L.I.G.S. : Airborne Gravimetry

Home ] Up ]

Brief History of the Gravimetry Project
    Gravity Survey Hardware
        Techniques

            Data Processing
                Testing  Results

The gravity prospecting (or simply gravimetry) is one of the methods of the geophysical prospecting. It is based on the investigation of the field of gravity properties. This field is produced by the Newtonian attraction of the variety mass which combine the selected area of the Earth crust.

A common picture of the gravity dissipation over Earth's surface may be describes by means of the rather simple formulas of so called normal gravity. But as soon as we try to get a more detailed description of the field of gravity in any area of the Earth's surface, we discover the deviations of this normal formula, i.e. gravity anomalies. Those anomalies are mostly explained by the heterogeny distribution of the masses in the upper layers of the Earth. Given the fact that the gravity anomalies depend on the density distribution in the Earth crust, it is possible to apply the the gravimetry for the geology and prospecting needs. The picture of the field of gravity in the investigated area gives the answer on the question: where and how are located the heterogeneity in the Earth's crust which provide the appearance of the gravity anomalies. By this way we can attempt to clarify the geological structures of the area, make the conclusion on the natural mineral presence and figure out the particularity of the geological structures laying.The gravimetry application is particularly successful on the oil field prospecting when the fields are attached to the salt domes. In most of cases the salt domes represent the sort of large geological structure with deep-wisely constant density. Moreover this density considerably differs with respect to surrounding variety. The results of the gravity prospecting of the salt domes are well known worldwide.

However, the gravity prospecting development is restricted by some technical reasons. The gravimetry tool is a very complicated and highly precise device. In order to get the accurate measurement we must provide the best possible conditions for surveying. The gravimeter must be installed on the special tripod. The measurement it self lasts several minutes in order to exclude some random measurement errors. All this reduces the productivity of the prospecting.  Meanwhile some areas on the Earth are still hard to achieve. The traveling expenses may turn senseless the surveying of the area. The airborne gravimetry may solve those problems. It dramatically increases the productivity and gives the easy access in almost any area of Earth if only this area would be within the range of the light airplane. [go top]

 

Brief History of the Gravimetry Project

1992-93

The survey vehicle with equipment

The first project has been concerned with the design and development of the land vehicle inertial navigation complex, based on the modified inertial navigation system. The applied data processed algorithm allowed to achieve the real time positioning accuracy which exceeded the most optimistic expectancies for the standalone INS.
1993

The base station in Kananskis country

The President of the World Association of the Geodesy Dr. K.P.Shwarz has suggested to Dr. Salychev to begin the joint gravimetry test in Calgary, Canada. The primary tests were carried out on the truck. The tests results has confirmed that the designed hardware is applicable for the gravity measurement. More over some promising results were obtained for the deflection of the vertical determination tests.
1994-1995

Beech King Air C-90

The first flight test of the survey system took place March, 8, 1993 over the Rocky Mountains in the Kananskis area on the King Air C-90 airplane. The further series of flights had a goal to prove the system accuracy, repeatability and reliability. The tests over Lesser Slave lake (Alberta, Canada) were carried out with the Cessna 310 aircraft owned by Canagrad Surveys Ltd. Basing on the obtained results the data processing procedures were considerably modified. That allowed to claim that the achieved accuracy is close to submilligal level. The positive replay of Dr. J.Brozeena from US Navy Lab confirmed that the initial stage of the gravity survey was successfully terminated.
1996

Cessna 310and flight tests participants

The challenge of the December 1996 test series was to achieve the full scale map. The oil fields in the Turner Valley looked attractive to start with because this area is well explored and the pretty dense net of the gravity ground point exists and is available. Given the first map received using our system, the next data processing modification has been done.

herald.GIF (71870 bytes)1997

The very encouraging results obtained during the Turner Valley flight tests pushed the K2 Energy Corp. to hire the gravity system for the gravity survey in the Black Feet Indian reserve in northern Montana. The 600 flown line-kilometers produced a map which allowed to K2 make a deduction on the oil exploration perspective in the area. Moreover Canagrad Surveys carried out a simultaneous magnetic survey. The tallying of both gravity and magnetic maps obtained by airborne survey and seismic data as well was astonishing and apparently for the first time in the world.

4peoples.JPG (42162 bytes)
1999.
From left to right :  Vladimir Voronov, Joe Jarvis (Nevada's top pilot), Bruno Nillsen , Oleg Salychev
In March 1999  the  new airborne gravimetry test was carried out in Nevada, USA. The Cessna 206 single engine airplane was used as a carrier. The detailed report of this flight session is coming.

 

Gravity Survey Hardware [go top]

The hardware equipment consists in the inertial survey system. The basic system applied for the project realization is a Russian inertial navigation system I-21, which originally is designed to be installed in the heavy aircraft. The I-21 includes highly precise sensors which are installed in a compact gyroplatform. It implements a three-axis-four-gimbals platform equipped by two floated gyroscopes and three floated accelerometers. The I-21 is equipped by two built-in high-speed processors for the navigation solution. The digital output of the INS data is accepted by a standard PC/AT computer and provides the navigation parameters: two coordinates, two velocities, four angles (pitch, roll, heading, gyro yaw), and INS time as well. However, in order to apply the navigation system for surveying the original hardware was considerably modified in part of the electronics and interface. The ISS equipment set is combined by the following modules:

  •  
  •  
  •  
  • inertial unit;
  • power supply unit;
  • indication and control units;
  • block of the gravimeter electronics;
  • set of cables and wires.

The total weight of the equipment set does not exceed 30 kg. The power consumption in warm up mode (10 min) is less then 1,5 kW, in regular navigation regime less then 0,75 kW.
The PC/INS data communication is scheduled by means of special signals, which can be accepted directly into the ISA bus and interpreted as a hardware interruption (10 PPS). The form and edge of this signal are identical to those of the GPS 1PPS, that is used for precise INS/GPS data synchronization.
In order to be applied for the gravimetry purpose a highly sensitive gravimeter-accelerometer has been designed, manufactured and installed on the INS gyro stabilized platform. This unit is completely independent of any other INS hardware in terms of its power and data output. The available gravimeter output rate is 1 Hz. Data are accepted by PC through serial port.
The dimensional and power features of the ISS are enough to fit it into the light airplane like Cessna 310. During the tests in the 1994-1997 the ISS had proved its reliability. We never had any ISS caused failure on several thousand flown kilometers. In combination with perfect data processing algorithm the system is able to produce almost 100% of useful data (at the reasonable flight conditions). However, the system is rather tolerant with respect to the weather.


Techniques
[go top]

Flight traverse has a shape of a grid formed by the perpendicular lines. As a rule the line direction is selected proceeding from the expected gravity anomalies increment. For example, if the area has the ranges of hills, the lines must be perpendicular to the ranges. According with our experience the line spacing of 1 km is enough to obtain a gravity map. Moreover it is necessary to make tie-lines for leveling. The tie-lines direction is perpendicular to the lines and spacing is 2 km. Hence the survey of the area as wide as 60* 40 km consists in the (40:1+1)=41 lines of 60 km length and (60:2+1)=31 tie-lines of 40 km, i.e. 41*60+31*40=3700 line-kilometers. Would the airplane is maneuverable enough to make a turn with 1,5 km radius for start the next line without additional evolution and if the local relief allows to perform such U-turn, than the flight traverse length get more (40*pi*1) + (30 * pi* 2)= 314 km and overall traverse length comes to 4000 km. It can be covered during 16 flight hours with a speed 250 km/hrs.  

Note, than the piloting technique for such a job is a very sophisticated deal. It demands from the pilot special skill and experience. Besides regular plane control operations, watching the air traffic, the pilot must monitor the flight direction, keep constant altitude, pitch and roll angles. By our experience the autopilot is not able to performs those tasks good enough in windy weather especially. This is why, the companies who make the geophysical air survey jobs have the own flight crew intentionally trained for such type of flights. Hence, if the airplane can take the fuel amount enough for 5 hours of flying and time to survey area and back is less than 1,5 hours (i.e. the airstrip is in 200 km away from the survey area), then one flight can cover 21% of the area, and it takes 5 flight to finish the entire work. At the normal weather conditions with rotation of the crew it is possible perform two flights per day and survey the area of 60* 40 km in 3 days.

TESTING RESULT INTERPRETATION. [go top]

Series of test flights carried out on different types of aircraft, over different areas and under the different weather conditions have demonstrated that the accuracy achieved is about 1mGal per 3km or better. These test flights were performed during a period from 1993 to 1997 and covered more than 50.000 flown lines-kilometers. The first flight series consisted in the flying of the repeated line only and had a goal to prove the equipment Image64.gif (8567 bytes)reliability and the quality and repeatability of results . In order to realize above checking, the comparison with known ground upward continued gravity data were applied. The ground data were provided by the University of Calgary. The gravity estimation results for the repeated line are shown by plots above left. Figure above right illustrates the comparison between gravity estimates and known upward continued data.

The next testing step was the grid flying. The assumed result of the airborne gravity survey is the free-air gravity anomalies map of the surveyed area. In the autumn of 1996 Canada celebrated the 50-th anniversary of the oil exploration. The Turner Valley is exactly the place were this happened and the fact that our first grid flight took place over this field in this time looks symbolical. The Turner Valley oil fields is an area with large amount of ground gravity points available. The map built using the upward continued free-air gravity is shown below (right) , while the map based on the system measurements is illustrated by the picture on the left. This is our first airborne gravity survey map. The comparison between these two maps shows the correspondence within 0.6-1mGal per 2-3 km.

The next testing step was the grid flying. The assumed result of the airborne gravity survey is the free-air gravity anomalies map of the surveyed area. In the autumn of 1996 Canada celebrated the 50-th anniversary of the oil exploration. The Turner Valley is exactly the place were this happened and the fact that our first grid flight took place over this field in this time looks symbolical. The Turner Valley oil fields is an area with large amount of ground gravity points available. The map built using the upward continued free-air gravity is shown below (right) , while the map based on the system measurements is illustrated by the picture on the left.

This is our first airborne gravity survey map. The comparison between these two maps shows the correspondence within 0.6-1mGal per 2-3 km.

The further flight series was carried out in northern Montana (1997) over the Black Feet Indian reservation. The dimension of the surveyed area is 80? 60 km. This area was absolutely unprospected in sense of gravity. The gravity survey was conducted simultaneously with magnetic data determination as well as some seismic treks. What is important to emphasize, that the gravity map has been created without prior knowledge of the magnetic and seismic data. We’ve only applied the elevation data in order to obtain the right scale of our measurements. The astonishing correspondence between the gravity, magnetic and seismic data in sense of prospected area geophysical interpretation was achieved. As a result, some block of the reservation was recognized as a perspective place for the oil exploration. This fact was reflected in Canadian media.

The last flight series carried out over Ponoka oil fields in Alberta, Canada. In this testing the information of the ground gravity points was not available. In order to check the measurement accuracy the two independent maps have been produced using the information on lines only and tielines only. The measurements for the lines and tielines were obtained from the independent flights which were flown in the different days. The lines grid spacing is 1 km, while the tielines is 2 km. These two maps are shown on Figure below and. The correspondence between two maps is obvious and hardly can be considered as an occasional.
Image66.gif (30991 bytes)The achieved accuracy is within 0,5...0,8 mGal per 2 km. It is a reasonable way to check of the system accuracy because each of maps was created using absolutely independent data set. Important deduction is a fact that the system accuracy along the single line has sufficiently poorer level that the map due to original leveling procedure which was created by our group.

[go top]