GeoExplo Ltda.
Airborne Geophysical Survey
Quality Control (QC)

Santiago Chile Back to Home Page
Tel(56-2)326-5116
E-mail: surveys@new-sense.com Dr. W.E.S.(Ted) Urquhart

Geophysical Airborne Survey Quality Control (QC)

Abstract

Compilation and interpretation of airborne magnetic surveys data are dealt with in the article. The Compilation topics such as: Database, Flight Path Plotting, Leveling and Mapping the Total Magnetic Intensity. The paper also looks at various topics in the interpretation of airborne magnetic survey data including: Interpretation, Interpreting Magnetic Data and Quantitative Interpretation

 

Table of Contents

Introduction
QC Mandate
Client Representative
QC Officer Responsibilities
Survey Specifications
Typical Tests Required for a Magnetic Survey

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Introduction

   All airborne geophysical survey data contains errors. These may be the result of the physical limitations of the equipment, survey procedures, environmental realities or human factors. The objective of the QC role is to insure that the usability of the survey result is not impacted by these errors and that the data noise remains below set thresholds. This is done by carefully monitoring the procedures of the contractor, the sensitivity of the survey equipment and platform, maneuver noise, other sources of data noise, natural variations in the measured field and the data processing methodology.

There is nothing to be gained by "punishing" the contractor over issues that at the end of the day have no impact on the results. On the other hand result sensitive issues must be perused with vigor. An experienced QC officer can tell the difference between these two issues to the benefit of both the client and the contractor. As well a senior QC geophysicist can help the contractor find solutions to data compliance problems. All this will result in a quality survey being completed safely on a timely basis, which serves the interest of all.

The relationship between the QC officer and the contactor's personnel is very important. Openness is imperative. If the QC officer is respected for his fairness, level of expertise and experience the survey crew will cooperate with him on sensitive issue. This kind of respect can only be gained as a result of years of experience in all aspects of the airborne survey business. A junior QC person can only enforce the letter of the contract because he does not have the level of experience to judge what is reasonable under the actual survey conditions. This will lead to costly delays, poor moral and often drives the crew into performing unsafe activities while trying to satisfy a QC officer that does not understand.

The client is paying a lot of money for a survey and deserves to get the most out of it. The contractor is trying to do it's best and has a reputation to protect. Thus all parties want a good result. A senior QC officer has the knowledge and the know-how to help the contractor deliver it's best effort on a safe and timely basis to the benefit of the client. There is a lot of specialized work in the QC role and a certain amount of judgment required to ensure that the client's expectations and the contractor's abilities meet. The better this job is done the more value the client will get from his investment in the survey.

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QC Mandate

   To insure that the client receives the best possible quality survey data set on a timely basis without jeopardizing the safety of the survey crew.
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Client Representative

   In most cases the QC officer is acting as the only on site client representative, as such he must act in a responsible way protecting both the interests and reputation of the client.
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QC Officer Responsibilities

  
  • Prepare Bid Documents in cooperation with client (optional)
  • Participate in the contractor selection process (optional)
  • Review contract specifications and survey objectives with the client
  • At startup (two or three days)
    • Review and modify (if necessary) Job Safety Plan (JSP) in cooperation with the client and contractor.
    • Review and modify (if necessary) Emergency Response Plan (ERP) in cooperation with the client and contractor.
    • Review and modify (if necessary) Search and Rescue Plan (SAR) in cooperation with the client and contractor.
    • Inspect the site for safety compliance
    • Review safety risks specific to the project and confirm that the contractor has taken remedial action
    • Participate in Safety meetings
    • Verify that all required insurance policies are in place and up to date
    • Verify that all necessary permissions and licenses are in place
    • Verify that the survey crew have the experience and skills to perform their tasks
    • Verify that the pilots and aircraft engineer have appropriate training and related licenses and permits
    • Participate in project orientation meeting
    • Set up reporting procedures and documentation
    • Verify that the survey flight plan is correct and in the right place
    • Review the planed flight height obstacle avoidance
    • If a planned variable flight surface is required verify that it is appropriate for the performance characteristics of the aircraft
    • Inspect aircraft and geophysical equipment for compliance with contract specifications
    • Verify that the installation of the geophysical equipments complies with civil aviation requirements
    • Inspect ground geophysical equipment for compliance with contract specifications
    • Inspect base station location, setup and compliance with data quality specifications
    • Review the contactors procedures for data security and backup
    • Prepare and deliver "Start Up" report
  • Aircraft tests and calibrations (a few days depending on weather conditions and contractor readiness)
    • In cooperation with the contractor select location for test line and maneuver tests
    • Review results of:
      • Heading error
      • Lag test
      • Maneuver tests (FOM)
      • Altimeter calibration
      • Other instraments test and calibrations
    • Request the necessary improvements should the results of any of the test fail to meet specifications and review new test results
    • Release aircraft for first production flight
  • After first production flight (a few days depending on weather conditions)
    • Inspect data for compliance with contract
    • Review any problems with the contactor for mitigation
    • Release aircraft for general survey
  • On site daily routine (up to two weeks depending on production)
    • Participate in job safety meetings and continue monitoring compliance with the JSP
    • Inspect ground monitor data for compliance
    • Inspect survey data compliance for:
      • data noise
      • flight path deviation
      • flight line separation
      • sample spacing
      • missing data
      • control line closure
      • diurnal drift
      • flight film quality and completeness
      • accuracy of flight logs
      • Approve data collected on a timely basis
      • Flag and inform the contractor and client of any non-compliant data for reflight on a timely basis
      • Review contractor progress report for accuracy
      • If modifications to the survey platform or equipment are made then new tests must be performed and the results verified for compliance.
      • Review the qualifications of any replacement crew
      • Review the contract compliance of any replacement equipment
      • Prepare and deliver QC report on a schedule agreed with the client
  • Remote monitoring (duration of survey)
    • After the survey is up and going and a routine is set there is no need for the QC officer to be on site. The data can be monitored remotely over the internet or through regular dispatches of data. The tasks will be the same as above except that the JSP will be monitored through the review of the safety meeting minutes. This is the most cost effective way of insuring quality.
  • Closure of survey (on site for about a week)
    • Make final inspection of data and reflight data
    • Inspect the coverage of the survey for compliance
    • Inspect the delivery media for the field data for readability
    • Inspect the delivery data for completeness and errors
    • Prepare survey QC report
    • Consult with the client for release of aircraft
    • Release aircraft
  • Processing
    • Review (and modify if necessary) processing procedures with contractor
    • Inspect data for loss of fidelity
    • Inspect the leveling results
    • Inspect the GPS special corrected data
    • Inspect results of cultural effect removal (if any)
    • Inspect data for processing errors
    • Inspect the final deliverables for compliance
    • Inspects the final delivery media for readability
    • Inspect the delivery data for completeness and errors
    • Recommend remedial action to the contractor
  • Closure
    • Review the contactors logistical report for completeness and accuracy
    • Review the contractors final report for completeness and accuracy
    • Prepare final QC report
    • Visit with client for debriefing if requested
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Survey Specifications

   Survey specifications vary depending on the equipment used. The following specifications are included only to give the reader an idea of what specifications might look like. The specific technical specifications for a particular projects are best drafted by the QC officer in consultation with the client to deliver a survey fit for the client's particular exploration needs.
 
    

1 Traverse Line Separation

     Line separation should not exceed 1.2 times and be not less than 0.8 times the nominal line spacing for distances greater than 10 times the line spacing along the flight path.
 
    

2 Control (tie) Line Spacing

     Control lines will be surveyed at an average interval as specified but may be located to avoid, where possible, areas of strong magnetic gradient.
 
    

3 Boundary Lines

     The boundary of the survey area will be flown to facilitate leveling of the magnetic data. The planned flight paths for all flight lines will be agreed in principle with the Client.
 
    

4 Flight Height

     The aircraft will maintain a mean loose drape with a terrain clearance of 80-120 m where feasible depending on local topography and pilot judgment with respect to safety considerations. Flight altitude will not exceed the stated operating windows over distances of more than 5 km along the flight lines accept where the pilot in his sole judgment believes that doing otherwise would put the crew at risk.
 
    

5 Magnetic Diurnal

     Data acquired when the Earth's magnetic diurnal activity exceeds 2.5 nT when measured from a linear chord of 3 minutes will be subject to reflight. No flight will commence within one half hour of a diurnal occurrence that exceeds the above limits.
 
    

6 Missing or Substandard Data

     Data will be recorded digitally in the aircraft and by the ground station. Isolated errors, spikes and short non-sequential gaps consisting of a few points will be corrected by interpolation, up to a maximum of .05% for any given flight line, providing no similar data gaps exist on nearby sections of adjacent flight lines.
 
    

7 Re-flights

     Any flight lines or parts of flight lines with data outside the above tolerances will be reflown at the contractor's expense. All reflown lines or portions of lines will be tied to the closest control lines at both ends.
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Typical Tests Required for a Magnetic Survey

The specific test and calibrations that would be required vary for each survey equipment package. The following list is presented here only to give the reader an idea of what might be expected. The accrual set of tests and calibrations are best designed by the QC officer and the client as well as incorporating the recommendations of the contractors that are being considered for the project.
 

  Magnetometer.

 

1.1 Heading Effect

The magnetic heading effect will be determined by flying a cloverleaf pattern oriented in the same directions as the survey lines and control lines. One pass in each direction must be flown over a recognizable feature on the ground in order to estimate the heading effect. Heading effect will be determined before the survey and after any modification or additions to the aircraft or the equipment installed in it have been made.
 

1.2 Maneuver Noise

The Contractor will determine the effects of aircraft roll, pitch and yaw. These tests will be carried out at high altitude over an area of low magnetic gradient in or near the survey area by carrying out rolls, pitches, and yaw maneuvers, each with a period of about 6 seconds on the 4 headings aligned with the direction of the flight lines and control lines. A compensation Figure-of-Merit (FOM) for the aircraft will be calculated by a RMS sum of the peak to peak amplitudes of these twelve magnetic signatures. The FOM will be determined before the survey and after any modification or additions to the aircraft or the equipment installed in it have been made. A RMS sum of 0.7 nT or less will be considered acceptable.
 

1.3 Lag Tests

The Contractor will perform lag tests to ascertain the time difference between the magnetometer readings and the operation of the positioning devices. Test flights should be flown in two directions at survey altitude across distinct anomalies. Lag tests will be carried but before commencement of survey production and after any major survey equipment alteration or replacement has been carried out.
 

  Altimeter

Checks of the altimeter calibration will be undertaken at appropriate intervals above the base air strip or some other suitable location with know elevation and flat terrain.
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