GB2151794A - A tippermeter - Google Patents

Abstract

A measurement apparatus which is used in determining the angle which the earth's total electromagnetic field vector makes with the horizontal plane tangent to the surface of the earth at a given location, which angle is known as the "tipper". The apparatus may be used for reconnaissance surveys by taking measurements along roads and other public areas to determine locations of high "tipper" that in turn relate to subsurface features which may indicate the presence of hydrocarbon deposits. The apparatus includes two detecting coils 11a, 11b which are mounted at right angles in the shape of an inverted tee. The coils are designed so as to be sensitive to changes of as little as 0.01 gammas in the earth's electromagnetic field at a frequency of 8 Hz. Electronic circuits having a special transfer function are provided to amplify the signals from the coils. The amplified signals are converted to dc pulses which are stored in output capacitors for readout with a voltmeter. <IMAGE>

Description

SPECIFICATION A tippermeter This invention relates to an improved measurement apparatus which may be used in determining the angle which the earth's total electromagnetic field vector makes with the horizontal plane tangent to the surface of the earth at a given location.

A number of electromagnetic techniques are well known in the art of geophysical exploration. One such method for the geophysical exploration is the magnetotelluric method. It is well known that the natural electromagnetic fields present in the earth's atmosphere in turn generate electric currents in the earth's subsurface. These currents, known as telluric currents, in turn generate an associated magnetic field whose effects may be measured at the surface.

In the magnetotelluric method, the impedance of the earth to electromagnetic fields of external origin is determined by making measurements on the earth's surface. Three directional magnetometers are set out along two perpendicular horizontal axes and one vertical axis to measure variations in the magnetic field, which field is generally denoted by the symbol H. Two horizontal dipole antennas are set out to measure variations in the electric field, which field is generally denoted by the symbol E.

The type of magnetometer used and the length of the antenna required depend on the frequency range which is of interest.

The mathematical basis for the magnetotelluric method was disclosed by Louis Cagniard in his article entitled "Basic Theory of the Magneto-Telluric Method of Geophysical Prospecting" (Geophysics, Volume XVIII, No. 2, pgs. 605-635, 1953). It is now well known that a specific relationship exists between the orthogonal components of the earth's magnetic field and the orthogonal components of the earth's electric field. This relationship allows one to estimate the impedance of the earth relative to different electromagnetic frequencies. An interpretation of the frequency dependence allows one to estimate the depth to which the electromagnetic energy penetrates the earth.In the magnetotelluric method, the variations in these fields over a large frequency range, as measured by the three magnetometers and two dipole antennas, are used to derive earth resistivity information from shallow depths to depths exceeding 30,000 feet.

One of the relationships determined in the magnetotelluric method is the deviation from horizontal of the earth's natural magnetic field vector. This vertical angle, the angle of deviation from the horizontal plane tangent to the earth's surface, is often referred to as the "tipper" and this is the meaning when used in this specification. The tipper is useful in identifying the presence of conductive anomalies in the earth's subsurface.

Another electromagnetic technique for geophysical exploration is the AFMAG (audiofrequency magnetic) method. The AFMAG method may be practised on the ground, but is more frequently used for large-scale, airborne reconnaissance surveys. As discussed in U.S. Patent 3,568,048, issued March 2, 1971 to Robinson, this method generally makes use of two detection coils mounted with their axes perpendicular to one another, each axis further being at an angle of 45 to the horizontal. When the natural magnetic field vector is inclined above or below the horizontal, the voltage induced in one of the coils is greater than that induced in the other coil.

The ratio of the two coil voltages is determined by the angle of tilt of the magnetic field vector.

The magnetic field components measured in the AFMAG method are in the audio frequency range, with the components of particular interest in the range of 100 to 2,000 cycles per second. Unfortunately, the depth of electromagntic field penetration in the earth is inversely related to the frequency of such fields. Because of the relatively high frequencies measured, this method will only detect anomalies located relatively close to the earth's surface.

As discussed above, the AFMAG method is generally used for reconnaissance surveys of very large areas, where the information desired is related to subsurface structures relatively close to the earth's surface. If it is desired to gather data on the electrical structure of the earth's subsurface to a depth of 2000 feet or more, magnetotelluric methods may be used.

However, the complex instrumentation required to gather information related to such depths using magnetotelluric methods may result in costs greater than can be justified unless the area to be investigated is very small, e.g., less than five square miles.

In the past, no cost effective apparatus has been available for mapping the electrical structure of the earth's subsurface to depths of up to 2000 feet over an area covering 100 square miles or more.

The apparatus of the present invention comprises two large detecting coils mounted at right angles to each other in the shape of an inverted tee. The coils are designed to measure the very weak natural electromagnetic field of the earth in a certain frequency range, with the accuracy required for geological interpretation, while maintaining the small size required for portability. The coils are tuned with parallel capacitors to a peak sensitivity at a frequency of about 5 Hz.

The signal from each coil is fed into a separate electronic circuit designed to fully amplify signals below a frequency of about 10 Hz while attenuating signals higher than 30 Hz. The bandwidth rolls off at dc on the low frequency end. The output of each electronic circuit is converted to a series of direct current pulses, and is then stored in a large output capacitor. The ratio of the outputs of the two output capacitors is the tangent of the tipper, or the angle of the total magnetic field vector relative to the horizontal plane tangent to the surface of the earth at the measurement site.

In taking measurements, the apparatus is positioned so that one coil is substantially parallel to the plane tangent to the earth's surface, and the other coil is perpendicular to that plane. By taking two measurements at each location, one with the coil parallel to the earth's surface aligned in a generally north-south direction, and one with that coil aligned 90 degrees from its alignment during the first measurement, or in a generally east-west direction, the vertical angle and relative strength of the total magnetic field vector can be estimated. The vertical angle of the total magnetic field vector, the tipper, relates directly to the resistive and conductive bodies that make up the electrical structure of the earth beneath the recording site.

The entire apparatus is small enough to be readily portable by a single person. Because of its small size, the apparatus can be used for reconnaissance surveys by taking measurements along roads and other areas accessible to the general public. The tipper may be used to map the electrical structure of the earth to depths of up to 2,000 feet in the sedimentary basin environments that are often considered prospective locations for subsurface hydrocarbon deposits. The information obtained by use of the apparatus can be particularly useful in mapping the location of chimneys or alterations of subsurface rock due to hydrocarbon leakage. These chimneys and alterations may indicate the presence of hydrocarbons in the subsurface.

The invention is described with reference to the drawings wherein: Figure 1 is an isometric view of the apparatus of one embodiment of the present invention.

Figure 2 is an electrical schematic diagram of an amplifier/active diode circuit suitable for use in the present invention.

Figure 3 is a partial block diagram, partial schematic diagram of the electrical circuit of one embodiment of the present invention.

The apparatus of the present invention is typically made up of three major parts, i.e., two large, generally cylindrical detector coils mounted perpendicularto each other, and amplifierlactive diode circuitry. Figure 1 shows the physical arrangement of one embodiment of the apparatus of the present invention. Enclosures 1 Oa, 1 Ob, as well as enclosure 15, discussed below, and other assembly hardware desirably may be made of a hard, nonmagnetic material such as plastic. Nonmagnetic material is selected because the presence of metallic enclosures proximate to the coils would distort the indications of the earth's electromagnetic field as measured by the coils. The two detector coils 11 a, 11 b are mounted within enclosures 1 0a, 10b, respectively.

The coils are mounted at right angles to each other in the shape of an inverted tee. The tee shaped arrangement is selected to minimize the coupling between the coils.

End panels 12 may be attached to the ends of enclosure lOb to increase the stability of the apparatus when placed on a flat surface. Enclosure 15, mounted atop enclosure lOb at the angle formed by the junction of enclosures 10a and lOb, houses the electronic circuits of the apparatus. Coils 11 a, 11 b are connected to the electronic circuits with shielded wires 14 (Figure 3) having grounded shields. The functions of reset button 51, power switches 52 and 55, voltmeter 53, and selector switch 54, all of which are shown in Figure 1, will be described in more detail below.

In the preferred embodiment, each coil 1 1a and 11 b is wound with 3200 turns of No.22 copper wire on a core. The length of each coil is 15 inches. Each core comprises a 2-1/2 inch diameter nonmetallic form, such as PVC pipe, filled with 24 one-half inch diameter rods of a material having a magnetic susceptibility of 800 or greater, such as ferrite or mu metal. The spaces between the rods are filled with epoxy resin. Although the preferred embodiment comprises a core of 24 one-half inch diameter rods in a nonmetallic 2-1/2 inch diameter form, the core may also be made of a solid metallic element or a series of stacked plates having the overall length and diameter of the form used in the preferred embodiment.

The coils so constructed are small enough to allow the apparatus of the present invention to be readily portable by a single person. The coil so designed is also sensitive to changes of as little as 0.01 gammas in the earth's electromagnetic field at a frequency of 8 Hz. This 8 Hz design point is selected because of the presence of an 8 Hz peak in the earth's natural electromagnetic field.

Figure 2 is a schematic diagram of the amplifier active diode circuit of the apparatus of the present invention. Only the detailed circuit for coil 1 1a is shown, the circuit for coil 11 b being a duplicate thereof. Coil 11 a is tuned with parallel capacitors 16 to have a peak sensitivity at about 5 Hz. The reason for tuning the coils to this 5 Hz peak sensitivity will be discussed below.

The tuned coil output is fed to positive input terminal 3a and negative input terminal 2a of operational amplifier 23 through capacitors 17 and resistors 18. Capacitors 17 provide a phase adjustment so that the current and voltage output of the coil are in phase at the input to operational amplifier 23. The positive input terminal 3a of operational amplifier 23 is grounded through resistor 24. Power is supplied to terminals 4a and 7a of operational amplifier 23 from conventional 9-volt power supply 28 (Figure 3). Capacitor 19 is inserted across terminals 1 a and 8a of operational amplifier 23 to filter out high frequency spikes.

Feedback resistor 25 between the output terminal 6a and negative input terminal 2a of operational amplifier 23 adjusts the amplification level of the circuit. Capacitor 26 inserted in parallel with resistor 25 provides additional filtering of high frequency spikes. The output of operational amplifier 23 may be measured at test point 27 which is connected to said output through resistor 30.

The output of operational amplifier 23 is connected to negative input terminal 2b of operational amplifier 31 through resistor 32. The positive input terminal 3b of operational amplifier 31 is connected to potentiometer 35. Potentiometer 35 is connected to the 9-volt power supply through resistors 36 and 37. The potentiometer is provided to allow adjustment of operational amplifier 31 for minimum noise and maximum signal output operation. Resistors 36 and 37 act to reduce the voltage range available over potentiometer 35 to the range required for adjustment of operational amplifier 31. Positive input terminal 3b of operational amplifier 31 is connected to ground through resistor 40. Capacitor 41 is connected across terminals 1 b and 8b of operational amplifier 31 to filter out high frequency spikes.

Power is supplied to terminals 4b and 7b of operational amplifier 31 from the 9-volt power supply.

The output terminal 6b of operational amplifier 31 is connected to diode 42. Diode 42 is located in the feedback circuit of operational amplifier 31 so that diode 42 will function as an active diode, converting the ac signal at its input to a series of dc pulses regardless of the strength of the ac input. Feedback resistor 45 and variable resistor 48, connected between the output terminal of diode 42 and negative input terminal 2b of operational amplifier 31, control the level of amplification across that portion of the circuit. Variable resistor 48 is provided so that the amplification of one apparatus, and thus its output for a given input signal, may be calibrated to be the same as the output for another apparatus in response to the same input signal.Capacitor 46 is connected in parallel with resistor 45 and variable resistor 48 to provide additional filtering of high frequency spikes. Diode 42 converts the amplified time varying signal from coil 11 a to a series of dc pulses. The output of diode 42 may be measured at test point 43 which is connected to said output through resistor 44. The output is fed to output capacitor 47 through resistor 50.

The components of the amplifier/active diode circuit described above are selected so that the circuit has a special transfer function yielding full amplification below a frequency of 10 Hz with a roll-off of approximately 50 db at 30 Hz. Thus, any signals detected by the coils higherthan 30 Hz are attenuated, whereas signals below 10 Hz are fully amplified. As discussed above, coils 11 a, 11 b are tuned to a peak sensitivity at 5Hz. With coils tuned to this frequency acting in combination with circuitry having a transfer function as described above, the apparatus has the desired sensitivity at the naturally occurring electromagnetic energy peak of 8 Hz, while being sufficiently insensitive to frequencies above 30 Hz to allow operation near 50 Hz or 60 Hz power sources without interference.

Referring to Figure 3, the apparatus of the present invention is operated as follows. Before taking a measurement, reset button 51 is operated. This action discharges the output capacitors 47a, 47b of the two amplifier/active diode circuits to ground.

Power switch 52 is operated to connect 9-volt power supply 28 to the operational amplifiers of the two circuits. The signals produced by coils 11 a, 11 bin response to the electromagnetic field of the earth are amplified and converted to a series of dc pulses by the circuit described above and shown in Figure 2, and stored in output capacitors 47a, 47b. After a short period of time, e.g., 20 or 30 seconds, the measurement is terminated by operating power switch 52 to disconnect the power supply from the electronic circuits.This "turns off" the operational amplifiers so that no more dc pulses are transmitted to output capacitors 47a, 47b. The voltage across capacitors 47a, 47b, which voltage has been built-up by the dc pulses, is then read on voltmeter 53 which is energized from power supply 28 through power switch 55. The voltage across either output capacitor may be read by selecting the desired capacitor with selector switch 54.

In normal field operations, a first set of measurements is made with the apparatus positioned so that coil 11 b (Figure 1) is parallel to the plane tangent to the earth's surface and aligned in generally a north-south direction. A second set of measurements is made with the apparatus positioned so that coil 11 b is parallel to the plane tangent to the earth's surface and aligned 90 degrees from its alignment during the first set of measurements, or in a generally east-west direction. The value of these measurements is combined to a single number using vector algebra. The vertical angle, or tipper, related to that number can then be interpreted to predict the subsurface resistivity of the measurement site using procedures well known from the more complex electromagnetic methods such as the magnetotelluric method.The information obtained using this apparatus can be particularly useful in mapping the location of chimneys or alterations of subsurface rock from hydrocarbon leakage, which chimneys and alterations may indicate the presence of hydrocarbons in the subsurface.

It will be apparent that various changes may be made in the details of construction from those shown in the attached drawings and discussed in conjunction therewith without departing from the spirit and scope of this invention as defined in the appended claims. For example, the transfer function of the amplifier circuits might be selected to have a narrower band width. Also, the amplifier/active diode circuitry could be constructed using digital components. The signals from the tuned coils would be passed through analog-to-digital converters before being fed to the digital amplifier/active diode circuits. The outputs of such circuits would be stored using digital storage devices rather than capacitors.

It is therefore to be understood that this invention is not to be limited to the specific details shown and described.

Claims (17)

1. A portable measurement device suitable for measuring the angle which the total electromagnetic field vector makes with the horizontal plane tangent to the surface of the earth at the point of measurement, comprising: first and second detecting coils for producing signals in response to the earth's electromagnetic field, each of said coils being wound with a conductive wire, each coil having two ends, said coils being positioned at right angles with respect to each other so that one end of said first coil is adjacent to the midsection of said second coil; and means for tuning said first and second coils to a peak sensitivity at a frequency of about 5 Hz; whereby said device may be positioned so that one of said coils is generally parallel to the horizontal plane tangent to the earth's surface, the other of said coils being perpendicular to said plane, in which such orientation the device measures components of the natural electromagnetic field of the earth over a certain frequency range, which measurements may be used in determining the tipper (as hereinbe fore defined).

2. A device of claim 1 further comprising two electronic amplifiers, each adapted to amplify the signal of one of said coils.

3. A device according to either of claims 1 and 2 which includes means for converting said signals to a series of dc pulses; and means for storing said dc pulses.

4. A portable measurement device suitable for measuring magnetic fields comprising: first and second generally cylindrical detecting coils for generating signals in response to the earth's electromagnetic field, each coil having two ends, said coils being mounted at right angles to each other so that the end of the first of said coils is proximate to the longitudinal centre of the second of said coils; and means for tuning said coils to a peak sensitivity at a frequency of about 5 Hz; two electronic amplifiers, each adapted to amplify the signal of one of said coils; means for converting said signals to a series of dc pulses; and means for storing said dc pulses; whereby said apparatus may be positioned so that one of said coils is generally parallel to the horizontal plane tangent to the earth's surface, the other of said coils being perpendicular to said plane, in which such orientation the apparatus measures components of the natural electromagnetic field of the earth over a certain frequency range, which measurements may be used to determine the angle which the total electromagnetic field vector makes with said horizontal plane at the location of the measurement.

5. A device according to any one of claims 2 to 4 wherein the components of said amplifiers are selected so that each amplifier has a transferfunction such that the portion of said signal below 10 Hz is fully amplified, and the portion of said signal above 30 Hz is attenuated.

6. A device according to any one of claims 3 to 5 wherein said means for converting said signals to a series of dc pulses comprises diodes.

7. A device according to any one of claims 3 to 6 wherein said means for storing said dc pulses comprises capacitors.

8. A device according to claim 7 which includes a voltmeter for indicating the voltage across said capacitors.

9. A device according to any one of the preceding claims wherein each of said detecting coils is wound with 3200 turns of No. 22 copper wire on a core.

10. A device according to claim 9 wherein said core comprises a 2-1/2 inch diameter solid element of a material having a magnetic susceptibility of 800 or greater, whereby each of said coils is sensitive to changes in the earth's electromagnetic field of 0.01 gammas at a frequency of 8 Hz.

11. A device according to claim 10 wherein the solid element comprises nonmetallic form, said form being filled with twenty-four 1/2 inch diameter rods of a material having the magnetic susceptibility of 800 or greater.

12. A device according to claim 11 wherein said material having a magnetic susceptibility of 800 or greater is ferrite.

13. A device according to any one of the preceding claims wherein said means for tuning said coils comprises at least one capacitor connected in parallel with each of said coils.

14. A device according to any one of claims 2 to 13 which includes a power supply for said amplifiers.

15. A device according to any one of claims 2 to 14 which includes nonmagnetic enclosures for said coils, tuning means, amplifiers, conversion means, and storage means.

16. A portable measurement apparatus suitable for measuring values of the earth's electromagnetic field, which values may be used to determine the angle which the total electromagnetic field vector makes with the horizontal plane tangent to the surface of the earth at the measurement location, said apparatus comprising:: first and second generally cylindrical detecting coils for generating signals in response to the earth's electromagnetic field, each coil having two ends, said coils being mounted at right angles to each other so that the end of the first of said coils is proximate to the longitudinal center of the second of said coils; at least one capacitor connected in parallel with each of said coils so as to tune said coils to a peak sensitivity at a frequency of about 5 Hz; two electronic amplifiers, each adapted to amplify the signal of one of said coils, said amplifiers having components selected so that each amplifier has a transfer function such that the portion of said signal below 10 Hz is fully amplified, and the portion of said signal above 30 Hz is attenuated; diodes for converting said signals to a series of dc pulses; capacitors for storing said dc pulses; and a voltmeter for indicating the voltage across said capacitors.

17. An apparatus according to claim 16 wherein each of said coils is wound with 3200 turns of No. 22 copper wire on a 2-1/2 inch diameter nonmetallic form, said form being filled with twenty-four 1/2-inch diameter rods of a material having a magnetic susceptibility of 800 or greater, whereby said coils are sensitive to changes in the earth's electromagnetic field of 0.01 gammas at a frequency of 8 Hz.