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Long-line Surveys

A long-line survey involves the use of a helicopter and a gravity meter that is operated remotely. The gravity meter is suspended by the helicopter by a support cable in a similar manner as normal cargo. The gravity meter usually requires a a combined external power source and data cable, and this is connected at the top end to a laptop computer and a battery.

Once airborne, the helicopter flies from station to station using a preloaded GPS system. The pilot lowers the gravity meter to the ground gently. The gravity operator stores the position and elevation of the helicopter at the same instant that it touches down. An offset is used to correct for the cable length and GPS antenna position, normally located on the helicopter. Software specific to the gravity meter system allows the operator to level the meter, unlock the beam arrestment screw, move the dial counter, and take a reading within a couple of minutes of touching down. Meanwhile, the helicopter has lowered and backed off to slacken the cable and remove any vibration that would interfere with the measurement. He continues to hover until the operator signals that the measurement is complete. The meter is picked up and they fly to the next station.

The type of project that best describes a long-line survey is one that involves a large project area, a station spacing of 500 meters or more, very rugged or inaccessible terrain, an available helicopter with good lines of support for fuel and maintenance, and slightly coarser expectations of data accuracy. Most of these conditions define a regional survey.

Some conditions where long-line surveys will not work include:

  • heavily treed areas with a full canopy - the meter can not be lowered to the ground and returned safely.
  • no available helicopter - some countries do not allow civilian helicopter operations.
  • the survey requires better than 0.1 mGal accuracies - long-line accuracies are closer to 0.25 mGal combined error analysis with a substantially larger failure rate. That is, some flights have to be reflown due to loop problems.
  • smaller station spacing - ground-based surveys become cheaper as you approach a 100-meter spacing.
  • budget - helicopter time accounts for as much as 90% of the cost of doing a survey
  • pilot fatigue - it is a very difficult job to hover for hours, especially in windy conditions. Additional pilots must be on stand-by because they time-out quickly.
  • high winds, fog, freezing rain conditions - helicopters can not fly in these conditions.

 

Papua, New Guinea

Meter on the landing pad

LaCoste & Romberg Helimeter Rigged for Swamp Work
Sepik River Area, Papua, New Guinea

 

 

LaCoste & Romberg Helimeter Reading for Take-off
Sepik River Area, Papua, New Guinea

Arctic / Winter Conditions

Longline gravity crew, NWT

Long-line Gravity Crew
Northwest Territories, Canada

Scintrex Helimeter

Scintrex Heligrav meter
Northern Alberta, Canada

 

Lacoste & Romberg Helimeter Winter Survey
Northern Alberta, Canada

Africa

Flight checklist, Ethiopia

Flight Checklist Preparation
Ogaden Desert, Ethiopia

Flight underway, Ethiopia

Survey Flight Underway
Ogaden Desert, Ethiopia

 

 

Mountain Regions

LaCoste & Romberg Helimeter with Brush Ring
Northern British Columbia, Canada

 

 
 
 
 
 
 
 

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Last Modified on Mon 10-Oct-2016

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