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SPECIAL TOPIC: MARINE SEISMIC & EM

Vertical dipole CSEM: 3D acquisition and data impact from infrastructure Kjetil Eide1* presents improvements to the method and a 3D field data example. Introduction Marine CSEM methods for subsurface investigations were developed nearly 30 years ago (Cox, 1981), and have found extensive applications within the offshore hydrocarbon exploration industry over the past 15 years (e.g. Constable, 2010). These methods detect contrasts in electrical conductivity, exploiting the fact that the electrical conductivity of hydrocarbon-saturated reservoirs is significantly smaller than in the surrounding sediments (Ellingsrud et al., 2002). While marine CSEM has proven itself as a valuable tool in exploration and mapping of frontier areas, there is growing increased interest in applying CSEM for near-exploration and reservoir monitoring applications. A vertical-based time domain EM exploration method (Barsukov et al., 2007) has been developed by the Norwegian geophysical company PetroMarker, founded in 2005. In the past the method has primarily been used for exploration, but since the vertical method relies on a stationary acquisition mode with very high accuracy in transmitter positioning it allows both repeatability and freedom of operation close to existing infrastructure and installations, and in vulnerable environments. Over the past few years there have also been improvements in receiver technology, allowing efficient 3D acquisition. We will present improvements to the method and a 3D field data example. We will also present investigations of the impact of the method on field data from infrastructure.

Figure 1 Vessel used for 2018 acquisition with transmitter and receiver systems. The transmitter electrodes are lowered into sea from the A-frames on the side of the vessel. Dynamic positioning ensures verticality of source dipole during transmission.

1

PetroMarker

*

Corresponding author, E-mail: kjetil.eide@petromarker.com

The vertical CSEM method operates in the time domain and uses a stationary vertical electric dipole (VED) source which transmits a transient electromagnetic signal. The potential of using vertical dipoles as transmitters was suggested at an early stage by, for example, Kaufman and Keller (1983) and Edwards et al. (1985). Goldman (1990) demonstrates the high sensitivity of vertical transient measurements towards resistivities in between resistive air and a resistive basement. When the vertical source is turned on a DC field diffuses outwards both into the sea water and into the subsurface. When the source is turned off, it induces a secondary electromagnetic field which diffuses from the subsurface and back to the receivers, which record the

Figure 2 CSEM receiver with active verticality correction. The vertical field is measured along the vertical centre pole within a tripod structure. A sensor in the top of the receiver measures the verticality of the dipole, and an active system adjusts the position to achieve verticality within 0.1°.

DOI: 10.3997/1365-2397.2019033

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vertical electric field component. The receivers are located at an offset range where the signal is above background noise levels, typically 500-5000 m. A general overview of the method is given by Helwig et al. (2013). With the vertical method the main sensitivity is observed at relatively short transmitter-receiver offsets up to 2000 m, and the ability to record near-offset data gives rise both to increased lateral resolution and deeper imaging compared to other CSEM technologies (Alumbaugh et al., 2011). The vertical source also reduces the airwave influence through limited air-water interface coupling (Constable and Weiss, 2006). Another important factor in the vertical CSEM acquisition is the stationary pulsing, allowing more energy to be transmitted by increasing the pulsing time on each location to optimize signalto-noise ratio environments. Vertical 3D CSEM acquisition While the vertical methodology has theoretical advantages for imaging small resistors, there are strict requirements on the acquisition set-up. The vertical E-fields are typically weaker than horizontal fields by two orders of magnitude, resulting in very low tolerance limits on the verticality and noise performance of a vertical E-field receiver. During the first decade of vertical CSEM data acquisition the size and deployment mode of the receiver systems limited data acquisition to 2D applications. In 2016 a new compact vertical receiver was launched (Helwig et al., 2016), allowing efficient 3D acquisition for the first time. The receivers are equipped with an active verticality correction for the vertical electric dipole sensors. Each receiver has four parallel vertical dipoles, and orthogonal horizontal sensors allow the recording of all electric field components. Receivers are dropped to the sea floor by gravity, positioned accurately by acoustic signals and then the verticality is adjusted for a potentially tilted

receiver frame. When the data acquisition has been completed an acoustic signal is transmitted to the receiver initiating the release of sand from the ballast tanks, and the receiver can be retrieved by the survey vessel. The vertical electrical dipole source is adjustable to the sea depth in the acquisition area. The transmitter has lower electrodes sitting on the seabed while upper electrodes are lowered circa 40 m below the vessel. Usually, two pairs of electrodes operate in parallel, and the combined output is 5000 amperes. The source has been operated in depths from 90 m to 1700 m. During pulsing the verticality of the transmitter electrodes is monitored with dynamic positioning to ensure the verticality of the source dipole. In field repeatability tests conducted at 300 m sea depth the lower transmitter electrode position can be repeated with an accuracy below 1 m. During June and July 2018 PetroMarker acquired 700 km2 of vertical CSEM data around the Gjøa field in the Norwegian part of the North Sea. Sea depth in the area ranged from 160 m to 390 m. The survey covered both licensed and open acreage and included calibration against both known discoveries and dry wells. Receivers were deployed in a grid with 1.7 km spacing, and the source placed in a shifted grid which provided short offsets of 700 m and 1000 m. The transmitter waveform was a P2 pulse sequence at 50% duty cycle with 9 s positive and negative pulses, each followed by a 9 s listening period. The pulsing was repeated for approximately 1 hour per transmitter location. After download of data from the receiver a high pass filter is applied to the time series to remove low frequency noise, and a robust stacking algorithm reduces the full pulse series to a single transient for each transmitter-receiver pair. Due to the static measurement any pulse that falls outside verticality or noise specifications can be discarded from stacking. The linearly sampled data is then binned

Figure 3 Gjøa 3D CSEM acquisition area, covering ca 700 km2.

Figure 4 Two pipelines from the Gjøa field traverse the acquisition area. A test transmitter position in very close proximity to the pipeline was acquired.

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Figure 5 Stacked time domain Ez responses. The vertical electric field is displayed as a function of time after transmitter shut-off. Transients are the result of circa 60 minutes of pulse stacking. Four different offsets between 700 m and 2700 m are compared for a transmitter close to the pipeline and away from the pipeline.

and averaged within logarithmic time intervals to further improve signal-to-noise. The noise level is estimated as the standard deviation of the mean of each time bin. The set of vertical E-field transients constitutes the primary input to the CSEM inversion, together with sea water resistivity profiles, measured during acquisition, and a bathymetry model. CSEM in mature areas with infrastructure In order to demonstrate the capabilities of CSEM in a near-field setting it is important to properly understand and mitigate the challenges caused by metallic infrastructure in proximity to the measurements. The highly conductive metal can interact with the polarized electric fields and produce a strong pipeline response in the data. In recent years there have been several improvements in capabilities enabling improved modelling of the response from thin conductors for land-based CSEM, and also inversion approaches for marine CSEM where data is affected by metal infrastructure (Morten et al., 2017). For time domain CSEM, the metallic structure can be considered a secondary source, setting up a secondary response that distorts the earth response. This pipeline response can be highly dependent on the geometry (Hamilton et al., 2010), with coupling to the polarization of the imposed fields set up by the CSEM transmitter. For vertical CSEM the primary vertical source has a different orientation than the horizontal pipeline. Two pipelines from the Gjøa field cross the acquisition area, as seen in Figure 4. Stabilized oil is carried south to the Troll Oil Pipeline II, while gas is exported westwards to the FLAGS pipeline. Pipeline effects are therefore primarily anticipated in the northern part of the area. For vertical CSEM, the strongest effect is expected when the pipeline follows the radial direction from transmitter to receiver.

The data from the test transmitter has been compared with surrounding transmitter positions to identify localized deviations in the areas where pipeline effects are expected. In Figure 5 responses from the test transmitter location are displayed together with responses from a reference transmitter in the same area. They reduce minimum influence from geology changes. The reference transmitter is located 2.3 km away from the pipeline. four different offset measurements in the range 700 m to 2700 m are displayed. This offset range is similar to the range of offsets used in 3D inversion. The different offset measurements also have different bearings relative to the pipeline. The transient response starts at time 0 when the transmitter is turned off. The early times off the response are characterized by a flat amplitude, which is the DC level associated with the transmitter on state. After a short time related to the characteristic diffusion length, the signal recorded at the receiver rapidly drops off. A notch in the transient represents the polarity change of the vertical field, and the late time decay reflects the resistivity distribution in the subsurface. After approximately 3 seconds, the decaying responses can be seen to approach the noise floor, as the response flattens and becomes non-smooth. The noise level is dependent on ambient noise conditions, sea depth and subsurface resistivity. For the test transmitter data no systematic distortion can be identified when comparing the test receiver data to the reference data. The decay from the test location qualitatively follows the same time development as the reference receiver for all the observed offsets, and the minor differences in amplitude can be explained by the offset differences and minor subsurface variations. When we compare the noise levels in the stacked data from the two transmitter locations, we observe that the test transmitter data has higher noise levels by approximately a factor of 2.

Assessment of pipeline effects in transient data For the Gjøa acquisition, a test transmitter location in near proximity to a pipeline was included to investigate the pipeline response. The transmitter was placed 45 m away from the pipeline, with several near-offset receivers surrounding the transmitter position.

3D inversion of time domain data 3D inversion of the time domain data has been performed by CGG Multi-Physics Milan its their proprietary code Otze for simulation of marine and time domain electromagnetic data (Scholl and Miorelli, 2018). Otze is based on a finite difference approach, and uses an implicit time stepping scheme for the 3D time domain FIRST

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Figure 6 Depth section with 3D CSEM inversion anomalies from the south of Gjøa dataset. Vertical resisitivity superimposed on seismic and resistivity log 35/12-1 (3D seismic courtesy of CGG).

computations. The code can model arbitrary transmitter-receiver configurations. 3D inversions are done with a preconditioned gradient-based method (Mackie et al., 1994). Data from the area south of Gjøa was inverted in 3D through unconstrained anisotropic inversion. The transmitter-receiver offset range for the transient input data was 500 to 3000 m, and the inversion start model was an anisotropic half-space with hori-

zontal resistivity 1.8 Ωm and vertical resistivity 3.6 Ωm. From the inversion a moderate resistivity anisotropy around 2 is observed, close to the start model value. It should be noted that while the vertical CSEM method has high sensitivity to vertical resistivity, the sensitivity to the horizontal resistivity is low. The inversion volume covers three dry wells: 35/12-1, 35/12-3 and 35/12-5. No elevated resistivity is observed from the inversion at the well locations, and this is confirmed by the resistivity logs. Offset from the dry wells the inversion resolves several resistive events in the lower cretaceous to triassic interval, suggesting the presence of additional exploration possibilities in the area. Figure 6 displays a depth section with two resistive events. Impact on 3D inversion from infrastructure Investigation of the 3D inverted resistivity volume has been performed to evaluate the pipeline impact on the imaging result. Pipeline effects are expected to appear as shallow anomalies in the vicinity of the pipelines, with associated deeper compensation artefacts. No strong artefacts have been identified around the

Figure 7 3D inversion result. Vertical resistivity depth slice at 500 m (top) and 1200 m (bottom). Dots indicate receiver positions while the solid lines are the two pipelines within the survey area.

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Figure 8 Vertical resistivity depth slice at 2350 m showing two resistive events in the lower cretaceous to triassic interval.


SPECIAL TOPIC: MARINE SEISMIC & EM

pipelines. In Figure 7 two shallow depth slices are shown, displaying vertical resistivity at depths of 500 m and 1200 m. The lateral variation in resistivity is relatively small in these shallow parts. Two deeper resistive events are seen from the depth slice at 2350 m displayed in Figure 8. The events are not laterally coinciding with the pipeline locations, and the resistivity maxima occur below receivers farther away from the pipelines.

Alumbaugh, D., Cuevas, N.H., Chen, J., Gao, G. and Brady, J. [2010]. Comparison of sensitivity and resolution with two marine CSEM exploration methods. 80th SEG Annual International Meeting, Expanded Abstracts, 3893–3897. Edwards, R.N., Law L.K., Wolfgram, P.A., Nobes, D.C., Bone, M.N., Trigg, D.F. and DeLaurier, J.M. [1985]. First Results of the MOSES experiment: Sea Sediment Conductivity and Thickness Determination, Bute Inlet, British Columbia, by Magnetometric Off-Shore

Conclusions The vertical CSEM method has undergone improvements in receiver technology which enable full 3D acquisition, while also demonstrating the suitability of the short offset vertical E-field data for 3D inversion. The stationary acquisition mode enables accurate deployment and high repeatability suitable for areas near field infrastructure. Data evaluation of measurements close to horizontal pipelines display very small effects from pipelines on the short offset data, and the impact on 3D inversion appears limited. Still, further modelling work and field data investigations will be conducted to properly analyse the effect of metallic infrastructure on the vertical data.

Electrical Sounding. Geophysics, 50, 153-160. Ellingsrud, S., Eidesmo, T., Johansen S., Sinha, M.C., MacGregor L.M. and Constable, S. [2002]. Remote sensing of HC layers by seabed logging (SBL): Results from a cruise offshore Angola. The Leading Edge, 21 (10), 972-982. Goldman, M. [1990]. Non-conventional methods in geoelectrical prospecting, Ellis Horwood series in applied geology, Ellis Horwood, 153. Hamilton, M.P., Mikkelsen, G., Poujardieu, R. and Price, A. [2010]. CSEM Survey over the Frigg Gas Field, North Sea. 72nd EAGE Conference & Exhibition, Extended Abstracts, P074. Helwig, S.L., Kaffas, A., Holten, T., Frafjord, Ø. and Eide, K. [2013].

Acknowledgements The author would like to thank CGG for permission to access the BroadSeis-BroadSource seismic data available in the Gjøa area and publish the example included in the article, and to thank CGG`s Multi Physics team in Milan for the 3D inversion work.

Vertical dipole CSEM: technology advances and results from the Snøhvit field. First Break, 31 (4), 63-68. Helwig, S.L., Wood,W., Gloux, B. and Holten, T. [2016]. A New Generation of Vertical CSEM Receiver. 78th EAGE Conference & Exhibition, Extended Abstracts, LHR1 06. Kaufman, A.K. and Keller, G. V. [1983]. Frequency and Transient Sound-

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Geophysics, 923-935. Morten, J.P, Berre, L., de la Kethulle de Ryhove S. and Markhus, V. [2017]. 3D CSEM Inversion Of Data Affected by Infrastructure. 79th EAGE Conference & Exhibition, Extended Abstracts, A409. Scholl, C. and Miorelli, F. [2018]. Otze – Airborne EM Inversion on unstructured model grids. AEGC, Abstract.

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