Abstract:
A submersible high current PWM-based source system for marine electromagnetic surveying for hydrocarbon exploration. The system converts a periodic, arbitrary digital signal to a high output current in a HED optimized for maximum dipole moment, using PWM modulation and real-time current regulation for precise output amplitude. The design and choice of components allows a wideband EM signal to be generated with sharp transition characteristics and substantially independent of the AC input power characteristics. This in turn enables for improved tailoring of an EM source signal for a specific subterranean prospect, and thus increased accuracy of the EM data.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is a National Phase entry of PCT Application No. PCT/GB2012/000088, filed Jan. 27, 2012, which claims priority from GB Application No. 1101576.5, filed Jan. 28, 2011, which applications are hereby incorporated by reference herein in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a source for electromagnetic (EM) surveying, in particular for seabed logging, and a method for producing a signal for EM surveying. 
       BACKGROUND ART 
       [0003]    Currently, the most widely used techniques for geological surveying, particularly in sub-marine situations, are seismic methods. These seismic techniques are capable of revealing the structure of the subterranean strata with some accuracy. However, whereas a seismic survey can reveal the location and shape of a potential reservoir, it cannot normally reveal the nature of the reservoir, for example, whether the reservoir contains hydrocarbons or water. 
         [0004]    However, while the seismic properties of hydrocarbon filled strata and water-filled strata do not differ significantly, their electromagnetic resistivities do differ. Thus, by using an electromagnetic surveying method, these differences can be exploited and the success rate in predicting the nature of a reservoir can be increased significantly. 
         [0005]    Consequently, a method and apparatus embodying these principles form the basis of the present applicant&#39;s EP1256019. 
         [0006]    This contemplates a method for searching for a hydrocarbon containing subterranean reservoir which comprises applying a time varying electromagnetic field to subterranean strata; detecting the electromagnetic wave field response; seeking, in the wave field response, a component representing a refracted wave; and determining the presence and/or nature of any reservoir identified based on the presence or absence of a wave component refracted by hydrocarbon layer. 
         [0007]    At present, electromagnetic surveying techniques are not widely used in practice. In general, the reservoirs of interest are about  1  km or more below the seabed. In order to carry out electromagnetic surveying as a standalone technique in these conditions, with any reasonable degree of resolution, short wavelengths are necessary. Unfortunately, such short wavelengths suffer from very high attenuation. Long wavelengths do not provide adequate resolution. For these reasons, seismic techniques tend to be preferred. 
         [0008]    However, while longer wavelengths applied by electromagnetic techniques cannot provide sufficient information to provide an accurate indication of the boundaries of the various strata, if the geological structure is already known they can be used to determine the nature of a particular identified formation, if the possibilities for the nature of that formation have significantly differing electromagnetic characteristics. The resolution is not particularly important and so longer wavelengths which do not suffer from excessive attenuation can be employed. 
         [0009]    The resistivity of seawater is about 0.3 ohm-m and that of the overburden beneath the seabed would typically be from 0.3 to 4 ohm-m, for example about 2 ohm-m. However, the resistivity of an oil reservoir is likely to be about 20-300 ohm-m. Typically, the resistivity of a hydrocarbon-bearing formation will be 20 to 300 times greater than a water-bearing formation. This large difference can be exploited using EM surveying. 
         [0010]    Due to the different electromagnetic properties of a gas/oil bearing formation and a water bearing formation, one can expect a reflection and refraction of the transmitted field at the boundary of a gas/oil bearing formation. However, the similarity between the properties of the overburden and a reservoir containing water means that no reflection or refraction is likely to occur. 
         [0011]    A refracted wave behaves differently, depending on the nature of the stratum in which it is propagated. In particular, the propagation losses in a hydrocarbon stratum are much lower than in a water-bearing stratum, while the speed of propagation is much higher. Thus, when an oil-bearing reservoir is present, and an EM field is applied, a strong and rapidly propagated refracted wave can be detected. This may therefore indicate the presence of the reservoir or its nature if its presence is already known. 
         [0012]    An EM source such as an electric dipole transmitter antenna on or close to the sea floor can be used to induce (EM) fields and currents in the sea water and in the subsurface strata. In the sea water, the EM-fields are strongly attenuated due to the high conductivity in the saline environment, whereas a subsurface strata with less conductivity potentially can act as a “guide” for the EM-fields due to lower attenuation. If the frequency of the transmitted signal is low enough (of the order of 1 Hz), the EM-waves are able to penetrate deep into the subsurface, and deeply buried geological layers having higher electrical resistivity than the overburden (such as e.g. a hydrocarbon filled reservoir) will affect the EM-waves. Depending on the angle of incidence and state of polarisation, an EM wave incident upon a high resistive layer may excite a ducted (guided) wave mode in the layer. The ducted mode is propagated laterally along the layer and leaks energy back to the overburden and receivers positioned on the sea floor. The term “refracted” wave in this specification is intended to refer to this wave mode. Methods of marine electromagnetic surveying for hydrocarbon exploration typically require transmissions to be made with currents at around 1,000 to 10,000 A, in order to obtain measureable responses. 
         [0013]    In addition to their use in hydrocarbon surveying before a well is drilled, sub-marine electromagnetic surveying techniques may also be used to appraise the characteristics of known hydrocarbon reservoirs before, during and after drilling, and to monitor hydrocarbon reservoirs before, during and after hydrocarbon extraction. Certain features of the method, for example, the frequency of the transmitted signal, may be adjusted according to the nature of the survey being undertaken and the physical parameters of the measurement area, in order to obtain information about certain regions of the subsurface or to tailor the measurements in other ways. However, the general principles of electromagnetic surveying remain the same regardless of the type of survey. 
         [0014]    It will be appreciated by those skilled in the art that the accuracy of data obtainable using electromagnetic surveying methods depends on a number of factors, including the nature of the survey signal, the existing knowledge of the physical measurement area and the accuracy achievable by the transmitters and receivers. The accuracy and usefulness of the results of any survey depend on the quality of the survey data. Thus there is a need to improve the accuracy and scope of the data obtainable using such methods. 
       SUMMARY 
       [0015]    Embodiments of the present invention provide a system and method for generating an EM signal, with increased accuracy in the controllable characteristics of the generated EM field, and in addition, an increased flexibility of frequency range for use in different surveys. The system and method of such embodiments allow improved tailoring of an EM source signal for a specific subterranean prospect and increased signal to noise ratios, resulting in increased accuracy of the EM data. 
         [0016]    According to embodiments of the invention, there is provided a transmitter system for generation of a high-current signal for marine electromagnetic surveying for hydrocarbon exploration, the system comprising a topside Power Supply Unit (PSU) arranged to transmit a suitably high voltage 3-phase signal at substantially constant amplitude, a Pulse Width Modulated (PWM) current source connected to the output of the topside PSU, wherein the PWM current source is capable of converting an input from the topside PSU into a stable internal DC voltage; thereby enabling conversion of an arbitrary digital periodic input to a suitably high current output signal, at least one towed Horizontal Electric Dipole (HED) antenna connected to the output of the PWM current source, wherein the HED antenna is arranged to be substantially neutrally buoyant when in use during the survey, a control system comprising a topside control unit having an operator interface and a PWM control unit having output current and voltage measurement devices and a time distribution network comprising a GPS receiver and network switches, arranged to acquire and distribute a time signal to the PWM control unit of the control system. The claims set out further features which may be comprised in the transmitter system. Embodiments of the invention also extend to a method of generating a high-current signal for marine electromagnetic surveying, as set out in the claims. 
         [0017]    The system and method of embodiments of the invention achieve improvements in EM surveying by providing a high-current wideband EM signal with sharp transition characteristics, produced by a HED antenna. Data obtained using such a signal is of increased accuracy. The transmitted EM signal is also largely independent of the AC power input characteristics. This increases the accuracy of the generated EM field, which also increases the accuracy of survey data. 
         [0018]    The system and method of embodiments of the invention can be particularly beneficial for the investigation of targets at large depths beneath the seabed, due to increases in the signal to noise ratio in the EM data acquired using the claimed system and method, which allows more information about the subsurface to be extrapolated from the data. Furthermore, the system and method of embodiments of the invention can improve surveys in areas with complex geology and small targets, because the embodiments can enable EM surveying to be carried out successfully with higher frequencies than are normally used, which results in the increased resolution necessary to successfully investigate such targets. 
         [0019]    The invention is further exemplified with reference to the following figures, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  depicts an overview of a system according to an embodiment of the invention. 
           [0021]      FIG. 2  depicts a schematic overview of a PWM current source suitable for use in the system according to embodiments of the invention. 
           [0022]      FIG. 3  depicts PWM. 
           [0023]      FIG. 4  depicts voltage and current curves. 
           [0024]      FIG. 5  depicts voltage and current curves. 
           [0025]      FIG. 6  depicts measured internal DC voltage and antenna output voltage during current transition for a PWM current source according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The topside PSU required by the system according to embodiments of the invention is typically located on a survey vessel. An example of this arrangement is shown in  FIG. 1 , where the survey vessel  1  carries the topside PSU  5 . Alternatively, the topside PSU could be supported on land or located on a moving or stationary support. If the survey is being carried out in relatively shallow water, for example in water depths of 500 m or less, the PWM current source may be located out of the water, near the topside PSU, for example on the same survey vessel. If the survey is being carried out in deeper water, the PWM current source may be submersible, so that it can be operated at depths near the HED. In that case, the PWM current source will be connected to the topside PSU via an umbilical, which may comprise 3-phase high voltage main power conductors, auxiliary power conductors and a fibre optic communication link. The umbilical may be armored and serve as a tow cable for the PWM current source and HED. 
         [0027]      FIG. 1  depicts one embodiment of the invention, in which the PWM current source is towed subsea. The seabed  13  is shown, and a possible location of a subsurface reservoir is marked with reference number  14 . Survey vessel  1  carries GPS receiver and system clock  2 , as well as topside system  3 . The topside system  3  comprises topside control unit  4  and topside PSU  5 . In this embodiment, a subsea towfish  7  comprises PWM control unit  8  (which in this embodiment is subsea) and PWM-modulated current source  9 , which includes a current source PSU. The subsea towfish is connected to the topside system via umbilical  6 , which in this embodiment also acts as the tow cable. An HED antenna  10  is towed. The HED antenna comprises two electrodes  11  and an antenna cable  12 . 
         [0028]    The subsea system may be submersible down to a depth of 4000 m below sea level. The umbilical  6  transports power and communication to the subsea components. The dedicated topside power supply unit located on the survey vessel provides power to the PWM current source and the subsea components. The control system including both topside and PWM control units provides operator interface, output signal measurement and time stamping, data file generation and system surveillance. A dedicated network is used to acquire and distribute time signals from a global positioning system (GPS) receiver to the PWM control unit. 
         [0029]    The topside PSU located on the vessel transmits power to the subsea unit. The topside PSU output voltage may typically be between 100 and 1000 V (i.e. higher than the HED voltage). In many EM source systems, the subsea output current amplitude is modulated using variable topside PSU output voltage and only the current direction is controlled subsea. However in the system according to the present invention, all output regulation can be done in the PWM current source as true current regulation. In that case, there does not need to be modulation of amplitude in the topside PSU, i.e. it can transmit at constant amplitude. This simplifies the control system and increases the precision in the subsea output current. 
         [0030]    The HED antenna consists of two electrodes connected to the PWM current source through low-impedance cables. The cables, electrodes and electrode spacing are selected for maximum dipole moment within the range of available output current and power. In addition, a buoyant material is added to the cables and electrodes to give the HED neutral buoyancy in sea water. 
         [0031]    The long cables in the HED add inductivity to the PWM current source load, and increase the minimum transition time from −Imax to +Imax and vice versa. For optimal performance at high marine EM survey frequencies (for example, 50-100 Hz), the internal filter chokes in the PWM current source can be adapted for a specific load inductance according to the HED length. This frequency range covers the main frequencies relevant for marine electromagnetic surveying for hydrocarbon exploration. 
         [0032]    Further advantages can be obtained if the HED is designed for maximum dipole moment within the maximum current and power from the PWM current source. For a given maximum current and power from the PWM current source, and for a given maximum efficient dipole length (dictated by data processing considerations), there will be an optimal configuration of antenna cable dimension (which affects antenna impedance), and an optimal electrode diameter and length. In addition, operational aspects should be taken into account when specifying the design of a HED for a survey. 
         [0033]    In the embodiment depicted in  FIG. 1 , the umbilical can serve both as tow cable and link for power and communication between topside and subsea units. Such an umbilical must be designed to sustain the load from its own weight and the weight of the subsea units, added to the drag force when towing at a depth of 4000 m at speeds of up to 2 knots. The cable will therefore contain armoring suitable for its role as a stress member, as well as 3-phase main power conductors, auxiliary power conductors for instrumentation, safety grounding and finally a communications link which is preferably a fibre optic link. 
         [0034]    The topside control unit provides operator interface, communicates with the subsea control system and receives time stamped measurements of output current and voltage. The measurements can be recorded in log files for use in post-processing of EM-data. Finally, the topside control system supervises operation of the other components in the source system. 
         [0035]    In one embodiment, the PWM control unit can be synchronized with GPS time through a dedicated network. The control unit contains a table with the desired output signal and sends sample by sample at the correct sample rate to the current regulator in the PWM current source. At the same sample rate, it measures output current and voltage from the current source, time-stamps the measurements and transports them to the topside control unit. The control unit can adjust the phase of the desired output signal to fit with a fixed point in time, depending on the processing scheme of the EM-data. 
         [0036]    It should be noted that it is not necessary for the current regulator to relate to absolute time. 
         [0037]    The core of the system is the current source converting the digital periodic, arbitrary signal to a high current output signal. Such a signal must be transmitted with a high enough current to obtain measureable responses in a marine EM survey, for example the signal current may be around 1,000 A or up to around 10,000 A. The current source is powered with 3-phase alternating current (AC) high voltage power from the topside power supply unit. The power frequency should be selected to form a good compromise between prevention of capacitive currents in the umbilical and current source transformer size and weight. The high voltage power may be transformed down to a lower voltage in a sigma-delta transformer and rectified in a 12-pulse rectifier bridge. Use of a relatively high frequency and the 12-pulse rectifier bridge minimises ripple in the resulting DC voltage. Capacitors can be used to further stabilize the DC voltage (reduce DC ripple), and to absorb the energy fed back from the antenna during rapid changes in the output current. 
         [0038]    In the PWM current source, high-power transistors are used as switches, and can be turned either on or off. Switching can be controlled by the PWM control unit. A high-power transistor unit is shown in principle in  FIG. 2 . The unit comprises a 3-phase transformer  15 , a 12-pulse rectifier bridge  16 , eight power transistors with surge protection one of which is identified by number  20 , inverter unit  19 , DC-link  17 , capacitor  18  and output current filter  21 . The outputs  22 - 25  are output 1+, output 2+, output 1− and output 2− respectively. The unit shown in  FIG. 2  comprises four inverter modules, to be controlled in pairs, each pair connected to an individual output. In general, the PWM current source may comprise protection functionality against short circuits and open circuits on inputs and outputs, which may be hardware based and/or software based, and which may be multiply redundant. 
         [0039]    If one transistor in a pair is on, then the other is off Thus, the voltage at the common point of the two transistors is then either 0 or +VDC. In practice, the transistors are switched with a high frequency and a choke is used as seen in  FIG. 2 . Then the voltage on the antenna output will be equal to the time-average of the voltage at the common point. This means that if the transistors are switched to active for 75% of the period, the resulting voltage on the antenna output will be +VDC*0.75. This factor is called the duty cycle. This principle is illustrated in  FIG. 3  for a 50% duty cycle and a 75% duty cycle. The current regulator measures the current in each transistor module and adjusts the pulse width and duty cycle to obtain the desired amplitude at any given time. This can be done at a very high sample rate. If the sample rate is significantly higher than the base frequency of the output EM signal, then the current regulation can be very fast and accurate. 
         [0040]    The PWM current source can use high-power Insulated Gate Bipolar Transistors (IGBTs) or power Metal-Oxide Semiconductor Field Effect Transistors (MOS-FETs) with transient voltage suppressors, in order to chop the internal DC voltage. 
         [0041]    For a smoother output signal, several transistors can be switched in parallel, but phase delayed. This results in a very high apparent switching frequency. This also reduces latency in current regulation and reduces output ripple. The output signal can be filtered both internally in the PWM current source and externally in the HED resulting in a very stable current signal. 
         [0042]    By using 4 inverter modules in the PWM current source, controlled in pairs, it is possible to transmit completely independent source signals on the two pairs. This can be used for dual output, with 2 non-parallel HEDs to generate independent EM fields in two directions. The two inverter pairs can also be interconnected, synchronized and controlled with a single source signal for maximum current in a single HED. 
         [0043]    The transition period from −Imax to +Imax and vice versa in the PWM current source may be limited by the inductance in the HED. A short HED will have a lower inductance and give a shorter transition period, hence increasing the maximum output signal frequency. 
         [0044]    However, this reduces the dipole moment and hence the strength of the generated EM-field. Using a HED which is impedance matched with the PWM current source and designed for maximum dipole moment within the maximum current and power from the PWM current source, can at the same time allow use of frequencies from 0-50 Hz, which is within the range of interest for marine EM surveying. 
         [0045]    The internal DC voltage of the PWM current source can thus be 3-8 times higher than the nominal output voltage, depending on the load connected. This, optionally combined with large capacitors, provides an energy reserve for extremely fast transition from −Imax to Imax and vice versa. The high DC voltage also means the system has large flexibility regarding load impedance. For EM use, it is preferable to transmit the maximum current achievable with the equipment, in order to obtain maximum dipole moment. 
         [0046]      FIGS. 4 and 5  illustrate example antenna voltages and currents against time.  FIG. 4  illustrates the existing state of the art and  FIG. 5  shows what can be achieved with a system according to embodiments of the present invention, wherein a higher voltage (3-8 times higher than the nominal output voltage) is provided at transition. The voltage curve in  FIG. 5  gives a shorter current rise time than that in  FIG. 4 , i.e. the invention can provide a higher bandwidth output signal compared to known systems. Current rise times of the order of 7 ms for a rise from −1500 A to +1500 A can be achieved using the system according to the present invention, which is a significant improvement over rise times achievable by previous systems, which are typically of the order of 140 ms for such a rise. 
         [0047]    In  FIG. 6 , the measured internal DC voltage and antenna output voltage during current transition for a PWM current source usable in a system according to embodiments of the invention is shown. The horizontal axis represents time and current/voltage/power are represented on the vertical axes. The sign labeled “t transition ” refers to the time required for a transition from −Imax to +Imax. 
         [0048]    The trace marked “Reference” is an example desired current profile of the device. The trace marked “Iout” is the measured output current to the HED. The trace marked “Uout” is the output voltage to the HED. The trace marked “Power out” is the measured output power from the PWM current source, and is the product of “Iout” and “Uout”. Finally, the trace marked “UDCint” is the internal DC voltage. We can see from the graph that when the current is switched from −Imax to +Imax, the output voltage “Uout” increases to a value approximately 3 times its nominal value. This signifies that the current regulator unleashes its maximum capability of turning the current flow in the HED. We also see that the “UDCint” increases slightly during the current transition, signifying that energy is absorbed from the HED due to its inductance. Finally, we see that the output power “Power out” has two spikes during the transition period. This signifies that the output power is increased to speed up the transition, whereas the power dip between the spikes comes when the output current passes zero. 
         [0049]    The system and method of embodiments of the present invention, by inclusion of a PWM current source according to the claims, provides a high-current wideband EM signal with sharp transition characteristics. The internal DC voltage stabilization, PWM signal generation and true current regulation of the output signal decouple the output signal from the AC input power characteristics, provide antenna load immunity and permit high accuracy EM field generation. 
         [0050]    Any references herein to “seabed” or “subsea” should be understood as extending to any relevant body of water, such as a lake, in which such surveys may be undertaken.