Patent Application
20100237870
This description relates to a multiple electric and magnetic field receiver system and apparatus for airborne geophysical surveying.
Geophysical electromagnetic (“EM”) prospecting techniques can be effective in determining the electrical conductivity of soils, rocks, and other bodies at and under the earth's surface.
Geophysical EM prospecting can be carried out using surface based equipment and airborne equipment. Airborne methods in which equipment is transported by aircraft such as helicopter, airplane or airship may be useful for large area surveys. For airborne electromagnetic (“AEM”) systems, survey data may be acquired while an aircraft such as an airplane or helicopter flies at a nearly constant speed along nearly-parallel and close to equally-spaced lines at an approximately constant height above ground. In some applications, geophysical EM prospecting of a seabed may be carried out using equipment located under the surface of a body of water.
Some geophysical surveying methods are active in that the equipment is used to transmit a signal to a targeted area, and then measure a response to the transmitted signal. Other geophysical surveying methods are passive in that signals produced from a target area are measured without first transmitting a signal to the target area.
An example of a passive geophysical EM prospecting method is Audio Frequency Magnetic (“AFMAG”) surveying in which the EM fields resulting from naturally occurring primary signal sources such as lightning discharges are measured. These EM fields propagate around the earth as plane waves guided by the ionosphere and earth's surface. Lightning activity occurring remote from the measurement point can produce signals with an approximately flat spectral density at frequencies between, for example, 8 Hz and 500 Hz, varying with geographical location, time of the day, seasons and weather conditions. An example of a passive AFMAG geophysical EM prospecting method is shown in U.S. Pat. No. 6,876,202.
A geophysical survey system comprising: a first sensor system towed by an aircraft, having at least one airborne sensor for measuring electric components of a low frequency natural electromagnetic field in a survey area; a second sensor system for positioning at a fixed position on the ground during a survey, having at least two ground based induction coil sensors for measuring magnetic components of a low frequency natural electromagnetic field in or near the survey area, the ground based sensors each being oriented to sense the magnetic components in different directions; and a processing system for calculating a set of first vector values over time in dependence on the electric components measured through the first sensor system and calculating a set of second vector values over time in dependence on the magnetic components measured through the second sensor system and comparing one or more characteristics of the first vector values and the second vector values to identify geophysical information about the survey area.
According to another example embodiments is a method of conducting a geophysical survey of a survey region, comprising: measuring, using a towed airborne sensor system an electric component of a low frequency natural electromagnetic field in a survey area; measuring, at a ground based sensor system during a survey, magnetic field components of the low frequency natural electromagnetic field in or near the survey area using at least two stationary induction coil sensors each being oriented to sense the magnetic field in different directions; and calculating a set of first vector values over time in dependence on electromagnetic field data measured through the airborne sensor system and calculating a set of second vector values over time in dependence on electromagnetic field data measured through the ground based sensor system and comparing one or more characteristics of the first vector values and the second vector values to identify geophysical information about the survey area.
According to another example embodiment is an airborne sensor system for geophysical surveying, the airborne sensor system being towable by an aircraft and comprising: a continuous tubular support frame defining at least one internal tubular passage; and a plurality of conductive antenna elements supported at spaced apart locations within the internal tubular passage for measuring electric components of low frequency natural electromagnetic field in a survey area, the antenna elements each having a different relative orientation and measuring the electric components in at least two different relative directions.
Example embodiments of the invention are provided in the following description. Such description makes reference to the annexed drawings wherein:
The tow assembly sensor system 410 includes a communications link such as a non-conductive cable 12 containing optical fibers to allow bidirectional data transmission between the electric field sensor assembly 10 and airborne data processing equipment 16 located in the helicopter. In some example embodiments, the suspension system 11 reduces the vibration of the conductive elements of the electric field sensor assemblies 10 and 13, so as to reduce the noise generated by motion in the geomagnetic and static electric field of the earth. The use of the non-conductive fiber cable 12 and non-conductive tow cable 14 mitigates against distortions and fluctuations of the electric field that could otherwise be caused by a vertical conductor between the helicopter and the electric field sensor assemblies. In at least some example embodiments, the use of an electric field sensor assembly which is physically large, instead of a smaller, enclosed, streamlined “bird”, enhances the data quality while keeping the weight of the system within the range that can be towed by a small helicopter.
As shown in
The ground-based magnetic sensor assembly 17 and data collection computer 19 of ground assembly sensor system 420 are designed using known techniques so that their combined noise floor is substantially less than the magnetic component of electromagnetic fields usually caused by distant natural sources at intervals of 1-10 s in the 10 Hz-1,000 Hz frequency range. For example, in some implementations the noise floor spectral density may be on the order of 3 fT/√Hz at 30 Hz, decreasing to the order of 0.2 fT/√Hz at 1,000 Hz. By way of further example, in some implementations the noise floor spectral density may be on the order of 10 fT/√Hz at 30 Hz, decreasing to the order of 1 fT/√Hz at 1,000 Hz. It is well known that this performance can be achieved by making the sensor coils sufficiently large. With this configuration and performance, the magnetic sensor assembly 17 provides vector measurements of the horizontal magnetic component of the naturally occurring electromagnetic field. In at least some example embodiments, such a ground assembly configuration provides substantially more useful information in the 10 Hz-1 kHz range than is provided by a commercially available total field magnetometer such as a cesium magnetometer.
The inner assembly 418 supports amplifier packages 8 and 9 and data acquisition package 90. As will be explained in greater detail below, amplifier package 8 receives input signals from conductive section 5 and amplifier package 9 receives input signals from conductive section 6. The previously mentioned amplifier package 20 associated with vertical electric field sensor assembly 13 receives input signals from conductive section 13A. The local commons or grounds of amplifier packages 8, 9 and 20 are electrically connected to conductive element 7, which is therefore the common element and the reference for all electrical field measurements such that conductive section 5 in combination with conductive section 7 and amplifier package 8 provides a first electric field sensor for measuring electric components of a naturally occurring magnetic field in a first direction; conductive section 6 in combination with conductive section 7 and amplifier package 9 provides a second electric field sensor for measuring electric components of the magnetic field in a second direction; and conductive section 13A (of vertical sensor assembly 13) in combination with conductive section 7 and amplifier package 20 provides a third electric field sensor for measuring electric components of the magnetic field in a third direction. In the illustrated embodiment, a data acquisition package 90 for receiving the outputs of amplifier packages 8, 9 and 20 is located in one of the outer tubular members 412 that houses reference conductive section 7. The amplifier packages 8, 9 are respectively located in tubular members 412 that house the non-conductive sections 3 and 4 between section 7 and each of the other conductive sections 5 and 6, and are positioned distant from conductive section 7 and close to the each of the other conductive sections 5 and 6. In some example embodiments, three GPS antennas 511, 512 and 513 are provided at spatially separated locations within or on the shell 1 to allow the orientation of the airborne electric sensor assembly to be determined. The conductive sections 5, 6, 7 and conductive section 13A of vertical field electric sensor assembly 13 are each electric field antenna elements that are supported in constant positions relative to each other during flight.
In at least some example implementations, the above described arrangement of the electric field sensor assembly 10 maximizes or allows for increased electrical signal strength by providing three conductive antenna elements 5, 6 and 7 of large self capacitance, separated from each other by a large distance, and isolated from airflow induced vibration, within an assembly 10 which is large, and yet light and with sufficiently low drag to be towed by a helicopter. The three conductive antenna elements 5, 6 and 7 are arranged in a non-colinear array enabling the airborne sensor system to independently measure at least two perpendicular electric components of the low frequency natural electromagnetic field. In example embodiments the support frame or shell 1 is a loop-like continuous tubular skeletal frame that can support spatially separated antenna elements in substantially constant relative positions in an internal continuous tubular passage while having a relatively light weight by defining a large central area or opening 440 through which air can pass. In example embodiments, the surface area, volume, and weight of the physical components of the shell 1 is small, considering the total spatial volume occupied by shell 1 (including opening 440,) is so that the conductive sections 5, 6, 7 have a large spatial separation from each other relative to the weight of the sensor assembly 10. Similarly, the suspension system 11 allows vertical e-field sensor assembly 13 (including its antenna element 13A) to be supported in a relatively constant position to the shell 1 with the sensor assembly 13 spatially separated from the horizontally oriented conductive elements that are located in shell 1.
Other example embodiments could contain two, four, or more conductive sections, with corresponding limitation or enhancement of the number of electric field directions that could be independently measured. Instead of sharing a common conductive element 7 among a plurality of electric field sensors, each sensor could be provided with its own reference conductive element. The vertical electric field sensor 13 could be omitted. The shell 1 could have a different number of straight or curved sections. Also, a more complex structure could be used, such as an octahedron structure. By way of example, a suitable octahedron structure for a tow assembly is described in U.S. patent application Ser. No. 12/645,915 filed Dec. 23, 2009 and International Patent Application No. PCT/CA2009/001865 claiming priority to U.S. Patent application No. 61/140,337, the contents of which are all incorporated herein by reference.
By way of further example,
Although the example embodiment of the electric field sensor assembly in
Other example embodiments could combine the electric field sensors with one or more coils that would sense magnetic fields. Specifically, in the example embodiment shown in
Other example embodiments of towed sensor assemblies 10, 500 uses a plurality of two stage elastic suspension systems 42 as shown in
Although each of
The example tow assembly structures shown in
The circuit of
In an alternative implementation, the capacitor 107 is used with calibration generator 109 to inject a small calibration signal into each antenna element. The frequency of the calibration signal is selected so that it is near the upper frequency limit of the entire system. A different calibration signal frequency is injected at each of the antenna elements. In the signals acquired from each antenna element, each calibration signal is separated from the others and from natural signals by known digital filtering techniques. When analyzed, the calibration signals provide a continuous measure of the sensitivity of each channel of the system and the coupling between channels.
In some implementations, the circuit of
In some implementations, an adjustment procedure is used with the circuit of
In an example embodiment, the input of amplifier package 8 is connected to receive signals from conductive element 5, the input of amplifier package 9 is connected to receive signals from conductive element 6, and the amplifier package is connected to receive signals from the conductive element 13A of vertical sensor assembly 13. The outputs of amplifier packages 8, 9, 20 and GPS antennas 511 and 512 and 513, are connected to the electronics package 90 by, in at least some example embodiments, thin shielded cables. The cables are thin, so as to reduce their capacitance with respect to the antenna elements and their effect on the electric field. The shields of these cables and the local commons or grounds of all the circuits in the sensor assembly 10 and sensor assembly 13 are electrically connected to conductive element 7, which is therefore the common element and the reference for all electrical field measurements. In some example embodiments, a differential amplifier circuit may be used in place of single input amplifier packages 8, 9—for example conductive elements 5 and 6 could be attached to the differential inputs of a single differential amplifier circuit with conductive element 7 being used as the common for the differential amplifier circuit.
In an example embodiment, data collected by airborne data collection computer 16 and the data collected by the ground based data collection computer 19 is ultimately transferred over respective communication links 22, 23 (which may be wired or wireless links or may include physical transfer of a memory medium such as laser discs or flash memory cards) to a data processing computer 21 at which the electromagnetic field data obtained from airborne sensor assemblies 10, 13 and ground based magnetic field sensor assembly 17 and the GPS data from GPS antennas 511, 512, 513, and 18 is processed by the data processing computer 21 to determine the electrical resistivity structure of the earth. In some example embodiments, some or all of the functions performed by data processing computer 21 may be performed at one or both of the airborne or ground based data collection computers 16 and 19.
In operation, in some example implementations the airborne sensor systems 10 and 13 can be flown at a substantially constant speed in a series of parallel lines over a survey area to make a series of measurements of the low or audio frequency range (for example in the range of 10 Hz to 1000 Hz) electric field in three approximately orthogonal directions. Simultaneously, the stationary magnetic field sensor assembly 17 is located on the ground within or close to the survey region to also make a series of measurements of the low or audio frequency range (for example in the range of 10 Hz to 1000 Hz) horizontal magnetic field in two orthogonal directions. The stationary magnetic field sensor assembly 17 should, in at least some example uses, be placed a sufficient distance from any industrial electromagnetic field sources such as power lines so that natural audio-frequency magnetic fields dominate the signals received at the location of the stationary magnetic field sensor assembly 17. For example, in one application the distance of the stationary magnetic field sensor assembly 17 from a major power lines could be at least 3 km.
Thus, as a survey of a region is conducted, the airborne data collection computer 16 receives and stores a stream of digitized data that is representative of the naturally occurring audio frequency electric field vector E(air)(t). Each of the airborne magnetic field measurements is stamped with a GPS location and time information received from the GPS antennas 511, 512, 513. At the same time, the ground based data collection computer 19 receives and stores a stream of digitized data that is representative of the naturally occurring audio frequency horizontal magnetic field vector H(ground)(t) as measured by the ground based sensor coils 422. Each of the ground based magnetic field measurements is stamped at least with time information received from the GPS sensor 18, and in some embodiments also with location information. Thus, each of the airborne and stationary data collection computer systems 16, 19 respectively collect data records that each include two or three channels of data, each channel corresponding to the electric or magnetic field measurement taken by a respective one of the sensor coils or antenna electrode pairs.
At the signal processing computer 21, the data records from each of the airborne and stationary systems 16, 19 are merged in dependence on the GPS signal time data associated with each of the records to generate records that include three channels of digitized airborne electric field data, two channels of digitized ground magnetic field data, and four channels of GPS data from antennas 511, 512, 513, 18 with each record corresponding to measurements taken at substantially the same time at both the ground and airborne sensor systems.
The measurements from the airborne system are converted into an orthogonal frame of reference and into units of field strength (V/m). Specifically, the calibration signal strengths are processed to determine the response of the electronics to a unit voltage or charge. A theoretical calculation is performed, using the physical dimensions and relative positions of the conductive elements 5, 6, 7 of the sensor assembly 10 to determine the voltage or charge produced on each antenna element by an electric field of 1 V/m in each axial direction in an orthogonal frame of reference fixed relative to the airborne system 410. The theoretical calculation and the calibration results are combined to obtain a matrix of coefficients that relate the electric field vector to the digitized amplifier outputs. The inverse of this matrix is found by known techniques. The vector of measured outputs is multiplied by the inverse matrix to obtain the electric field vector in the airborne system frame of reference. This calculation is performed for every sample interval in the digitized electric field data.
In one example embodiment, the GPS data from antennas 511, 512, 513 is processed to yield the geographical position and altitude differences between the three sensors to a precision on the order of 0.1 m. These differences are used in data processing computer 21 to calculate the orientation of the electric field sensor assembly 10 (including the vertical field sensor 13) relative to geographic directions e.g. North-East-Down. This orientation information is used to calculate a matrix which transforms a vector expressed in the airborne system frame of reference, to a vector expressed in a geographic frame of reference. The electric field vectors calculated as in the previous paragraph are then multiplied by this matrix to transform them to a geographical frame of reference (North-East-Down). The calculation is repeated for every sample interval, obtaining a time series of electrical field components in the North, East and Down directions.
In some example embodiments, the vertical electric field sensor 13 is not present. In that case, the calculations described above are performed, using the approximation that the vertical component of the electric field is zero. In at least some implementations, this is a good approximation since the contribution of the vertical electric field sensor to the calculated horizontal components is in proportion to the tilt of the airborne sensor assembly 10, which can be kept small by appropriate flight procedures.
The orientation of the sensor coils 422 in the ground based assembly 420 are measured in geographical coordinates, e.g. by an operator using a magnetic compass with a correction for magnetic declination. The magnetic field components measured by magnetic field sensor assembly 17 are rotated into a North-East fame of reference using techniques similar to those described for the electric field sensors. Alternatively, the sensor coils 422 in magnetic field sensor assembly 17 are installed on the ground so that their axes bear North and East respectively, so that no rotation is needed.
In one example embodiment, frequency-domain processing is then performed on the data records either through applying narrow-band filters or applying Fast Fourier-transforms on multiple consecutive time blocks (by way of non limiting example, time blocks could each be 0.5-2 seconds long), or by use of known cascade decimation techniques (see e.g. Wight, D. E., F. X. Bostick, and H. W. Smith, Real time Fourier transformation of magnetotelluric data, report, Elecr. Geophs. Res. Lab., Uni. of Tex. at Austin, 1977) resulting in a time series of data that represents the electric and magnetic field in each of the axis directions at specific audio frequencies. This data includes a real and imaginary number representation of the electric field components for each of the North, East, and Down axis as measured in the air, and each of the North and East axis as measured on the ground. Certain frequencies can be filtered out—for example 60 Hz noise is removed in some embodiments.
In some example embodiments, the frequency domain processing is performed on the voltage or charge outputs of the airborne electric field sensors, and the magnetic field outputs of the ground sensors, and the subsequent calculations to convert to electric and magnetic field strength, and then to the geographical frame of reference, are performed on the frequency domain data. The various processing operations can be performed in a variety of ways and orders which are mathematically equivalent.
The result of the above calculation is a series of frequency domain electric and magnetic field measurements in a geographical frame of reference. Given the data derived as described in the previous paragraph, known techniques can be used to calculate the transfer functions between the horizontal magnetic field components at the ground station, and the horizontal electric field components form the airborne system. (Again see the Wight paper referenced.)
It is known that the theory of the magnetotelluric survey technique, confirmed by observations, shows that the electric component of the electromagnetic field is strongly affected by changes in the electrical conductivity structure of the earth below the point of observation, while the magnetic field is only weakly affected. Thus, the magnetic component at the location of the airborne sensor assembly 10 can be approximated by the magnetic component measured by magnetic field sensor assembly 17 at the fixed ground station. The transfer functions referred to in the previous paragraph can therefore be interpreted using known magnetotelluric theory to determine the electrical conductivity structure of the earth as a function of depth and to detect subsurface bodies and structures. (See for example (1) Vozoff, K., 1972, The magnetotelluric method in the exploration of sedimentary basins: Geophysics, 37,98-141. and (2) Anav, A., Cantarano, S., Cermli-Irelli, P., and Pallotino, G. V., 1976, A correlation method for measurement of variable magnetic fields: Inst. Elect. and Electron. Eng. Trans., Geosc. Elect., GE14, 106-1 14, which are hereby incorporated by reference).
Some sample implementations may include fewer sensors than described here. For example, the airborne sensor assembly 10 could measure only one horizontal electrical field component. The data can then be processed using known approximations to the techniques described above, for example those used in the interpretation of surveys known in the industry as “Scalar CSAMT” (Scalar Controlled Source Audio Magneto Telluric).
The interpretation described in the previous paragraphs allows the electrical resistivity of the earth, as a function of depth, to be determined even when there are no horizontal variations in the electrical resistivity. The data collection and processing techniques and methods described above in respect of airborne sensor assembly 10 can also be applied to airborne sensor assembly 500. In one non-limiting example implementation there is provided a geophysical survey system comprising: a first sensor system towed by an aircraft, having three sensors for measuring the electric component of low frequency (10 Hz-1,000 Hz) natural electromagnetic field in a survey area, the sensors each having a different relative orientation and measuring the electromagnetic field in a different relative direction; a second sensor system, located at a fixed position on the ground, having two sensors for measuring the magnetic component of the low frequency (10 Hz-1,000 Hz) natural electromagnetic field in or near the survey area, the sensors being oriented to sense the magnetic field in two perpendicular horizontal directions; and a processing system for calculating a set of first vector values over time in dependence on the electromagnetic field data measured through the first sensor system and calculating a set of second vector values over time in dependence on the electromagnetic field data measured through the second sensor system and comparing one or more characteristics of the first vector values and the second vector values to identify geophysical information about the survey area.
It will be appreciated by those skilled in the art that other variations of the embodiments described herein may also be practiced without departing from the scope of the invention. Other modifications are therefore possible.
This application claims the benefit of and priority to U.S. Patent Application No. 61/160,722 filed Mar. 17, 2009, the content of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61160722 | Mar 2009 | US |
