Progress In Electromagnetics Research Symposium Proceedings, Cambridge, USA, July 58, 2010 617

Simultaneous Joint Inversion of Seismic and Magnetotelluric Datafor Complex Sub-salt Depth Imaging in Gulf of Mexico

M. Viriglio, M. De Stefano, S. Re, F. Golfre` Andreasi, and F. F. C. SnyderWesternGeco, Italy

Abstract We present the first application of the 3D simultaneous joint inversion (SJI) be-tween seismic and marine magnetotelluric data over the northern Gulf of Mexico. Interpretingthe complex salt structures is a key to understand and create accurate tomographic velocitymodels, which in turn, are necessary to properly position and image the subsalt targets in theframework of the geophysical exploration. SJI collects seismic and non-seismic information intoa single-objective function to be inverted, as opposed to the multiple functions inverted by bothsingle-domain approaches. SJI enhances the ambition of improving the existing velocity modelsfor prestack depth migrations and the consistency of seismic and non-seismic representations ofthe subsurface in complex salt geometries.

1. INTRODUCTION

Various approaches have been proposed ([13]) for multidomain and multimeasurement integration,both in processing and interpretation. As a general view, integration can take place at differentlevels of an exploration workflow: to constrain processing, inversion, or simply when comparinginterpretations or co-rendering them. Nowhere is this approach more important than in the GreenCanyon-Garden Banks-Keathley Canyon-Walker Ridge areas where salt complexity is challenging,even with the latest wide-azimuth acquisition and processing methods.

Properly interpreting the numerous coalescing allochthonous salt canopies that cover potentialreservoir structures is a key to understand and create accurate tomographic velocity models. Thesalt structures are particular challenges to deepwater exploration in this part of northern Gulf ofMexico and in order to properly position the subsalt targets for geophysical exploration, a consistentrepresentation of the velocity model is the most important requirement for depth imaging.

2. SIMULTANEOUS JOINT INVERSION: THE METHOD

Simultaneous Joint Inversion (SJI) is a robust and integrated process to invert multiple geophysicalparameters within one unique cost function. Beyond the algorithm, this requires integrated work-flows across traditionally distinct geophysical domains (Seismic, Gravity, and Electromagnetics).

The SJI workflow presented here combines MMT and seismic measurements integrated withinthe inversion phase: in SJI inversion, one single objective function is inverted, as opposed to themultiple functions inverted in the single domains approach ([1, 2]). The kernel of the objectivefunction is built by three different elements as displayed in Equation (1) ([1]): residual collectionfrom different domains (d1, d2), single domain constraints (m1, m2) and several interdomainconstraints (i). From this point of view, the role of SJI is to combine the residuals, collate theconstraints for single domain models, set the constraints between the models of different domains,and finally invert for the two models involved.

In this contest, SJI uses the same regularization and preconditioning that the single domainsuse for standard inversion. This is one of the key benefits provided by the SJI: each domain isregularized (and/or preconditioned) as in single domain inversions. First order (Gradient filters) orsecond order (Laplacian filters) have been tested and used, but in general any regularizator couldbe used in order to drive the coherence between adjacent cells of the same model. Laplacian filtersfor both seismic and MMT have been used in the following examples.

The algorithm inverts for all the models providing updates for the different domains. From theinversion point of view, the SJI is fully described by the single objective function in Equation (1)where we used a summation of different costs as opposite of [2] where multiplication has been used:

(m1,m2) = 1d1(r1) + 2d2(r2) + 3m1(m1) + 4m2(m2) +ni=0

i(m1,m2) (1)

is the unique function to minimizemi are the models to invert

618 PIERS Proceedings, Cambridge, USA, July 58, 2010

i are the contributes from the residualsi are the contributes from the regularizations1, 2, . . . , n are the cross-links between unknownsi are the weights for each component

3. THE MT SIGNAL AND DATA PROCESSING

The electromagnetic source for magnetotellurics is the natural time-varying geomagnetic field. Auseful measure of its level is the Ap index. It is a measure of the general level of geomagneticactivity over the globe for a given day. It is derived from measurements made at a number ofstations world-wide of the variation of the geomagnetic field due to currents flowing in the earthsionosphere and, to a lesser extent, in the earths magnetosphere. The strength of solar activitiesaffects the quality of MT signal. The quality of the MT response has a correlation with the goodMT source signal. Although solar activity is generally not predictable, the geomagnetic amplitudepulsed at a frequency of approximately 69 days. A longer recording window can increase thelikelihood of capturing peak MT pulses. Another benefit of a longer recording window is increasingthe stacking. Stacking can reduce the random (incoherent) noise and improve the signal-to-noiseratio. The receivers have some common noisy segments, e.g., instrument deployment and recovery.There are also some noise sources which can affect the receivers in different ways, e.g., complicatedocean environment, motional noise, spikes from the atmosphere and ionosphere, pipe lines etc.These noisy segments of data were cut off from the time series and the quietest segments of datawere then used for processing.

A robust remote reference processing approach [4] was used to calculate the MT impedancetensor from the time series data. The use of a remote reference receiver is the usual way to reducethe incoherent noise in the time series and therefore to improve the data quality.

4. SUBSALT IMAGING

Subsalt seismic imaging is a very common problem due to complex lateral and vertical velocityvariations, scarce penetration of seismic energy, wavefield scattering, multiples, conversions, strongray-path distortion, and irregular illumination with multipath. Solving for complex velocity modelsis the first goal of the SJI technology when integrated with MMT domain.

MMT measurements provide additional data on the high resistive anomaly of the salt. MMTis suited to investigate within and below the salt formation which provides a strong resistive effectboth on apparent resistivity and phase but more importantly MMT is an inductive method capableof detecting the resistivity contrast at the base of salt and it provides a key benefit for velocitymodel building within and below basalt.

5. SYNTHETIC MODELS

The synthetic tests are based on the standard SEG 3D synthetic model and have three differentelements: a water layer, a space variant background velocity (sediments), and the salt formationwith constant velocity. We built a proper resistivity model [ m] using well logs informationpopulating the seismic structural framework, and we inverted velocities and resistivities with SJI.In this way, we emulate the common case of very fast and resistive salt structure with different dipsand depths, one of the main challenges for today marine applications. Figure 1 shows the velocityand the resistivity models: the two domains use different grids according to the resolution of themeasurement. SJI provides a better inversion of the salt anomaly, and a more stable velocity ofthe background; the SJI velocity model is also more accurate below the bottom of the salt.

6. REAL DATA APPLICATION IN WALKER RIDGE NORTHERN GULF OFMEXICO

Since the formation of the Jurassic Louann salt during continental crust pre-breakup and subse-quent early salt movement due to gravity spreading toward the cooling and sinking oceanic plate,the Gulf of Mexico has been destined as a tectonically favorable exploration province, albeit inplaces very complex. This complexity comes in part from the repeated and diverse salt movementepisodes and the resulting deep water allochthonous canopies; our remote sensing attempts to im-age favorable hydrocarbon-bearing structures beneath. Properly imaging and interpreting the saltand substructures are primary tools to understanding and creating accurate geologically driventomographic velocity models, which in turn, are necessary to properly position and image thesesubsalt targets.

Progress In Electromagnetics Research Symposium Proceedings, Cambridge, USA, July 58, 2010 619

Figure 1: LHS: Comparisons between single domaininversions (top left) and SJI models (bottom left):SJI improves the focusing of the MMT inversions ofthe salt formation (black line). RHS: Match of SJIbase of salt and synthetic one. Proper model thebase of salt is a key for enhancing the subsalt depthmigrations.

Figure 2: Workflow for the SJI. Clockwise from topleft: initial structural framework, salt removal, SJIinversion, and SJI interpretation.

Figure 3: Left: Initial resistivity model. Middle: SJI inverted resistivity. Right: SJI resistivity update.

A robust approach involving seismic and non seismic measurements is very important in theGreen Canyon-Garden Banks-Keathley Canyon-Walker Ridge areas because the salt complexity isextremely challenging, even with the latest seismic acquisition methods. Using geological informa-tion and multidomain integration to augment seismic data is proving successful. These methodshelp better facilitate interpretating the entire salt, which is important not only for a better saltmodel and reservoir image, but also to better understand possible reservoir compartmentalizationmechanisms, abnormal pressure cells, and other exploration risks.

Figure 3 shows the SJI resistivity results: inverting MMT data together with seismic reflectionshelped to detect/confirm the base of allochthonous salt at the macro-scale, and also the top of thedeeper and resistive autochthonous salt. Given the high-resolution of the seismic migration, thenew bottom of the salt has been interpreted using the seismic velocity and migration.

Figure 2 shows some 3D views of velocities (colored) and seismic (black and white) overlays. Forthe 3D SJI proof of concept, we started from an existing seismic reflection tomographic model (topleft) from which we removed a portion of the salt (top right) obtaining the initial velocity modeland ran several iterations of targeted 3D SJI with reflection seismic and MMT data, obtaining thefinal SJI velocity model (bottom right). The bottom left image shows how the 3D SJI has led to anew 3D interpretation of the allochthonous salt base (colored) and positioning of the autochthonous

620 PIERS Proceedings, Cambridge, USA, July 58, 2010

Figure 4: Subsalt zoom of the PSDM sections using single domain velocity model (LHS) and SJI velocitymodel (RHS). Red arrows points to the base of the salt. White arrows point ot the improved subsalt migratedevents.

top salt (blue). The new interpretations have been used for new depth migrations to confirm theimprovements in the new SJI model.

The SJI results are shown in Figure 4 by means of 3D depth migrations. SJI provides a morefocused base of the salt, enhancing the quality of the subsalt migrated events.

7. CONCLUSIONS

We integrated the inversion of seismic and MMT data with the 3D simultaneous joint inversion,showing an application in the northern Gulf of Mexico, and producing new interpretations of theallochthonous and autochthonous salt and thus an improved imaging with new depth migrations.SJI enhances the quality of the seismic migrations, reduces the inversion uncertainties, and mostimportantly defines a new strategy for subsalt interpretation, thereby enhancing the role of non-seismic methods.

ACKNOWLEDGMENT

The authors would like to thank Stephen Alwon, Michael OBriain, Don Watts, and Marta Wood-ward for their advice and help for the project, Luca Masnaghetti for the internal review of thispaper, and WesternGeco GeoSolutions and Electromagnetic divisions for the seismic, gravity, andelectromagnetic dataset.

REFERENCES

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3. Gamble, T. D., W. M. Goubau, and J. Clarke, Magnetotellurics with a remote reference,Geophysics, Vol. 54, 5368, 1979.

4. Wenyi, H., A. Abubakar, and T. M. Habashy, Joint electromagnetic and seismic inversionusing structural constraints, Geophysics, Vol. 74, No. 6, 99109, 2009.