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WO2008032065A2 - Orientation d'un récepteur - Google Patents

Orientation d'un récepteur Download PDF

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Publication number
WO2008032065A2
WO2008032065A2 PCT/GB2007/003464 GB2007003464W WO2008032065A2 WO 2008032065 A2 WO2008032065 A2 WO 2008032065A2 GB 2007003464 W GB2007003464 W GB 2007003464W WO 2008032065 A2 WO2008032065 A2 WO 2008032065A2
Authority
WO
WIPO (PCT)
Prior art keywords
data
phase
receiver
electric
dipole
Prior art date
Application number
PCT/GB2007/003464
Other languages
English (en)
Other versions
WO2008032065A3 (fr
Inventor
Rune Mittet
Odd Marius Aakervik
Original Assignee
Electromagnetic Geoservices Asa
Copsey, Timothy Graham
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Electromagnetic Geoservices Asa, Copsey, Timothy Graham filed Critical Electromagnetic Geoservices Asa
Priority to EP07804257A priority Critical patent/EP2067058A2/fr
Publication of WO2008032065A2 publication Critical patent/WO2008032065A2/fr
Publication of WO2008032065A3 publication Critical patent/WO2008032065A3/fr
Priority to NO20091452A priority patent/NO20091452L/no

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
    • G01B7/30—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring angles or tapers; for testing the alignment of axes
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
    • G01V3/12—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with electromagnetic waves
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
    • G01C15/00—Surveying instruments or accessories not provided for in groups G01C1/00 - G01C13/00
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
    • G01V3/08—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices
    • G01V3/083—Controlled source electromagnetic [CSEM] surveying

Definitions

  • the present invention relates to a method for determining the orientation of receivers which have been deployed for use in a sub sea survey, in particular 5 for use in an electromagnetic (EM) survey.
  • EM electromagnetic
  • Controlled Source Electromagnetic (CSEM) surveying methods for the direct detection of hydrocarbons also known as Sea Bed Logging (SBL) uses an active EM source to probe the underground for thin, high resistive
  • Hydrocarbon filled reservoirs will typically have a resistivity that is of one to two orders of magnitude higher than a water filled reservoir and the surrounding shale or mud rock. This difference is sufficient to support a partially guided or ducted field in the reservoir which will subsequently leak energy up to EM field receivers placed on the seabed. 5
  • the SBL experiment consists of dropping electric and magnetic sensors onto the seabed along a predetermined sail line and subsequently towing a horizontal electric dipole source along this line.
  • the sail line starts at approximately 10 km before the first receiver and ends at approximately 10 0 km after the last receiver.
  • all receivers have at least active source data with source receiver offsets of 10 km.
  • the receivers will spin around their respective vertical axes on the way from the vessel at the sea surface to the seabed and therefore they have an arbitrary orientation when they reach the seabed. 5
  • Gyroscopes which have the level of accuracy
  • a method of determining the orientation of an electric and magnetic receiver deployed remotely which comprises: measuring electric and magnetic data in response to an applied active source; resolving the data into components in-line (x- component) to the electric dipole of the receiver and orthogonal to this (y-
  • the data is transformed from the time domain to the frequency domain and normalised; and the rotation angle ⁇ between the direction of the source dipole and the receiver dipole is calculated.
  • one of the characteristic responses of the resistive layer is an increased amplitude in the measured electric field.
  • Data from a receiver with a towline over a generally conductive formation can be chosen as a reference.
  • An anomaly can be identified by normalizing the amplitude for each receiver 10 using the reference dataset. Relative amplitudes that are large compared to unity may indicate a resistive body in the subsurface.
  • the depth of the resistive body can not be found with this qualitative technique.
  • the absolute phase In order to find the depth to a resistive body, the absolute phase must also be known. 5 The depth of a resistive body can be found either by depth migration or inversion techniques. Both types of methods require absolute phase data.
  • Measurement of absolute phase require accurate time measurements and in particular that the clock time stamping the transmitter current is synchronized with the clock time stamping the receiver data. There is 0 information in the combined measurement of transmitter current and receiver data that make it possible to do a quality control of this synchronization. However, problems may occur if the synchronization of clocks is lost and measurement of absolute phase is difficult. 5 It is therefore a further object of the present invention to provide a means for approximating absolute phase measurements in order to determine the depth of any resistive body detected.
  • a method of approximating absolute phase measurements which comprises: : measuring electric and magnetic data in response to an applied active source; resolving the data into components in-line (x-component) to the electric dipole 5 of the receiver and orthogonal to this (y-component); transforming the data from the time domain to the frequency domain; identifying the minimum horizontal offset between source and receiver; comparing at all frequencies the phase at this minimum offset with a pre calculated phase for zero offset obtained from forward modelling; and calculating an average time difference to 10 be applied to measured data to approximate absolute phase measurement.
  • the data measured by the receivers may be normalised with reference to the phase of the transmitter signal. Alternatively, it may be further normalised with reference to the absolute value of the dipole moment. 5
  • the wavefield is preferably applied by means of a transmitter located at or near the seabed and may be at a frequency between 0.01 and 20Hz.
  • the EM wavefield may be transmitted at a number of discrete frequencies to allow a range of measurements to be taken.
  • the EM wavefield may be transmitted at a 0 wavelength between IS and 50S, where S is the thickness of the overburden above the considered strata.
  • the invention may be carried into practice in various ways and one approach to the determination of the orientation of a deployed receiver and an 5 approximation of the absolute phase from the receiver will now be described in detail. This is by way of example in order to illustrate the calculation.
  • the two horizontal electric components measured at the receiver are e x ( x r
  • the towline is chosen to be our desired x-direction. This will also be referred to as the inline direction.
  • the 5 receivers are dropped to the sea bed and in general the receiver's x- direction will not coincide with the towline direction. However, the measured components can be rotated so that the new x-direction along the dipole axis of the receiver coincides with the towline direction.
  • x s , ⁇ ) h x ( x r
  • Absolute Phase 5 A realistic analysis of the electromagnetic field close to an electric dipole in sea water, relevant for SBL data, must be performed with both an air layer above and a formation below the transmitter. This is the procedure followed when QC or correction tables used for. processing are built.
  • V 2 A ⁇ x ⁇ x 5 ) + klA(x ⁇ x s ) - ⁇ J(x 5 ) 0 (17) where J(x s ) is the source-current density.
  • the electric field is given by the electric scalar potential, ⁇ (x
  • x s ), and the magnetic vector potential as, E(x ⁇ x s ) -V ⁇ (x ⁇ x s ) + i ⁇ A(x ⁇ x s )
  • V • A(x ⁇ x s ) i ⁇ (x ⁇ x s ) .
  • Figure 1 shows measured field data prior to inline rotation with figure Ia showing the amplitudes of measured fields and figure Ib showing phase of the measured fields;
  • Figure 2 shows measured field data after inline rotation with figure 2a showing amplitude and figure 2b showing phase;
  • Figure 3 shows the distribution of phase for the acoustic field in an x-z cross section through the source plane;
  • Figure 4 shows a depth trace through the source location from figure 3;
  • Figure 5 shows a trace in the x-direction from figure 3 at a distance of
  • Figure 7 shows a depth trace through the source location from figure 6;
  • Figure 8 shows a trace in the x-direction from figure 6 at a distance of 10m below the source location
  • Figure 9 shows the radiation pattern for a horizontal electric dipole (HED);
  • Figure 10 shows measured field data after inline rotation and final 180 degree correction with figure 10a showing amplitude and figure 10b showing phase;
  • the measured electric and magnetic data are normally transformed from the time domain to the frequency domain.
  • the desired source receiver offsets are specified. From the navigation data you can find the time corresponding to each source receiver offset.
  • the signal is extracted for some periods around these 5 central times and these traces are transformed to the frequency domain. The actual number of periods will naturally depend on the base frequency for the survey. Traces with the same time intervals are extracted from the transmitter signal. 0
  • the electric and magnetic data are then normalized by the phase of the transmitter signal. The result is electric data measured in V/m and magnetic data measured in A/m with absolute phase. Alternatively, the electric and magnetic data can further be normalized with the absolute value of the dipole moment which is electric current times dipole length. 5
  • Figure Ia shows the amplitudes of the measured horizontal electric fields. Both the E x component and the E y component.
  • the source receiver offsets are from - 10 km to 10 km.
  • the amplitudes of the two components are of equal size and the traces therefore lie on top of each other.
  • the receiver has
  • Figure Ib shows the phases of the horizontal electric fields.
  • the two 5 components are shifted 180 degrees with respect to each other.
  • a 180 degrees phase shift is the same as a difference in sign.
  • the reason for the difference in sign is that one component is oriented in the positive towline direction and one component is oriented in the negative towline direction. If the receiver is oriented at -45 degrees or +135 degrees, then
  • both components should have the same sign.
  • both E x and E y point in the positive towline direction.
  • both E x and E y point in the negative towline direction.
  • this receiver must point in either the +45 degrees direction or in the -135 degrees orientation. How to resolve the true orientation direction is 5 discussed below.
  • Figure 2a shows the amplitudes of the horizontal electric fields after the inline rotation angle is found from equations (12) and (13) and the application of this angle in equation (1).
  • the E y component is now close 0 to two orders of magnitude smaller than the E x component.
  • the actual rotation angle in this case is +44 degrees or alternatively -136 degrees.
  • the E x component in Figure 2a is either 0 degrees or ⁇ 180 degrees with respect to the towline direction.
  • 5 Figure 2b shows the phase of the inline horizontal electric field (E x ). It is this curve that will be used to resolve the final problem of determining the true orientation.
  • the procedure used here concentrates on the minimum source receiver separation. The reason is that propagation effects are minimal when the source receiver separation is minimal. It is then
  • x s ) is valid for a homogeneous acoustic 0 medium.
  • Figure 3 show the distribution of the phase in the x and z plane.
  • the source location is (0,0,0).
  • the cross section is in the source plane.
  • the x-axis is denoted "distance” and the z-axis is denoted "depth”.
  • the wavenumber is real and we have,
  • phase velocity c ph is 866 m/s and the frequency is 0.25 Hz.
  • This particular phase velocity value is chosen for comparison with the electric field in seawater at the same frequency.
  • the phase starts out at 0 and it increase linearly with0 distance from the source in all directions.
  • Figure 4 is a trace in the depth direction from Figure 3.
  • the x-position is 0.
  • Figure 5 is a trace in the x-direction from Figure 3.
  • the depth is 10 m below the source location.
  • the linear increase with distance is evident in both5 Figure 4 and Figure 5. If the depth is further increased for the trace in the x-direction, a hyperbolic move out would be evident.
  • the linear increase in phase with offset reflects the constant phase velocity.
  • the phase, ⁇ is given directly by
  • Figure 3 can be inte ⁇ reted as a travel time map for the acoustic field. It is also clear that when the phase velocity increases, the gradient in phase with respect to source receiver offset decreases since the gradient is inversely proportional to the phase velocity. If the phase velocity becomes very high, the gradient in phase with respect to source receiver offset goes 10 to zero over a distance of 10 km. If the phase map is interpreted as a travel time map, the small gradient reflects the fact that at high velocity it takes little time to travel to the largest offset.
  • Figure 6 show the phase of the inline electric field for the same cross
  • FIG. 15 section as the acoustic field in Figure 3.
  • the frequency is 0.25 Hz and the conductivity is 3.33 S/m.
  • the inline electric field is given by equation (27), and is due to an electric dipole of length 270 m. This phase distribution is clearly different from that of the acoustic field.
  • Figure 7 is a trace in the depth direction from Figure 6.
  • the x-position is 0.
  • Figure 8 is a trace in the 0 x-direction from Figure 6.
  • the depth is 10 m below the source location.
  • Figure 6 will have the same phase velocity in the far field region.
  • the inline electric field is here in phase with the source current. From Figure 9 we see that this is in accordance with the 10 radiation pattern of an electric dipole in a conducting medium. For larger offsets the phase increase with distance.
  • the phase is shown in Figure 10b.
  • the phase is now close to 180 degrees 5 at zero offset and dropping to close to zero degrees in front of and behind the transmitter.
  • the phase gradient is smaller on the real data in Figure 10b than for the synthetic data in Figure 8. The reason is that for the synthetic data we used a model with seawater only.
  • the real data is influenced by propagation of the electric field in a formation that has a
  • the zero offset inline electric phase is 5 frequency dependent.
  • the 0.25 Hz zero offset phase is 175 degrees.
  • the 0.75 Hz zero offset phase is 166 degrees.
  • the 1.25 Hz zero offset phase is 159 degrees. It turns out that the most important parameters determining this zero offset phase are transmitter length, L, distance from transmitter centre to receiver, R 0 , angular frequency, ⁇ , and seawater conductivity, 0 ⁇ w . All these quantities are measured in an SBL survey.
  • the zero or minimum offset phase is much less sensitive to top formation conductivity and total water depth. If the source is not towed directly above the receiver, a true horizontal zero offset can 5 not be realized. However, a minimum horizontal offset can still be found. The smallest K 0 value will be for this minimum horizontal offset. It is the total distance between the centre of the transmitter and the receiver that is important for the phase properties. This distance can be obtained from navigation since the source elevation is measured. In the following we will
  • 484764v1 1 1 refer to the minimum horizontal offset also as zero offset. If the synchronization between transmitter clock and receiver clock is lost for some reason, then there is a possibility to recover from this failure by close inspection of the electric data at zero offset.
  • the procedure requires a 5 four dimensional table to be pre calculated by forward modelling. This table must contain absolute phase as a function of transmitter length, distance from the transmitter centre to receiver, frequency and seawater conductivity. By comparing the zero offset phases for all frequencies of the inline real data with the tabulated value, a single, averaged, time delay or 10 time advance, ⁇ , may be deduced.
  • Figure 12 show the phase of the cross line magnetic field, H y (x r
  • the formation, frequencies and transmitter length is the same as for the inline 0 electric field in Figure 11. From Figure 12 it is clear that the zero offset cross line magnetic phase is much less frequency dependent than the zero offset inline electric phase.
  • the 0.25 Hz zero offset phase is -179.5 degrees.
  • the 0.75 Hz zero offset phase is -178.8 degrees.
  • the 1.25 Hz zero offset phase is -178.1 degrees.
  • the zero offset cross line 5 magnetic phase show much less variation with transmitter length, distance from the transmitter centre to receiver, frequency and seawater conductivity than the zero offset electric phase.
  • the zero offset magnetic phase is not very sensitive to top formation conductivity and total water depth.
  • the largest effects on the zero offset cross line magnetic phase can be observed when the frequency is above 1 Hz and the distance from the transmitter centre to receiver is more than 100 m.
  • the zero offset magnetic phase can be -170 degrees to -160 degrees.
  • the zero offset magnetic phase is usually between - 180 degrees and -175 degrees.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Remote Sensing (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • Environmental & Geological Engineering (AREA)
  • Geology (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • Geophysics (AREA)
  • Electromagnetism (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Geophysics And Detection Of Objects (AREA)

Abstract

L'invention concerne un procédé permettant de déterminer l'orientation d'un récepteur électrique et magnétique déployé à distance, consistant: à mesurer des données électriques et magnétiques en réponse à une source active appliquée; à traduire les données en composants en ligne (composante x) par rapport au dipôle électrique du récepteur et orthogonaux (composante y) par rapport à celui-ci; à transformer les données à partir du domaine temporel à un domaine fréquentiel et à les normaliser; et à calculer l'angle de rotation ϑ entre la direction du dipôle source et le dipôle du récepteur.
PCT/GB2007/003464 2006-09-15 2007-09-13 Orientation d'un récepteur WO2008032065A2 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
EP07804257A EP2067058A2 (fr) 2006-09-15 2007-09-13 Orientation d'un récepteur dans un levé électromagnétique
NO20091452A NO20091452L (no) 2006-09-15 2009-04-14 Mottakerorientering

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0618240.6 2006-09-15
GB0618240A GB2441787A (en) 2006-09-15 2006-09-15 Method of determining the orientation of an electric and magnetic receiver deployed remotely

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WO2008032065A2 true WO2008032065A2 (fr) 2008-03-20
WO2008032065A3 WO2008032065A3 (fr) 2008-06-05

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GB (1) GB2441787A (fr)
NO (1) NO20091452L (fr)
WO (1) WO2008032065A2 (fr)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8471555B2 (en) 2008-11-04 2013-06-25 Exxonmobil Upstream Research Company Method for determining orientation of electromagnetic receivers

Citations (3)

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WO2000013046A1 (fr) * 1998-08-28 2000-03-09 Den Norske Stats Oljeselskap A.S. Procede et appareil pour determiner la nature de reservoirs souterrains
GB2395563A (en) * 2002-11-25 2004-05-26 Activeem Ltd Electromagnetic surveying for hydrocarbon reservoirs
WO2007018810A1 (fr) * 2005-07-22 2007-02-15 Exxonmobil Upstream Research Company Procede destine a determiner des orientations de recepteur

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WO2000013046A1 (fr) * 1998-08-28 2000-03-09 Den Norske Stats Oljeselskap A.S. Procede et appareil pour determiner la nature de reservoirs souterrains
GB2395563A (en) * 2002-11-25 2004-05-26 Activeem Ltd Electromagnetic surveying for hydrocarbon reservoirs
WO2007018810A1 (fr) * 2005-07-22 2007-02-15 Exxonmobil Upstream Research Company Procede destine a determiner des orientations de recepteur

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ELLINGSRUD S ET AL: "Remote sensing of hydrocarbon layers by seabed logging (SBL): Results from a cruise offshore Angola" LEADING EDGE, THE, SOCIETY OF EXPLORATION GEOPHYSICISTS, TULSA, OK, US, vol. 21, no. 10, October 2002 (2002-10), pages 972-982, XP002328147 ISSN: 1070-485X *
MITTET R ET AL: "Inversion of SBL data acquired in shallow waters" EAGE CONFERENCE AND EXHIBITION, XX, XX, 2004, pages 1-4, XP002378403 *
MITTET R ET AL: "ON THE ORIENTATION AND ABSOLUTE PHASE OF MARINE CSEM RECEIVERS" GEOPHYSICS, SOCIETY OF EXPLORATION GEOPHYSICISTS, TULSA, OK, US, vol. 72, no. 4, July 2007 (2007-07), pages F145-F155, XP001542243 ISSN: 0016-8033 *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8471555B2 (en) 2008-11-04 2013-06-25 Exxonmobil Upstream Research Company Method for determining orientation of electromagnetic receivers

Also Published As

Publication number Publication date
WO2008032065A3 (fr) 2008-06-05
EP2067058A2 (fr) 2009-06-10
NO20091452L (no) 2009-06-11
GB2441787A8 (en)
GB0618240D0 (en) 2006-10-25
GB2441787A (en) 2008-03-19
GB2441787A9 (en) 2008-03-27

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