MX2008006700A - Optimisation of mtem parameters - Google Patents

Abstract

A method of optimising electromagnetic surveying comprising applying current to a current source, receiving a signal at one or more voltage receivers and recording the signals received, characterised by varying one or more acquisition parameters as a function of the source-receiver separation.

Description

OPTIMIZATION OF MULTI-TRANSITORY ELECTROMAGNETIC PARAMETERS DESCRIPTIVE MEMORY The present invention relates to multi-transient electromagnetic studies (MTEM) to estimate the response of the earth to electromagnetic pulses, thereby detecting hydrocarbon-containing or water-containing formations. In particular, the present invention relates to the optimization of parameters for multi-transient electromagnetic studies (MTEM). Porous rocks are saturated with fluids. The fluids can be water, gas or oil or a mixture of the three. The flow of current in the earth is determined by the resistivities of the rocks, which are affected by the saturation fluids. For example, porous rocks saturated with brine are much less resistive than the same rocks filled with hydrocarbon. By measuring the resistivity of geological formations, it is possible to determine if hydrocarbons are present. This is very useful, because if tests using other methods, for example seismic exploration, suggest that a geological formation has the potential to carry hydrocarbons, resistivity measurements can be used before starting drilling to provide some indication of whether the formation in fact it contains hydrocarbons or if it contains mainly water.

An example of a technique based on resistivity to identify hydrocarbons uses electromagnetic techniques in the time domain. Conventionally, electromagnetic investigations in the time domain use a transmitter and one or more receivers. The transmitter may be an electrical source, that is, a grounded bipolo or a magnetic source, such as a current in a loop or muti-loops of wire. The receivers can be bipolos grounded to measure potential differences, or loops or multi-loops of wire or magnetometers to measure the magnetic fields and / or the time derivatives of magnetic fields. The frequently transmitted signal is formed by a step change in the current either in an electrical or magnetic source, but any transient signal can be used, including, for example, a pseudo-random binary sequence (PRBS). A PRBS is a sequence that is switched between the two pseudo-random time levels that are multiples of one elementary time step At. The switching frequency of PRBS is fs -MAt. A PRBS has a wide frequency bandwidth, whose upper limit is half the switching frequency / s. In recent years, a promising new study technique based on multichannel transient electromagnetic signals has been investigated. The article "Hydrocarbon detection and monitoring with a multichannel transient electromagnetic (MTEM) survey" by Wright, D., Ziolkowski, A., and Hobbs, B., (2002), The Leading Edge, 21, 852-864, describes the multichannel transient electromagnetic method. In this case, there is a Source, usually a current applied between a pair of grounded electrodes, and receivers, usually measuring the potential difference between electrodes along a line. This is also described in WO 03/023452. The multi-transient electromagnetic studies produce geophysical data that are similar in some aspects to the seismic reflection and seismic refraction data. The diffusion of electric currents in the earth, however, is fundamentally different from the propagation of sound waves through the earth itself and the resulting responses differ profoundly, especially in the form of change of response with displacement or resistivity of the material of the earth. covering. The objective of a study of MTEM is to obtain a map of subsurface resistivity variations. The ability to make this map depends entirely on the quality of the measurements made. The present invention recognizes this and establishes a structure for quality control of the MTEM data to enable good quality data to be obtained for processing and subsequent investment to map sub-surface resistivities. According to a first aspect of the invention, a method for optimizing the electromagnetic study is provided, which comprises applying a current to a two-pin current source, receiving a signal at one or more voltage bipolo receivers and recording the received signals, characterized in that the method involves varying one or more acquisition parameters as a function of source-receiver separation. The invention resides in the realization that the optimal data acquisition parameters for MTEM studies can vary significantly as a function of source-receiver separation. This has not been appreciated previously. This embodiment has allowed the selection of optimal measurement parameters, where previously only the experience and an estimation element were used. This is an important advance in the technique. The acquisition parameters that are varied may be at least one of the switching frequency / s at the source and a sampling frequency fr in the recording system. The switching frequency / s and the sampling frequency fr are inversely proportional to the square of the source-receiver separation, and in this case, the step to vary the switching frequency and / or the sampling frequency can be performed inversely to the square of the separation of the source -receptor. The spacing between the source electrodes and the spacing between the receiving electrodes may vary, preferably as a function of the depth of the target study. According to another aspect of the invention, there is provided an electromagnetic study system comprising a bio-pole source of current, one or more bipolo voltage receivers and a recorder for recording the received signals, characterized in that one or more parameters of acquisition used by the source and / or each receiver is selected as a function of source-receiver separation. The acquisition parameters can be at least one of a switching frequency / s at the source and a sampling frequency / r in the recording system. The switching frequency and / or the sampling frequency can be selected inversely as the square of the separation of the source-receiver. Preferably, a plurality of receivers are provided and the source is operated to provide a current at a plurality of different frequencies, each frequency selected as a function of the separation of one of the source receivers. The source can be operated to provide current signals on a different bandwidth scale. Alternatively, the source may comprise a plurality of different sources each operable to provide current in a different frequency bandwidth. The current source can comprise at least one current bipole source. The / each voltage receiver may comprise at least one voltage bipolo receiver. The separation between the source electrodes and the separation between the receiving electrodes can be selected as a function of the profanity of the objective study.

Various aspects of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a source / receiver configuration of MTEM; Figure 2 is a graph of the amplitude of the pulse response of the earth as a function of time. Figure 1 shows a source-receiver configuration of Typical MTEM with a current bi-pole source and its two electrodes A and B, and a line of receivers in line with the source, measuring the potential between the pairs of receiving electrodes, for example C and B. Associated with each pair of receiving electrodes is a recording instrument to digitally sample and record the received signal. A variable current with time is injected between the two source electrodes and measured and recorded digitally in each receiver. The receivers are normally connected to a computer that can interrogate them and download the recorded data. The current input can be a simple step for a shallow objective, or, more likely, some other function, such as the pseudo-random binary sequence (PRBS). The variable voltage response with earth time is measured and recorded in each receiver. In data processing, the measured voltages are deconvolved from the measured input current to obtain the impulse responses of the Earth. These responses are subsequently reversed to obtain the subsurface resistivity variations of the earth. The quality of data processing and investment depends on the quality of receiver and source measurements. Data with low quality can not be corrected in processing and investment. Therefore, it is necessary to make sure that the data acquired in the field is good enough. In practice, this can be a significant challenge due to the large number of potentially variable acquisition parameters, see Table A. In fact, in practice, it is necessary to maintain some substantially constant acquisition parameters and carefully control changes in the others.

TABLE A Parameters that have an impact on the measurements of MTEM It can be seen that the peak voltage of the ground response is related to the acquisition parameters by the following equation: V «106.? ? x .1. -P-t Volts. f (1) The factor of r5 in the denominator makes it very difficult to obtain a good signal at large source-receiver shifts, specifically if the resistivity of the coating layer p, the average resistance of the surface of the earth to the target, is low. In order to resolve the rise and fall of the target, it has been found that the maximum displacement should be about four times the target depth; that is, rmax ~ 4d. For example, for a system of forty channels (N = 4?) The following presentation parameters should be used: • Axs = d / \ 0 (2), Axr = d / \ 0 (3). rmB = 5Axs. { = d / 5) (4) • r- = 5Ax + 39? r (= 4Ad) (5) As the objective depth increases, so does the maximum displacement, which dramatically decreases the voltage in the receiver in accordance with equation (1). The situation is somewhat mitigated by the scale of A? S and A? R. For a particular prospect, these parameters are normally kept reasonably constant, although for larger displacements it is useful to maximize A? S, from equation (5), A? S = r. However, the other parameters are more variable, ie the current /, the switching frequency of the source fs, the sampling rate of the receiver fr, the number of PBRS NPBRS samples, the hearing time TAUD, the number of samples of NAUD hearing, and the number of samples recorded per cycle Nt and the number of cycles recorded is a NC run? c The present invention is based on the recognition that some of the acquisition parameters may vary with a source-receiver shift.

Current I The resistance of the received signal is directly proportional to the current / placed on the ground and the signal to noise ratio is therefore proportional to /. If the signal-to-noise ratio is a problem especially at large source-receiver shifts, it is important to maximize the current level of the source within the applied voltage limit. This is done by reducing the contact resistance of the ground at the source electrodes. A number of well-known methods can be used, including the use of electrodes in parallel, watering the electrodes, and adding bentonite.

Source switching frequency and In the case of ground the impulse response of the earth has the form as shown in figure 2, where t0 is the cut in time, or data start and tP¡C0 is the time to peak of the impulse response of the earth. In the source impulse it travels across the surface of the earth at about the speed of light and reaches the receivers almost instantaneously. This is an air wave. This is followed by the impulse response of the diffusion earth. The signal received is the convolution of the total impulse response, the air wave and the response of the earth, with the input signal. From equation (1) it can be seen that the amplitude of the received signal is inversely proportional to the source switching frequency fs. Thus, the signal-to-noise ratio may increase as the source current switching frequency fs decreases. This is particularly important in large source-receiver shifts. Having said this, there is a limit to how low fs should be: the minimum time between the commutations Í must be smaller compared to the peak time of the earth's impulse responses: í. = y «tp¡co - 10 (6) J s Typically, it is needed ? (7) 10 Therefore, it is better to use the switching frequency more low and still allow the peak of the impulse response of the earth Separate from the air wave.

To optimize the measurements, and in accordance with this invention, it has been appreciated that it is not normally possible to obtain a good resolution and good signal-to-noise ratio in all travels with a single switching frequency /. Instead, it is usually necessary to vary / with the displacement. In fact, for the MTEM measurement configuration of Figure 1, the switching frequency f can in principle, be different for each source-receiver pair.

In marine cases, the "air wave" has a different shape from the acute impulse that occurs in the case of soil. Its form depends on the depth of water, the depths of the source and receiver below the marine surface, and the separation of the source-receiver. In principle, marine data can be considered the same as in the case of soil, but with the impulsive floor air wave replaced by the wave duration greater, which is superimposed on the ground impulse response.

Sampling rate of the receiver r In all the source-receiver shifts the data should ideally satisfy two criteria (1) the peak of the impulse response of the earth must be separated from the air wave, this is required for the resolution of characteristics shallow, and (2) the impulse response length T r 7n _ zn must be greater than four times the peak time Typical ~ t 0 that is, 'or > 4 T - This is essential for data inversion to solve the objective. For an average space, in this case the space below the surface of the earth, the time to the peak increases as the square of the displacement of the source-receiver r (m) and inversely as the resistivity p (ohm m): kr / = (8) pico P The constant k has the value 477.10 in SI units. In short displacements, for example rm? n, and for large resistivities p this time is short and a higher receiver sampling rate is required. On long trips, for example rmax, the pulse is much higher and the sampling rate of the receiver may be lower. On long trips the signal is weak and the source switching frequency should be as low as possible.

There is no point in the oversampling of the data received, but the data received must be sampled appropriately, so that the sampling rate of receiver r can be equal to, or greater than, the switching frequency of the source: Ideally, f r - f s, but in practice this can not be possible due to limitations in the receiver's electronics. If this / is the case, it should be convenient if / out an exact multiple of 5: r / = mf, (10) where m is an integer.

Number of? / Pggs of PBRS samples n The number of PRBS samples in the source is ^ PUBS = 2 ~], where n is known as the PRBS order. Whenever the frequency the switching of the source is low enough, the gain of the processing in the signal amplitude after the deconvolution is almost N equal to PRBS and much higher to N PRBS. To obtain adequate data for the minimum cost, a single long PRBS is used and only registered a record.

Listening time TAuo V number of hearing samples NAUD After the deconvolution the impulse response recovered must be long enough; that is, the recoverable length of the impulse response must be greater than four times the peak time, as explained above. The time and number of hearing of the Listing samples are defined as: • T - t > 4 (. - /) TO UD 0 peak 0 (1 1 • N - T If. A UD A UD r (12) Number of samples recorded per cycle? / J If the source switching frequency and velocity of =) sampling at the receiver are equal (ie, if r s, the total number of Samples recorded is equal to the number of PRBS samples plus hearing samples: • N = N + N T PRBS A UD (1 3) If the source switching frequency and the sampling rate at the receiver are not equal (that is, if f r = mf), the number Total of samples is greater: / N = - N + N = m N + N, ",.

T PRBS A UD PRBS A UD (14) s Number of cycles recorded in a run Nc, c If the registration system has a memory that is too much small, it may be impossible to obtain an adequate signal-to-noise ratio with a single PRBS cycle that is, with only one sample register? / r per channel In this case a cycle run NC? c is recorded and the resulting traces are added, or stacked, so that each channel increases the ratio of signal to noise before deconvolution The signal to noise ratio increases as? ¡N r / r It is clearly more efficient to maximize NPRBS and minimize Ncic This can be achieved only if there is enough memory in the register boxes Operational considerations It will be clear from the foregoing that the ratio of the longest displacement rmax to the shortest rm? N is around 10 Since the frequency of commutation f s and the sampling rate f r can both vary As the displacement square, these two frequencies may vary by approximately two orders of the magnitude of the displacement plus short to longer In the arrangement of Figure 1, it will not be possible for a single source to switch on different frequencies simultaneously, although It is possible to measure and record all receivers simultaneously place, to meet the above requirements, a frequency scale Source switching for each source position should be used, each source switching frequency selected to direct a particular scale of receivers based on the source-receiver shift. For the example of the individual source of Figure 1, this means that the source can typically transmit signals of different frequency bandwidths, which is determined by the switching frequency, and the receiver / registration systems can be recorded using the speed corresponding sampling. The data can be classified according to the displacement and proceed with the appropriate bandwidth source signal. Alternatively, multiple sources with non-overlapping frequency bandwidths can be used. In this case, the signals can be transmitted simultaneously. However, when this is done, the combination of the receiver / register system can be configured to allow different frequency bandwidths to be separated. In any case, the registration system must have the flexibility to overcome the scale of frequency widths that the MTEM data possess. One skilled in the art will appreciate that variations of the described arrangements are possible without departing from the invention. Alternative configurations can be clearly possible. Likewise, the previous description of the specific modality is made by means of example only and not for purposes of limitation. It will be clear to the person skilled in the art that minor modifications can be made without significant changes to the described operation.

Claims (12)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for optimizing the electromagnetic study comprising applying current to a current source, receiving a signal at one or more voltage receivers and recording the received signals, characterized in that the method involves varying one or more acquisition parameters as a function of the separation of the source-receiver.

2. The method according to claim 1, further characterized in that the varying acquisition parameters comprise at least one switching frequency at the source and a sampling frequency in the recording system.

3. The method according to claim 2, further characterized in that it comprises varying the switching frequency and / or the sampling frequency inversely to the square of the source-receiver separation.

4. The method according to any of the preceding claims, further characterized in that it comprises varying the spacing between the source electrodes and the spacing between the receiving electrodes.

5. The method according to claim 4, further characterized in that the separation varies in proportion to the depth of the study of the objective and / or the separation between the source and the receiver.

6. An electromagnetic study system comprising a current source and one or more voltage receivers for receiving and recording received signals, characterized in that one or more acquisition parameters used by the source and / or the or each receiver is selected as a function of the separation of the source-receiver.

7. The system according to claim 6, further characterized in that the acquisition parameters are at least one of a switching frequency at the source and a sampling frequency in the recording system.

8. The system according to claim 7, further characterized in that the switching frequency and / or the sampling frequency are selected inversely to the square of the separation of the source-receiver.

9. The system according to any of claims 6 to 8, further characterized in that the separation between the source electrodes and the separation between the receiving electrodes is selected as a function of the depth of the target study and / or the separation between the source and the receiver.

10. The system according to any of claims 6 to 9, further characterized in that the plurality of receivers is provided and the source is operated to provide a current in a plurality of different frequency bandwidths, each frequency bandwidth selected as a function of the separation of one of the source receivers.

11. The system according to claim 10, further characterized in that the source comprises a plurality of different sources each operable to provide current on a different frequency scale.

12. The system according to any of claims 6 to 11, further characterized in that the current source comprises at least one bipolar current source and the or each voltage receiver comprises at least one bipolar voltage receiver.