WO2014169318A1

WO2014169318A1 - Reducing the drift rate of airborne gravity measurement systems

Info

Publication number: WO2014169318A1
Application number: PCT/AU2014/000194
Authority: WO
Inventors: Roger Duplessis WRIGHT; Richard Stephen WRIGHT
Original assignee: Nrg Exploration Cc; Savage, Paul Alan
Priority date: 2013-04-15
Filing date: 2014-03-01
Publication date: 2014-10-23
Also published as: AU2014253659A1; CA2883020C; CA2883020A1; AU2014253659B2; ZA201501276B

Abstract

A temperature control device for use with airborne measurement in geological and geophysical surveys. The device includes a thermally insulated housing for mounting into an airborne vehicle and within which electronic survey equipment is located in use, and temperature control means for maintaining the temperature within the housing at a selected value. Preferably the housing has control apparatus mounted thereto for selecting the temperature within the housing. The temperature control means may be capable of raising and/or lowering the temperature within the housing relative to ambient temperature. Preferably the temperature within the housing is maintained at a value of between 10°C and 25°C, at the option of the operator thereof. Typically the device may house a gravity meter, for example. The invention extends to a method of obtaining geophysical data using measurement equipment housed in a temperature controlled housing.

Description

REDUCING THE DRIFT RATE OF AIRBORNE GRAVITY MEASUREMENT SYSTEMS

Introduction

This invention relates to apparatus for improving the accuracy of airborne measurement equipment, in particular equipment used for measuring terrestrial physical parameters for the purpose of geological and geophysical surveys.

Background

The accuracy of airborne electronic equipment such as accelerometers is important if reliable data is to be obtained during measurement. Airborne surveys are preferable to land based surveys in many locations, particularly in rough terrain, or terrain not well serviced by roads and other terrestrial transport facilities. Also, where it is desired to survey a large geographical area, the advantages of an airborne survey are immediately apparent in that the area can be surveyed relatively rapidly, as compared with on the ground surveys.

However, any survey needs to have an acceptable level of accuracy in order to be useful. Airborne surveys, whilst being advantageous for the reasons outlined above, are complex to undertake and yet still achieve an acceptable level of accuracy because the platform (in other words the aircraft) on which the measurement equipment is mounted is not stable. The aircraft is in continuous motion, accelerates and decelerates, changes altitude, and so forth. Also, ambient conditions such as wind, temperature and moisture vary as the aircraft moves over the terrain being surveyed. A typical survey might, for example, last a number of hours, and the temperature variation from the start of a flight to the end of the flight might vary significantly, particularly in inhospitable areas where many such surveys are conducted.

When ambient atmospheric conditions in which an aircraft is operating vary over the course of a day, or from day to day, the measurements obtained by the equipment on board that aircraft may drift, that is, two measurements taken at the same location, but spaced apart in time, will vary. Measurement drift is an acknowledged problem, and there have been various suggested solutions to counteract the problem. For example, US Patent Application No 2008/0092653 deals with reducing or eliminating the effect of water vapour on relative gravimeters.

Drift is a complex problem to deal with in that drift is often not linear where a particular ambient parameter has changed over the course of time. It is also not particularly easy to determine whether or not drift has in fact taken place, or the degree of drift that has occurred. In many survey situations, there is no absolute reference point for the equipment, so whilst the equipment would appear to be operating perfectly, the fact that drift has occurred is not apparent, and neither is the degree of drift. In some surveys the survey team may attempt to estimate drift by flying, at the start of a survey session, over a particular location on the ground, take measurements, and then at the end of the survey fly over the same location, take the same measurements, and then the difference will be the drift. However, this method of correcting for drift is only really applicable for short duration surveys, say 30 minutes or less. That is not satisfactory when a survey is due to last for a much longer period, say 5 hours. Also, as mentioned above, drift is not necessarily linear, and therefore the scaled correction may in fact alter some measurements much more than required, and alter others far less than required.

It is an object of the present invention to provide means for improving the accuracy of airborne survey equipment.

Summary of the Invention

According to the invention there is provided a temperature control device comprising: a thermally insulated housing for mounting to an airborne vehicle and within which electronic survey equipment is located in use, and temperature control means for maintaining the temperature within the housing at a selected value.

Preferably the housing has control apparatus mounted thereto for selecting the temperature within the housing. The temperature control means may be capable of raising and/or lowering the temperature within the housing relative to ambient temperature. Preferably the temperature within the housing is maintained at a value of between 10°C and 25"C, at the option of the operator thereof.

The electronic survey equipment may be a gravity meter, and the airborne vehicle may be a fixed wing aircraft or a helicopter.

These and further features of the invention will be apparent from the description of a preferred embodiment thereof, given below by way of example. In the description reference is made to the accompanying drawings, but the specific features shown in the drawings should not be construed as limiting on the ambit of the invention.

Brief Description of the Drawings

Fig 1 shows a schematic illustration of a fixed wing aircraft used for geological survey which is fitted with a temperature control device according to the invention;

Fig 2 shows a cross-sectional side view through a temperature control device according to the invention;

Fig 3 shows a perspective view of a gravity meter of the type to be fitted within the temperature control device;

Fig 4 is a graph showing the difference between the pre and post flight readings; and Figs 5 to 7 show graphs indicating the improved results obtained by keeping the equipment at constant temperature within a temperature controlled cabinet according to the invention.

Detailed Description of a Preferred Embodiment

As shown in Figure 1, a fixed wing aircraft 10 is depicted which has sensing equipment 12 on board. Typically, a fixed wing aircraft or helicopter carrying out geological surveys will have a range of different sensing equipment on board so multiple aspects of the terrain over which the aircraft is traversing can be surveyed simultaneously. One of the items of survey equipment is a gravity meter 16 which is carried on board the helicopter within a temperature control cabinet 18, diagrammatically shown in Figure 1, but shown in more detail in Figure 2.

As shown in Figure 2, the temperature control cabinet 18 has an internal volume of approximately 100 to 150 litres. The cabinet 18 is essentially a sealed unit formed of thermally insulated walls 20, which may conveniently be formed of inner and outer skins comprised of Kevlar, or similar high strength materials having a suitable high quality insulation material 21 sandwiched between them. The gravity meter 16 is shown in Figure 2 by dotted lines, spaced away from the walls 20.

The cabinet 18 includes an inspection port 22 which is openable to provide access to the gravity meter 16. The cabinet is mounted on an aluminium base 24 which in use is mounted to the floor of the aircraft 10.

Refrigeration is provided to the interior of the cabinet by means of a refrigeration unit 26. The refrigeration unit 26 comprises a compressor unit 28, a condenser 30, and an evaporator 32. The refrigeration unit is essentially mounted to the outer wall of the cabinet 18, and cooling air is pumped into the interior of the cabinet by means of fans 33 through ports 34. The compressor and condenser selected for the operational unit was a WAECO Cold Machine 94 unit, coupled to a WAECO VD-16 evaporator unit. The power inputs required for these two units are approximately 60 watts respectively. The Cold Machine 94 unit comprises a fully hermetic high-performance compressor with AEO electronics and integrated low-voltage protection, low-voltage cut-off, a brushless DC fan, fin condenser, and self-sealing valve couplings. These units are well capable of maintaining the temperature of a 150 litre insulated cabinet at the temperature required for operation of the invention.

Temperature sensors 36 monitor the interior temperature of the cabinet, and are used to control the refrigeration unit to ensure the temperature of the interior of the cabinet is kept at a selected value. A temperature selection switch 38 is used by the operator to select the temperature at which the interior of the cabinet is to be maintained. That temperature will typically be about 15°C. The temperature in the cabinet can be read from a temperature gauge 37.

The refrigeration unit will typically be powered, either directly or indirectly, using the aircraft power supply system.

The invention is not limited to use with gravity meters, or gravity meters 16 of the type shown in Figure 3. The gravity meter shown in Figure 3 is illustrative of the type of equipment that could benefit from the invention, and the following description illustrates the improvement in survey results that use of the invention is able to achieve.

Traditionally the drift allowed in a gravity survey or survey specification is less than 0.5mGal per hour. This value was recognised as typical because this is the sort of drift one can realistically expect from most of the gravity systems, although some airborne gravity manufactures quote more accurate results, these results would have been obtained under ideal conditions, that is, constant temperature and movement conditions in a laboratory. The GT systems of the type illustrated in Figure 3 have four thermally insulated chambers nested one within the other. The ambient temperature however seeps through the four chambers affecting the various components. Each component behaves differently as the temperature drifts. For example the gyro required to maintain platform stability has a certain drift with temperature change, the accelerometers used to measure the platform accelerations drift with temperature change, and obviously the most sensitive unit, the actual gravity sensor drifts with temperature change.

In practice, if drift exceeds a predefined specification then the complete flight will be rejected. However, it can be difficult to even estimate whether drift has occurred, and if so, what the extent of that drift has been.

The drift may theoretically be calculated by doing a pre-run data collection flight (20 minutes) and then a post run data collection flight (20 minutes) with the aircraft on an identical spot (marked out on the ground with tape or paint). The drift is then applied in a linear fashion over the complete flight. As mentioned previously, this drift compensation is really only accurate for short flights 30 minutes to 1 hour which are not cost effective. Also the drift is not linear in reality and over longer flights up to 5 hours, although the total estimated drift at the end of the flight could be with in 0.5mGal per hour limit, it is the variable drift due to ambient temperature change that is the problem. There are other means of trying to estimate this drift, such as looking at cross-over intersections between traverse and control lines (tie lines) in conjunction with the linear drift. This method is also unsatisfactory because there could be noise due to other external forces such as turbulence or flight altitude or direction change. Figure 4 is a graph that shows the difference between the pre and post flight readings for seven flights with the refrigeration cabinet 16 installed and seven flights without the cabinet 16 installed, the sample flights chosen was with similar ambient temperature fluctuations. The graph depicts the total drift for a flight. The results with the cabinet installed are far more constant and predictable than those without the cabinet installed. As described above it is very difficult to define or estimate the total drift on a system, with the refrigerated cabinet installed the drift is far more constant and definable.

All current airborne gravimeters rely on a level platform to which the gravity measuring device is attached. The system normally consists of a three axis gyro stabilized inertia! platform the gravity sensing unit is held level within 10 arc seconds by a "Schuler Tuned Inertial Platform" it makes use of accelerometers and gyroscopes and a number of complex feedback loops.

A maximum angle in the X axis or Y axis of 4.5 arc minutes or 0.075° will result in an error reading of approximately 1 mGal. In real time while on line and collecting data errors are as high as 1 to 2 Arc minutes but are normally post processed out to the required accuracy of 10 arc seconds (0.002777777778°).

As the accuracy of the platform depends on various sub-systems of the Schuler tuned platform system as shown in Figure 3

1. Torque Motors X and Y TM x, y axis

2. DTG Dynamically Tuned Gyro

3. ASx,y,z Angular sensor X, Y, Z axis

4. AC x, y, z Horizontal Accelerometers

5. FOG Fibre Optic Gyro z axis

All of these different items of equipment are in turn affected by temperature changes. If all items are maintained at a constant temperature the gravimeter will operate to a far greater degree of accuracy.

The inner most chamber houses the gravity sensing unit (GSU) see Figure 3. The GSU is in a hermetically sealed container thus it cannot dissipate heat easily and is most affected by temperature changes (12mGal/Degree C) thus the ambient temperature play a large role on the stability of the GSU. The inner most heating chamber is kept at a constant temperature of 60°C.

Prior to post flight processing the data interpreter will typically use a flag of 2 arc minutes or 0.0333334684° to determine the accuracy of the platform during flight.

After post flight processing the data interpreter will typically set a flag at 5 arc seconds or 0.0013888889° as a pass indicator or an indicator of good stable platform and thus good gravity data. The applicant extracted the results of the flag in the X and Y axis prior to post processing on a line by line basis for 52 lines from two projects, one with the refrigerated cabinet installed and one without the refrigerated cabinet installed. The mean of the standard deviations down the line as well as the mean of all 52 lines in the X and Y axes were plotted against each other to show the improvement to the misalignment of the platform when the refrigerated cabinet was installed. The improvement was approximately 22% across the board.

Figure 7 shows the effects of ambient temperature variations on the GSU. The profile above shows the change in ambient temperature (top curve, Celsius), cause a very small change in temperature in the inner GSU chamber (middle curve, Celsius) this small change of temperature cause significant change in the output reading in mGal of the Gravity Meter (lower curve).

The GT system operating range is between -10 to +40 degree's. Surveys are often carried out in areas where the ambient is above 40 degree (ambient in the present case is the cabin of the aircraft, and this heats up much faster than the outside air temperature).

The graphs shown in Figures 6 to 8 clearly show the improved results obtained by maintaining the equipment in a refrigerated temperature controlled environment within the aircraft.

Clearly the cabinet does not need to be in the form indicated in the drawings, and the refrigeration unit could be completely separate from the cabinet, with cooled air piped into the cabinet, for example.

Claims

An airborne measurement device comprising: a thermally insulated housing for mounting to an airborne vehicle and having an internal chamber specifically adapted to receive electronic survey equipment therein in use, and temperature control means for maintaining the temperature within the housing at a selected value.

An airborne measurement device according to claim 1 wherein the housing has control apparatus mounted thereto for selectively varying the temperature within the housing. An airborne measurement device according to claim 2 wherein the temperature control apparatus is capable of raising and/or lowering the temperature within the housing relative to ambient temperature.

An airborne measurement device according to claim 3 wherein the temperature within the housing is capable of being maintained at a value of between 10°C and 25°C, at the option of the operator thereof.

An airborne measurement device according to any preceding claim wherein the internal chamber is adapted to receive a gravity meter.

A method of conducting airborne geophysical measurements including the steps of:

• Mounting measurement apparatus in a temperature controllable housing in an aircraft adapted to be used for conducting said geophysical measurements;

• Maintaining the temperature within the housing at a selected value;

• Conducting a series of airborne measurements using said aircraft and said measurement apparatus whilst maintaining the temperature within the housing at said selected value; and

• Downloading data from said measurement apparatus and analyzing said data.