Patent Application
20090001986
The present invention relates to systems and methods for calibrating electromagnetic receivers for use in geophysical surveys.
The exploration of hydrocarbons, such as oil and gas, is usually performed in the form of geological survey. The survey is done during the reservoir assessment and development stage to avoid unnecessary drilling. Hydrocarbons and geological structures that tend to bear hydrocarbons can be detected based on the fact that their mechanical and electromagnetic (EM) properties are different from those of a background geological formation.
Among the EM properties, the resistivity (ρ), which is an inverse of the electrical conductivity (σ), is particularly useful. This is because hydrocarbon-bearing bodies, such as oil-bearing reservoirs, gas injection zones, and methane hydrates, may have different resistivities as compared with a background geological formation. For example, hydrocarbon-bearing reservoirs typically have a resistivity that is one to two orders of magnitude higher than the surrounding shale and water-bearing zones. A resistivity mapping or imaging can be used to locate the zones of interest in contrast to the background resistivity. This method has been used successfully in land or sea bed logging.
The resistivity mapping can be achieved by receiving EM signals that have traveled through the geological structures. The received data in EM logging are affected by a number of parameters, for example, the distance between the EM signal source and the receivers, EM field frequency, polarity of the EM waves, depth and thickness of the reservoir, and other factors (e.g., resistivity of sea water and surrounding geological formations).
The EM signals used in such surveys may be naturally occurring or may come from artificial sources. Among the various EM survey methods, magneto-telluric (MT) method takes advantage of naturally-occurring EM fields in geological formations. Because carbonates, volcanics, and salt all have large electrical resistivity in contrast with typical sedimentary rocks, MT measurements can produce high-contrast images of resistivity maps, and are particularly useful in examining large-scale basin features and for characterizing reservoirs below basalt (volcanics) layers beneath the sea bed.
Most recent EM methods use artificial EM sources that produce time-varying EM fields. The EM fields may include an EM pulse generated by turning on and off the EM transmitter. In this case, the receivers effectively measure a pulse response of the geological structures. The EM fields may be in the form of low-frequency EM waves with a fixed frequency, or with a combination of different frequencies.
Another EM survey method, referred to as the controlled source electromagnetic (CSEM) method, uses an artificial EM source to send controlled EM fields to a geological formation. As illustrated in
Ideally, the receivers, which may comprise electrodes and/or antennas, should be able to measure various components of the EM fields. That is, each of the receivers should correctly measure one component of the EM fields, including electrical field (Ex, Ey, Ez) and magnetic field (Bx, By, Bz). However, this is not always the case due to various reasons. For example, the orientation of the receivers may have changed during deployment at the sea bottom, in a borehole, or at the earth surface, such that the receivers no longer measure exactly orthogonal components. For example, the electrode pair 21a, 21b may be tilted or bent on the seafloor. Similarly, the vertical arm 22c may move with the current of the sea water.
Typically, the signals measured by the receivers are a linear function of the field: S=a F+h, wherein a is a gain and b is an offset. However, the relationship may be a complex function, with a real and an imaginary parts, when both the amplitude and the phase of the signals are measured. In any event, the gain a and offset b of a receiver are typically determined in the laboratory prior to deployment. However, the gain and offset can change with time or environmental factors (such as pressure or temperature), or they may change during handling and deployment of the equipment. Thus, the pre-deployment calibration maybe insufficient to ensure that a receiver will function as intended.
In addition, certain receivers may have unique problems that cannot be anticipated or resolved with pre-deployment calibration. An example of this situation can be found in an electric field receiver of the type described in French Patent No. 84 19577 issued to Jean Mosnier and PCT Patent Publication No. WO 2006/026361 A1 by Steven Constable (one example is shown in
Therefore, there exists a need for methods that can be used to calibrate the responses of EM receivers in-situ, to determine the correction factors or parameters for the gains and offsets, to verify the proper functioning of the receivers after they have been deployed, or to provide correction factors in data analysis.
In one aspect, an electromagnetic receiver includes at least one sensor for measuring electromagnetic signals, and a calibration antenna configured to generate an electromagnetic signal at a first frequency.
In another aspect, a method for calibrating an electromagnetic receiver includes energizing a calibration antenna disposed within the receiver to generate an electromagnetic signal, and detecting the electromagnetic signal using at least one sensor disposed within the receiver.
In another aspect, a method for making an electromagnetic survey includes deploying a plurality of electromagnetic receivers on a seafloor, and for each receiver, energizing a calibration antenna in receiver to generate a calibration electromagnetic signal and detecting the calibration electromagnetic signal using one or more sensors in the receiver. The method also includes generating a controlled source electromagnetic signal external from the receiver, detecting the controlled source electromagnetic signal with one or more of the receivers, and for each receiver, correcting the detected controlled source electromagnetic signal using the detected calibration electromagnetic signal.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
Embodiments of the present invention relate to methods for calibrating electromagnetic receivers. In accordance with embodiments of the invention, a electromagnetic source for providing a known electric and/or magnetic field for calibrating the sensors in a receiver may be included in a sub sea receiver A sensor in a receiver may be an electrode, an antenna, a magnetometer, or a combination thereof.
In accordance with one or more disclosed examples, a receiver may include a calibration antenna or electrode capable of generating an EM field when powered by a suitable power source. The generated EM field is of a known strength such that it can be used to calibrate the sensors in the receiver in the measurement device. Thus, at chosen times, e.g., after deployment of the device, before or after each survey data acquisition, at preset time intervals, or at appropriate times during a subsea survey, the calibration antenna or electrode may be energized, creating a known EM field in the vicinity of the device. The signals induced in the sensors in the receiver are recorded and compared to the expected responses of the sensors to the known EM field. This comparison can be used to determine calibration parameters (such as offsets, gains, misalignments, or other parameters describing the responses of the receivers), which may be used to calibrate the sensors before measurements or used to correct the measurements acquired by the receivers during the processing of the survey data. These corrections and/or calibrations may be applied locally, using the on-board processor or control unit built into the device. Alternatively, the data may be corrected in the processing phase, where the data from all of the receivers analyzed.
A calibration unit may be applied to various types of EM receivers, including a receiver disclosed by Jean Mosnier and Steven Constable noted above.
In some cases, it may be desirable to tune the receiver impedance Z to that of the surrounding seawater, however, this often is impractical because the seawater resistance may not be known beforehand. Furthermore, the resistance (or conductivity) of seawater can vary with time, temperature, salt concentration, etc. In addition, the process of deploying such a receiver to the sea floor may cause an otherwise perfectly tuned receiver to become imperfect. For example, the receiver, when deployed on a weak sea floor, may sink into the sea floor, as illustrated in
For the above and other reasons, it may be impractical to rely on pre-deployment calibration to ensure that the receivers will behave as intended after deployment. The disclosed examples, instead, rely on in-situ calibration to ensure that the receivers are properly calibrated under the measurement conditions. Alternatively, embodiments of the invention can provide calibration parameters (factors) that can be applied to the measured signals to correct for errors that arise from receiver imperfection under the measurement conditions.
In accordance with some examples, a receiver may include a calibration unit.
For example, with the type of receiver shown in
Further, in some applications, a receiver may be intentionally set to lower impedance relative to the impedance of the sea water in order to allow more current to flow through the electrodes. This has the effect if increasing the detection sensitivity of the receiver. Such receivers are disclosed in a co-pending application Ser. No. 11/770,902 by Besson, et al., entitled “Methods for Electromagnetic Measurements and Correction of Non-Ideal Receiver Responses,” filed on Jun. 29, 2007 (attorney docket No. 115.0017). The imperfection in impedance match in this case can be corrected during data analysis based on the calibration responses recorded by the receiver when the calibration antenna is energized.
In some examples, the strength of the calibration unit (e.g., 54 in
One skilled in the art would appreciate that the emitting antenna (e.g., 52 in
The calibration antenna can be powered by batteries built into the receiver. Alternatively, the calibration antenna may be energized by power transferred through a cable from an external power source, such as in the case where the receivers are linked by cable. In a typical sea-bottom CSEM survey, the receivers are not typically tethered, and the emitting antenna may be powered by batteries. This may limit the amount of energy that can be used in the calibration antennas, and therefore the strength of the EM fields used for calibration. However, this is usually not a problem, because the calibration antennas are located close to the sensors in the receivers.
In some EM surveys, the measurements are performed with a variety of frequencies to take advantage of the frequency-dependent responses. For example, it is known that lower frequency EM signals can traverse deeper into the formations (i.e., large skin depth), while the higher frequency EM signals produce better signal-to-noise ratios. In accordance with some embodiments of the invention, the calibration antennas may be powered at a range of different frequencies in order to calibrate the receivers over a range of frequencies. A range of frequencies can be achieved by a variety of means known in the art, including sequential excitation at different frequencies, frequency sweeps, frequency chirps, time domain excitation, etc.
While the above example uses a receiver as shown in
Furthermore, embodiments of the invention are not limited to determining the contact impedance and making adjustments. In addition to the calibration of receiver impedance described above, such calibrations may also allow correction for any changes due to temperature, pressure, and/or due to vector infidelity, such as caused by electrode arms being non-orthogonal when they come to rest on the sea floor. In addition, the use of the local calibration may also allow numeric tracking the vertical electrodes (e.g., 21c in
In addition, calibration methods may use a wide range of frequencies (or short time constants for time domain source waveforms) of the calibration signals to enable the measurement of local perturbations to the main electric field caused by the local environment around the receiver. Such local perturbations may include, for example, resistive bottom or nearby ridge or fracture which will cause a local static distortion of the measured EM fields. Because the depths, orientations, and locations of such local perturbations relative to the receivers are not known before deployment of the receivers, they cannot be calibrated beforehand. Having the ability to transmit calibration signals at different frequencies, it becomes possible to either map such local perturbations or to collect parameters at different frequencies for data correction in the inversion process.
The calibration signals are detected and recorded by the sensors in the receiver (step 63). The recorded responses may be used to adjust the receivers before measurements (step 64). In one example, the adjustment may be to match the impedance to that of the surrounding environment. In another example, the impedance may be intentionally mismatched to focus the current. In yet another example, a receiver may be repositioned by an ROV based on the calibration data. Alternatively, the receiver may perform the measurements without first fine-tuning the receivers (step 65), and then the recorded responses of the receiver to the calibration signals may be used to correct for receiver imperfections in the measurement data (step 66). Note that in the alternative approach, the measurements may be performed before or after the receiver calibration is performed.
Note that the method shown in
Advantages may include one or more of the following. A receiver for EM measurements in accordance with disclosed examples may include a calibration unit. Such receivers are capable of performing in-situ calibration. The calibration results may be used to fine tune the receivers before the measurements are made. Alternatively, the calibration results may be used on measurement data that have been acquired.
Being able to perform in-situ calibration makes it possible to ensure that a receiver is properly calibrated at the measurement site before the measurements are made. Furthermore, many factors that impact the characteristics of a receiver cannot be ascertained beforehand. In this case, in-situ calibration offers the real alternative to ensure that the receivers are properly calibrated before the measurements are made.
Some disclosed examples of the invention use a range of frequencies to perform the receiver calibrations. In this case, the multiple frequencies allows the user to identify and/or correct for local perturbations at the measurement sites. Such local perturbations may include fractures, layers of unusual resistivities, dipping formations, etc.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. For example, although the exemplary embodiments of the present invention have been described in terms of receivers used in sea bed logging, one of ordinary skill in the art would appreciate that the receivers and methods of the present invention may also be applied to other types of measurements such as MT measurements and inland subsurface surveys. Accordingly, the scope of the invention should be limited only by the attached claims.
