tectonics. The Cretaceous suite is a minor igneous event related to faulting and the high geothermal gradient in the active Al Jawf Graben. The most recent activity during the Neogene corresponds to an outcropping basalt sequence. The lithologic composition of the basalt cover is very heterogeneous both vertically and horizontally. Similarly to other basalt-covered areas, the high velocities generate strong ray bending in the near surface and result in poor penetration of the seismic energy. Moreover, the internal heterogeneities in the basalt cover as well as the rugosity of the basalt layers generate scattering effects. Sub-basalt imaging is one of the most challenging scenarios for seismic exploration worldwide and alternative or complementary geophysical solutions are often needed to address the problem. Use of magnetotellurics (MT) and other electromagnetic (EM) methods has proven to be effective in other basalt-covered areas both as an alternative imaging tool to look deep below the basalt cover, such as in the Deccan Plateau, India (Strack and Pandey, 2007), and to improve the seismic velocity model as in the Columbia River Basin, Washington to improve the seismic imaging results (Colombo et al., 2008). Multi-geophysics exploration In 2010 Saudi Aramco carried out multi-geophysics surveys in different areas of Saudi Arabia (Colombo and Keho, 2010). One of the tests involved a regional seismic line crossing a graben structure partially covered by basalts in the NW of Saudi Arabia (Figure 1). The purpose of this survey was to demonstrate the benefits that multiple geophysical data can provide to the solution of long standing seismic imaging challenges. TDEM, MT and gravity data were acquired along a pre-existing 2D seismic line crossing a major graben structure partially obscured by outcropping basalt cover. The structure can be classified as a half graben with a major fault displacement located below the basalts and not imaged by seismic. The objectives of the experiment were to define the thickness and the internal variations of the basalt flow and to define the deep structure of the graben, particularly identification of the major faults. The experiment includes reprocessing of the seismic data in time and depth by means of simultaneous joint inversion of seismic data with TDEM, MT and gravity. The joint inversion and seismic reprocessing is ongoing. 100 km Figure 1: A digital elevation model showing the location of the MT stations for one of the multi-geophysics projects in NW Saudi Arabia.. The basalt-covered area corresponds to the central section of the line where the soundings are denser. The acquisition specifications for the non-seismic methods are shown in Table 1. A denser spatial sampling was used to explore the basalt-covered area (core area) relative to the rest of the profile. SEG San Antonio 2011 Annual Meeting 619

2 Data TDEM, MT and gravity quality was good to very good. Expected problems related to high contact resistance for the low-frequency MT electric-field measurements did not manifest despite the fact that the survey was conducted in late July and August with temperatures typically above 40 C. TDEM data occasionally suffered from poor penetration through the highly resistive cover, so the loop size was increased during the survey from 50 m to 100 m. TDEM was generally able to detect a conductive layer below the outcropping basalts. Three-dimensional (i.e., out of plane) effects were generally small except for the SW section of the line where clear evidence of an off-line intrusive body was found. The intrusive body seemed to affect mostly the low frequency MT data. reproduce statics in this mode. The 2D inversion was completed in 50 iterations obtaining a final RMS residual of Figure 2 shows the comparison between the calculated pseudo-sections (i.e., frequency domain) of field data and forward MT responses at the last inversion iteration. The results indicate an excellent fit between observed and predicted data. TM mode Methodology Spacing (m) Specifications MT 1,000/2, Hz-10-3 Hz AMT 500/2,000 10,000Hz-1 Hz TDEM m loop only core area Gravity 250/ mgal repeatability Table 1: Acquisition parameters for the non-seismic measurements coincident with the 2D seismic line. The basalt-covered surface area is where the spatial sampling is denser. TE mode Data processing and inversion MT data were processed for all the frequency bands (0.001 to Hz) with robust processing methods (Larsen et al., 1996). Given the 2D geometry of the acquisition, the calculated impedances were rotated to N60 E (TM mode) and N30 W (TE mode) consistent with the expected dip and strike of the geologic structures. TDEM data processing followed a standard sequence consisting of editing outliers followed by stacking. Typically, 960 transients were vertically stacked for each sounding location. TDEM data were separately inverted using a 1D approach. The resulting 1D resistivity models were used to predict the high-frequency MT responses (100-10,000Hz) and to correct for large MT curve statics. The MT data were inverted for the full available bandwidth covering seven decades of frequency using 2D nonlinear conjugate gradient (NLCG) inversion (Rodi and Mackie, 2001) and starting from a homogeneous resistivity halfspace. Residual static shifts were jointly inverted with the MT data by providing differential weighting to TE and TM modes. TE modes were allowed more freedom to change during the inversion, as the forward solution cannot Figure 2: Comparison between pseudosections of field data and forward responses after 50 inversion iterations for MT TM and TE polarization modes. The pseudosections show an excellent fit between field and modeled data over seven decades of frequency (vertical axis shows period, i.e. 1/f, in s; horizontal axis shows the distance along the profile, in km). SEG San Antonio 2011 Annual Meeting 620

3 Results A fine mesh parameterization was used for the shallow section of the model (layer thickness between 10 and 20 m for the first 500 m of the model). This allowed accommodation of the residual static shifts during the inversion, and resolution of very fine details of the shallow resistivity structure. The first thin basalt layer, a conductive layer beneath it, and a lower thicker resistive layer were clearly imaged by the inversion (Figure 3). The details of the imaged resistivity structure can be tracked on the MT curves by observing the high frequency variations of the apparent resistivity and phase data (Figure 3-top). The outcropping basalt flow appears laterally heterogeneous with a maximum thickness of approximately 200 m. This result is consistent with basalt thickness data from a deep uphole survey run in the area where the closest uphole to the line shows a basalt thickness of 185 m (Ley, personal communication, 2010). TDEM inversion showed similar results to the MT inverted resistivities, both in terms of internal variations and thickness of the basalt cover (Figure 4a). Moreover, the deeper resistive horizon imaged by the MT inversion in the center of the profile suggests the presence of a second thicker basalt layer, a sill or a similar shallow intrusion. Rho (Ohm.m) basalts sediments 2 nd basalts? Ph (deg) Period (s) Elev. (m) m.s.l. x (km) Figure 3: MT inversion results in the center of the profile corresponding to the basalt-covered area. The high frequency content of the AMT-MT data (0.001 to Hz; horizontal axis shows period, i.e., 1/f) reveals fine details of the basalt cover including the existence of a deeper resistive layer (possibly a second basalt or a sill). The sample MT soundings (top) display the fit of the modeled responses (solid lines) with the field data (dots) regularized by D+ smoothing (Parker, 1980). The deep MT inversion results provide important information about the deep structure of the graben and of the deep seated faults (Figure 4). Indeed, the main fault controlling the graben on the NE side is clearly identified by the MT inversion results (Figure 4-c) together with other important structural features. The high resolution gravity data acquired along the line also confirm the position of the deep structures revealed by the MT inversion results. Conclusions The multi-geophysics investigation of a basalt covered area in NW Saudi Arabia demonstrated the added value that SEG San Antonio 2011 Annual Meeting 621

4 integrated non-seismic techniques provide to the exploration of complex geologic settings. This is particularly true for basalt-covered areas where broadband electromagnetic methods such as magnetotellurics can investigate deep below the basalt cover and provide important information about the geometry of faulted basins. The detailed structure of the basalt cover and the internal variations are also obtained by a combination of passive and active EM methods. This information can be further utilized in combination with the gravity data to improve seismic velocity model building. We are currently performing this additional task by applying simultaneous joint inversion of seismic travel-time data with TDEM, AMT-MT and Gravity data. Acknowledgments We would like to thank the Saudi Arabian Oil Company (Saudi Aramco) for permission to present these results. We also would like to thank Robert Ley of Saudi Aramco for the continuous support offered throughout the project. Figure 4: Multi-geophysics exploration of a regional transect across the basalt-covered area of NW Saudi Arabia: (a) Time-Domain EM (TDEM) inversion results detailing the thickness and the internal variations of the shallow basalt cover, (b) gravity profile (complete Bouguer anomaly) showing the asymmetric location of the sediment-filled graben relative to the basalt cover, (c) AMT-MT 2D inversion results in color overlying the seismic data with a schematic interpretation of the major faults of the graben and of a reference horizon, (d) the seismic data shown in c) without the MT inversion overlay. SEG San Antonio 2011 Annual Meeting 622

5 EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2011 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES Colombo, D., and T. Keho, 2010, The non-seismic data and joint inversion strategy for the near surface solution in Saudi Arabia: SEG, Expanded Abstracts, 29, Colombo, D., M. Mantovani, S. Hallinan, and M. Virgilio, 2008, Sub-basalt depth imaging using simultaneous joint inversion of seismic and electromagnetic (MT) data: a CRB field study, SEG, Expanded Abstracts, Larsen, J. C., R. Mackie, A. Manzella, A. Fiordelisi, and S. Rieven, 1996, Robust smooth magnetotelluric transfer functions: Geophysical Journal International, 124, , doi: /j x.1996.tb05639.x. Parker, R. L., 1980, The inverse problem of electromagnetic induction: Existence and construction of solutions based on incomplete data: Journal of Geophysical Research, 85, , doi: /jb085ib08p Rodi, W., and R. L. Mackie, 2001, Nonlinear conjugate gradients algorithm for 2-D magnetotelluric inversion: Geophysics, 66, , doi: / Strack, K. M., and P. B. Pandey, 2007, Exploration with controlled-source electromagnetics under basalt cover in India: The Leading Edge, 26, , doi: / SEG San Antonio 2011 Annual Meeting 623