Development of EM simulator for sea bed logging applications using MATLAB

Transcription

1 Indian Journal of Geo-Marine Sciences Vol. 40 (2), April 2011, pp Development of EM simulator for sea bed logging applications using MATLAB Hanita Daud 1*, Noorhana Yahya 2, & Vijanth Asirvadam 3 1,2 Fundamental and Applied Sciences Department, Universiti Teknologi PETRONAS, Malaysia 3, Electrical and Electronics Engineering Department, Universiti Teknologi PETRONAS, Malaysia [ * hanita_daud@petronas.com.my] Received 23 March 2011; revised 28 April 2011 Present study is a 1D modeling of electromagnetic waves for sea bed environment, developed using MATLAB software. It focuses on two main areas; the first is on the simulator that is able to model plane layer modeling of the sea bed environment, by setting the deep of sea water, sediments and size and location of the hydrocarbon trap. This model shall be used as a forward modeling of sea bed environment. The second focus is on the effect of electromagnetic waves called direct waves, reflected waves and refracted waves on the sea bed environment where potential hydrocarbon is present. Present work is based on Sea Bed Logging (SBL) application that uses marine controlled source electromagnetic (CSEM) sounding technique to detect and characterize hydrocarbon bearing reservoirs in deep water areas. SBL uses a mobile horizontal electric dipole (HED) source called transmitter and an array of seafloor electric field receivers. Transmitter transmits a low frequency electromagnetic signal from 30m-40m above the sea bed into water layer and to underneath the sea bed. Array of the receivers will receive the signal in the form of direct waves, air waves, reflected waves and refracted waves and is measured in the form of amplitude and phase. These signals depend on the resistivity structure beneath the sea bed. Hydrocarbon is known to have high resistivity value of Ωm in contrast to sea water layer of Ωm and sediments of 1-2 Ω m. These signals were plotted to show the Electric Field amplitude versus offset (AVO) and comparisons were made. Keywords: Forward modeling, Direct waves, Guided Waves, Reflected waves, Sea bed logging, acoustic, map Introduction Since 1970, seismic method has become the most accurate and frequently used for hydrocarbon exploration because it can detect potential hydrocarbon reservoir beneath the seafloor 1. It maps the boundary layers based on the different acoustic properties 2. This technique typically uses acoustic waves to map boundaries between layers with contrasting acoustic properties. A sound source that is attached to the ship sends sound waves through the water. As the sound waves are release, the rock layers beneath the seafloor reflect this sound. The weakness it has is not able to discriminate between the presences of water or hydrocarbon in the traps 2. New technique was introduced and it could overcome problem encountered by seismic technique. This new technique is known as Seabed Logging (SBL) where it uses Electromagnetic (EM) wave to detect hydrocarbon beneath the seafloor 3. The technique identifies resistive reservoirs by measuring the energy received at long source-receiver offsets distances. This technique uses a mobile Horizontal Electrical Dipole (HED) transmitter and array of seafloor electric receiver. The HED transmitter will transmit the low frequency EM waves through the layer beneath the seafloor. The receiver will collect and record all the data that been reflected back by the layer. This technique has very bright future in hydrocarbon exploration especially for offshore application, because it provides better imaging of layer and the ability to distinguish between hydrocarbon and water 4,5,6. This paper discusses 1D plane layer modeling of sea bed environment and the magnitude and phase of direct wave, reflected wave and guided waves obtain from the electric field received by the receivers. Materials and Methods Sea bed logging technique uses electromagnetic (EM) waves to map boundaries beneath the seafloor based on subsurface resistivity contrast 3. Due to this property, sea bed logging can distinguish the presence of hydrocarbon or water traps. Even though, this technique has increasingly being used, it is very costly to solely use it to conduct hydrocarbon detection survey. Normally, seismic method will be used first to detect the presence of liquid underneath the sea bed followed by SBL to confirm whether the

2 268 INDIAN J. MAR. SCI., VOL. 40, NO. 2, APRIL 2011 liquid is water or hydrocarbon traps. In short, SBL is used to complement seismic method in hydrocarbon detection. Sea Bed Logging Method According to 3 seabed logging uses very low frequency Electromagnetic (EM) waves that are from 0.1 to 0.5 Hz. In this paper we are discussing only at 0.25 Hz, the frequency that is commonly used by many geophysicists. Using of high frequency EM wave will cause high attenuation as the function of distance and this will affect the end result. The receivers are placed at appropriate locations relative to the source and the electric dipole transmitter is typically towed at an elevation of 30 m to 40 m above the sea bed. HED transmitter is towed starting 10 km before the first receiver and end 10 km after the last receiver. The transmitter emits low-frequency EM energy (0.25 Hz) into the subsurface. This lowfrequency electromagnetic energy is able to propagate to reservoir depths where it is guided with low attenuation over long distances. Lines or grids of seabed receivers detect EM energy that propagated through the sea and the subsurface. Significantly, some of the energy is guided with low attenuation by resistive bodies, such as hydrocarbon reservoirs. The EM waves will then be reflected up and the signals will be detected by the receivers. Data that was recorded by each receiver then will be used for processing and modeling, including inversion and depth migration of EM data result in maps, cross sections and 3D volumes that show the location and the depth of resistive bodies 3. Schematic diagram of air water-sediment geometry and receivers (Rx1-Rx4) layout on seabed environment during towing of the electromagnetic source is ascribed in Fig The schematic is divided into several layers such as air, water, overburden, hydrocarbon reservoir, and half space. As illustrated in Figure 1, the solid line arrows denote reflected transmission of electromagnetic signals by the airwater surface. Dash arrow denotes direct transmission of electromagnetic signals through sea water and along the sea floor. Lastly the dot arrows denote reflected transmission of electromagnetic signals by a buried high resistivity of hydrocarbon reservoir layer. According to 5 the propagation (α) and attenuation (β) constants in conductive medium for frequencies below 10 5 Hz are defined as where ω, µ and σ represent angular frequency, magnetic permeability and conductivity, respectively. Due to non magnetic rocks in sedimentary basins then µ = µo (magnetic permeability in free-space). Due to this, in the case of fixed geometry, EM energy attenuation depends only on frequency, conductivity and sourcereceiver distance. Ultra low frequency system as in SBL, the transmitting energy rapidly attenuates in seawater and seafloor sediments saturated with conductive saline water (Fig. 2). Therefore, the direct energy transmitted through seawater dominates the recordings only at short source-receiver offsets. Air-wave (down going field) dominance depends on the source frequency, seawater depth, seawater and subsurface resistivity distribution, and sourcereceiver distances. In the case of water depth (>1000m), the air wave starts to dominate at far offsets (e.g. > 6-8 km). In high-resistivity subsurface layers ( Ωm), EM energy propagates at a higher velocity as guided waves with less attenuation and is transmitted back (up going field) to the receivers at the seafloor. Up going field from a high resistivity subsurface layer will dominates over directly transmitted energy, when the sourcereceiver offset is comparable to or greater than approximately twice the depth to this layer from the seafloor. 5 Fig. 1 Schematic Diagram of EM Transmitted and Reflected Waves with Air, Water, Sediments and Hydrocarbon Layers. Fig. 2 A Normal Incidence Wave Propagation

3 DAUD et al.: DEVELOPMENT OF EM SIMULATOR FOR SEA BED LOGGING 269 Forward Modeling of Electromagnetic Wave A simulator is developed to model a plane layer of the sea bed environment, by setting the deep of sea water, sediments and size and location of the hydrocarbon trap. This model shall be used as a forward modeling of sea bed-environment. Forward modeling is a technique of determining what a given sensor would measure in a given formation and environment by applying a set of theoretical equations for the sensor response 7,8. In forward modeling technique, model for geological model is developed. The purpose of this simulation is to generate the synthetics data and this synthetic data are then compared to the real data acquired in the field 9. If the two data agree within an acceptable level of accuracy, the geological model that is developed before can be used as accurate model of the subsurface. If not, the new synthetic data are computed and again, it will be compared with the real data. The process will continue until the synthetic data computed are match with the real data obtained with acceptable level of accuracy. 9 Electromagnetic Waves Reflection and Refraction Reflection is when waves, whether physical or electromagnetic, bounce from a surface back toward the source 10. A mirror reflects the image of the observer. Whereas refraction is when waves, whether physical or electromagnetic, are deflected when the waves go through a substance. The wave generally changes the angle of its general direction. Electromagnetic reflection and refraction by transmission through planar boundaries can be divided into two parts which are normal incidence and oblique incidence 7,11. Normal incidence is when angle of incidence, θi = 0 ο, the reflection coefficient of a plane wave is independent of the wave polarization. A reflected wave propagates back into the medium in which the incident wave propagates. Figure 2 illustrates a normal incidence. The wave number k1 and intrinsic impedance η1 of medium 1 is given as: (1) Applying simultaneous solutions for and as in (5) and (6) respectively in term of gives us (5) (6) (7) (8) From the equations above, Γ is the reflection coefficient and τ is the transmission coefficient. From 7,11 reflection at oblique incidence means that for all angles of incidence except 0 ο (normal incidence), the reflection of a plane wave depends on the wave s polarization. There are two wave s polarizations; the first is called transverse electric (TE) polarization on which E-field vector is perpendicular to the plane incidence. The second is called transverse magnetic (TM) on which the E-field is parallel to the plane of incidence. Figure 3 shows the TE polarization with θi not equal to θt. The reflection coefficient Γ and the transmission coefficient τ for oblique incidence are: (9) (10) and for medium 2 (2) (3) (4) Fig. 3 Reflected and refracted rays and orientation of the E and H fields for Transverse Electric (TE) polarization.

4 270 INDIAN J. MAR. SCI., VOL. 40, NO. 2, APRIL 2011 Results and Discussions The simulator was developed 12 by using MATLAB GUI function but all the calculations, graphs and result estimation were developed using MATLAB programming 1. Many assumptions are being made while conducting this simulation. The environment is assumed to be free from internal and external disturbances, no bathymetry effect, no various shapes of hydrocarbon reservoirs as well as other aspects which we may find in real world survey. As this is our primary work, we will improve the model in the future by taking into considerations of real sea bed environment that has many challenges in it and introduce Gaussian Noise and compare it with noiseless environment. We shall also include the effect of temperature, pressure and salinity. It is known that as temperature decreases, the density will go higher and this will cause higher losses to the EM waves. For pressure and salinity as the value increase it will also cause higher losses due to rise of conductivity and reduce of resistivity. Sea Bed Logging Simulator The simulator as in Figure 4 is developed for hydrocarbon mapping and to generate electromagnetic waves components. It has few parameters to be provided by the users; parameter of mediums, size of medium, and source. After all the parameters are entered, result will be generated. Users may choose to see forward modeling results or model estimation results. Parameter of medium is where users may assign resistivity values of air, seawater, sediments and hydrocarbon. Size of medium is where users can design the desired model. Hydrocarbon reservoir can be positioned at any desired location with any length, thickness and suitable x coordinate position. Sea water depth, sediments and hydrocarbon thickness as well as model length can be set with this simulator. Source is where the transmitter is placed from sea bed. This simulator also has noiseless and White Gaussian Noise options. If noiseless option is used, noise is excluded from the synthetics data at receivers but if White Gaussian Noise is used noise is added to the synthetics data at receivers. Fig. 4 Sea Bed Logging Simulator

5 DAUD et al.: DEVELOPMENT OF EM SIMULATOR FOR SEA BED LOGGING 271 To demonstrate the simulator, we set sea water depth of 1000m, sediment thickness of 1000m, hydrocarbon thickness of 400m with 2000m length and with coordinate of 5000m (means that hydrocarbon is 5000m from origin). The model length is set m. Figure 5 shows the 1D plane layer of the sea bed after clicking the show model button. Figure 6 shows another 1D plane layer modeling of sea bed, with sea water depth of 3000m, sediment thickness of 500m, hydrocarbon thickness of 200m with 5000m length and with coordinate of 2500m. Model length is set as 20000m. Forward Modeling of Electromagnetic Waves The experimental set-up for forward modeling of electromagnetic waves will be based on Figure 7. Fig. 5 1D plane Layer Modeling of Sea Bed Logging 1 Fig. 6 1D plane Layer Modeling of Sea Bed Logging 2

6 272 INDIAN J. MAR. SCI., VOL. 40, NO. 2, APRIL 2011 Fig. 7 Experimental Set-up for Forward Modeling In the simulator, there are total of 11 receivers and we may choose the required receivers from pull down menu to see the effect of EM waves at that receiver. The source is towed 1 km before the first receiver until 1km after the 11 th receiver. During the towing process all the 11 receivers will capture the EM wave s components. For demonstration purposes, we will demonstrate EM waves from receivers 5, 6, 7 and 8 as shown in Figure 7. Air resistivity is set as 10 5 Ωm, sea water resistivity as 0.33 Ωm, sediment resistivity as 2 Ωm and hydrocarbon resistivity as 250 Ωm. Sea water depth is 1000m, sediment thickness is 1000m, hydrocarbon thickness is 400m with 2000m Fig. 8 Magnitude and Phase of Direct EM Waves for the 4 Receivers Fig. 9 Magnitude and Phase of Reflected EM Waves from Seafloor Electric Field for the 4 Receivers

7 DAUD et al.: DEVELOPMENT OF EM SIMULATOR FOR SEA BED LOGGING 273 Fig. 10 Magnitude and Phase of Reflected EM Waves from Hydrocarbon Electric Field for the 4 Receivers length and x coordinate of 4500m. Model length is 10000m. Source amplitude is set as 100 V/m with frequency of 0.25Hz and source depth of 960m (or 40m above sea bed). The magnitude and phase of direct EM waves, air waves, reflected EM waves from seafloor and reflected and guided EM waves from hydrocarbon electric field for receivers 5, 6, 7 and 8 will be shown below. Figure 8, shows that the magnitude of direct waves at all receivers are as high as 100V/m as expected and all are in phase. This model has been set with noiseless environment; therefore the data should have minimum attenuation. Figure 9 shows the reflected EM waves from the seafloor electric field and the magnitude has reduced almost half of the applied waves. Figures 10 Fig. 11 Magnitude and Phase of Guided EM Waves from Hydrocarbon Electric Field for the 4 Receivers and 11 confirmed the presence of hydrocarbon layer beneath receiver 6 and receiver 7 that is at about 5000m offset in between these two receivers. It should be noted that electric field and magnetic field below receiver 5 and receiver 8 is null due to the absence of the hydrocarbon underneath these receivers. Conclusion 1D modeling using MATLAB simulator was developed to model 1D plane layer of the seabed and to inspect the behaviors of EM waves for certain forward model. For the forward model as in Figure 7, reflected EM waves and guided EM waves

8 274 INDIAN J. MAR. SCI., VOL. 40, NO. 2, APRIL 2011 from hydrocarbon electric field are detected only at receivers 6 and 7 and none on receivers 5 and 8. This is because these two waves carry information from the hydrocarbon reservoir which is set to be underneath of these two receivers. For this model we can safely confirm that hydrocarbon is present below receivers 6 and 7 with an approximate length of 2000m. Acknowledgement Authors would like to express our gratitude to those who have contributed to the success of this paper directly or indirectly. We also would like to express our thankfulness to Universiti Teknologi PETRONAS for the financial support. References 1 Chapman Stephen J. MATLAB Programming For Engineers - Pasific Groove : Brooks/Cole, Dyke Kate Van, Fundamentals of Petroleum Texas : Petroleum Extension Services, Vol. Fourth Edition. 3 EN Kong H. Westerdhal Seabed Logging: A possible direct hydrocarbon for deepsea prospects using EM energy. Oslo : Oil & Gas Journal, May 13, 2002 edition. 4 F. Roth F. A. Maao Improving Seabed Logging Sensitivity in Shallow Water Through Up-Down Separation - Trondhiem, Norway : EGM 2007 International Workshop, Anwar Bhuiyan, Tor wicklund, Stale Johansen, High Resistivity Anomalies at Modgunn Arch in the Norwegian Sea, Technical Article, first break volume 24, January S.E. Johanse H.E.F. Amundsen, T. Rosten, S. Ellingsurd, T. Eidismo and A.H. Bhuyian, Subsurface Hydrocarbons detected by electromagnetic sounding, First Break, Vol Ulaby Fawwaz T. Electromagnetics for Engineers. New Jersey: Pearson Education, Oilfield Glossary: Forward Modeling [Online]. Schlumberger. 11 5, Krebes E. S. Seismic Forward Modeling. University of Calgary, Calgary : CSEG RECORDER, April Electromagnetic Waves [Online] membership/html/k-6/as/physics/5/asp5_2a.html 11 Physics Lecture Notes [Online] ~cantrell/ee6317/lectures/fresnel-slides.pdf 12 Mukh Tar, Herwan S A., 3D modeling of EM Waves, Final Year Project for Electrical and Electronics Department, Universiti Teknologi PETRONAS.