The first two articles of this series on potential field (PF) geophysics dealt with recent technological improvements in gravity and magnetics survey systems. This article addresses issues from a project management perspective where questions such as Why are we doing this? What are we going to get out of it? and, How is it done? are put forth and answered. Realistic project management and planning is crucial, specifically because the scope of PF projects is generally larger than most seismic surveys, and more integration of disciplines is required.
Steps in the Project
PF surveys are conducted for much the same reasons as are conventional seismic surveys: to provide an answer to a problem. The steps involved in the average PF project are:
- determine what results can be obtained from a PF survey
- conduct a feasibility/modelling study to confirm expected results
- develop a description of the work to be done
- establish a cost estimate of the work
- obtain permission from management for the work
- cut an AFE for the project
- define the purpose of the survey and the expected results
- set up an organization to do the work
- design the survey and write the specifications
- send bid out to contractors and evaluate the returns
- reconfirm AFE expenditures with management
- award the contract
- QC the acquisition and processing
- interpret the data
- synthesize the data with the geologists, seismic interpreters and engineers
- make recommendations for the next step in the exploration program
The following is a discussion of some of the foregoing aspects.
Magnetic and gravity surveys can be carried out at any time of the year, as they are either not at all or, little dependent on surface conditions. Both methods are completely non-invasive. The environmental impact of both methods is negligible to non-existent.
The traditional role of PF is seen as providing very cost effective regional tectonic information. As such, it is successfully and widely used in international exploration. Somewhat surprising to many. the approach is also used increasingly in mature basins. This is due to a shift in exploration perspective. Exploration teams now want to obtain comprehensive information, which encompasses regional tectonics, to make better informed exploration decisions.
Depending on the geological conditions, either magnetics or gravity, or both, can provide for an effective and inexpensive way of high grading exploration leases. This approach is used frequently on large international blocks where timely land relinquishment is of essence. This method of high grading can also be applied to available crown land thereby optimizing the land position of a company at a cost which pays for itself through much lower lease prices.
Specific exploration applications such as direct reef or reef trend detection in favourable geological settings provide for another very cost effective exploration approach.
The Feasibility Study
The range of geological sub-surface variations which can be resolved by means of gravity or magnetics surveys is quite well defined by physics. Recent technical advances in acquisition technology as well as major improvements in data processing and analytical capabilities allow for major improvements in resolution and accuracy compared to applications just a few years ago.
A wealth of intra-sedimentary geological information can now be extracted from high quality magnetic and gravity surveys, which could not be resolved previously, either because of data acquisition deficiencies or the lack of analytical tools. There are clear cut limits as to which geological events can be resolved with the respective methods. The limits of the methods have to be appreciated in the choice and recommendation of a particular method. If there is any doubt about resolving power, it is highly recommended to model the probable geophysical response. The exploration team may want to engage a PF specialist in the course of the project scoping to carry out a short feasibility study involving modelling with known and assumed geological parameters pertinent to the problem at hand.
It is further advisable to conduct a test project in marginal cases before proceeding with a large scale project.
Cost of PF Surveys
Aeromagnetic data acquisition rates vary a great deal depending on where and when a survey is to be carried out. Given reasonable diurnal conditions and weather, an average 3,000 line kilometers per week is quite feasible. A marked production decrease is to be expected for surveys in the amoral and sub-auroral zones and for surveys carried out in the winter months or near major mountain regions.
Typical costs for a magnetic survey of 10.000 line km or more are about $11.50 per km for data acquisition and $4.50 $ 6.00/km for processing, mapping, complete data analysis and interpretation.
The cost range for gravity surveys varies between approximately $50.00 and $170.00 per station (including processing and interpretation), depending on survey conditions and requirements.
PF surveys for the most part serve as first line exploration steps. To maximize the benefits, such projects should be planned well ahead of any costly seismic program. Scheduling is even more important when it comes to larger scale projects, to ensure a timely delivery of final project results. Partial final processing and interpretation of a survey still in progress is generally not recommended, as it only adds on cost and can be avoided by proper scheduling.
Good contractors carry out basic data processing at the survey operations base as the survey progresses and the processed field data set should be available within a few days of survey completion. The time frame required to generate the final processed data set depends on the overall size of the survey and the time required to remove cultural interference encountered in the survey. As a rule of thumb, the final processed data set should be ready within approximately two to three weeks of the completion of a 10,000 line km survey. Larger surveys will take somewhat longer to process, but not necessarily proportionally longer.
The expected estimated time for data analysis and interpretation depends a great deal on the requirements specified and the products desired. One should allow approximately two to three months total for final processing, analysis and interpretation of a survey with 10,000 to 20,000 line km.
Purpose and Objectives of the Survey
The purpose and objectives for a survey must be clearly stated. Is this a regional reconnaissance to define a basin or a survey to outline pinnacle reef buried at a certain depth. Perhaps the purpose is to determine basement fault throws and strike direction or to determine the depth to magnetic basement. Depths and target size along with anticipated lithologies should always be stated.
The decision to use gravity or magnetics in hydrocarbon exploration needs to be made from the end user perspective. It is therefore necessary that the user (project manager) is either knowledgeable about the methods, or that competent advice is sought. In order to gain the most from the survey, direction and leadership needs to be provided by the end user of the results. The main concern for the project leader should be project planning and the synthesis of the results with other geophysical/geological information. The details of the various project components can be looked after by the exploration team and knowledgeable specialists.
As an example, an exploration manager may need to high grade a lease block in order to bring costs for seismic into budget range. Management then will define the approach to be taken with the exploration team in terms of information to be obtained to achieve the goals. The project scoping will deal with regional, structural and tectonic information requirements, as well as timing. It should be noted that all parameters and specifications pertain to the project as a whole and should therefore be prepared from that perspective. Parameters, specifications and requirements for the individual components, such as data acquisition, processing and interpretation are very important by themselves, but the maximum impact will be achieved by harmonizing them with the desired end result in mind.
Parameters, Specifications and Products
The design should be carried out by project management and PF specialists. Project design is to take into account all available and/or assumed geological parameters to assure that the project goals are being achieved. The person responsible for the preparation of the project parameters and specifications needs to be finely tuned into the requirements of the end user concerning the expected results. He/she also requires a good knowledge of currently available service industry capability and prices as well as expertise in conducting the various phases in an efficient and cost effective manner.
The following is a condensed list of some of parameters for data acquisition, processing, and interpretation. Detailed parameters, of course, depend on the requirements of a particular survey and are beyond the scope of the article.
- line orientation
- line spacing
- tie line frequency
- flight altitude
- allowable diurnal variation
- sampling rate
- maximum mistie for total intensity
- maximum navigation/elevation deviation
- resolution & accuracy of instruments
- survey system compensation
- calibration tests
- restrictions on acquisition condition
- data recording formats
- verification procedures
- data compilation procedures
Processing Parameters – processing procedures
- filter parameters
- editing constraints
- display scales
- scale and format for profiles
- map product formats
Gravity – much the same as above
- base tie network
- loop ties for gravity
- elevation control/accuracy
- inner terrain corrections
- Drift and ET corrections
- field notes
- Nettleton tests
For processing and interpretation, specifications often need to be kept quite general. It is recommended to allow for fine tuning as the work proceeds. However, final deliverables need to be clearly specified to ensure the users requirements for scales and formats are met for hard copy and digital products.
An effective way to focus on parameters and specifications is to list the anticipated products of the project. By defining the results it is quite easy to select the appropriate input parameters. The following is a partial list of the results of a PF project.
Output from the Field
- field tape of the aeromagnetic data
- data verification profile plots
- flight tracking video tapes
- field notes from gravity operator
- calculations for inner terrain corrections
- surveyors notes for positioning and levelling
Output from Processing
- gridded data set
- total field maps
- residual maps
- first and second derivatives
- upward and downward continuation maps
Output from Interpretation
- depth to magnetic basement
- rock distribution/geologic contacts map
- fault lineaments
- geologic cross-sections from models
Output from Synthesis
- correlation with production
- maturation and migration models
- analogies to production
- prospective areas of the exploration block
- exploration strategies
- recommendations to management
Invitation to Tender and Bid Evaluation
Unless there are specific reasons or the project is rather small, it is recommended to go through a normal bid process. Although it requires a serious effort to prepare an invitation to tender, there are several benefits in return. For one, a good contractor often comes up with valuable suggestions which may greatly benefit the project. Bids also provide excellent information about survey systems, processing and analytical software capability, as well as alternatives for the tasks at hand. Last but not least, the cost component is being addressed in an informed manner.
The evaluation of the bids is critical. Too often bids are evaluated on the basis of cost only, without much or any regard for the actual substance of the proposal. If the bid evaluation process is to be meaningful, it has to encompass all information of each proposal received, not just the financial information, to be able to compare quality levels of service. The bottom line of the bid evaluation should not be the lowest price, but the maximum value.
The Acquisition and Processing Contract
To ensure that a project fulfils the users requirements, project parameters and specifications are an inseparable part of a proper contractual document. Most companies have seismic acquisition, processing and interpretation agreements in place, but no contracts for less frequently used PF services. As PF acquisition is a complex undertaking, it is highly recommended to develop a PF contract document before entering into an agreement with a contractor. The contract should address the work, general and specific project requirements, safety, delineation of survey area, survey standards, contract period, product and information delivery, as well as the standard legal elements.
There are a number of excellent data acquisition companies, and Canadian companies rank top in the world. The data acquisition component is naturally the most expensive component of any gravity or magnetics project, and accordingly deserves utmost attention during the actual survey phase itself. For over twenty years I have been puzzled why some companies would have gravity or magnetics data acquisition carried out without ever bothering to see if they get their moneys' worth or if specifications and requirements were met by the contractor. A parallel situation in the seismic data acquisition industry is quite unthinkable. Given the fact that the success of the entire project depends on the quality of data acquisition and basic processing, every user is well advised to have competent QC in the field and during the processing stage of the project.
Most PF surveys are carried out on a turn-key basis, providing for excellent cost control. Processing and interpretation are usually also done at an agreed per km price. Quality control of the processing and interpretation is best accomplished when the end user takes a direct interest in the work progress.
Data Analysis, Interpretation and Project Results
Once the data comes from the field, it is ready for processing, interpretation and synthesis. (Processing here means taking data from the field and generating maps and profiles for use by the PF interpreter.) Aeromag data generally have to have cultural interference signal removed from the field data; otherwise, after re-levelling, it is ready for processing. A Gravity data set is often tied in with previous surveys before it is ready for processing.
A good PF interpreter will come up with nearly all results which can be reliably extracted from a given data set. It also needs to be stressed that the quality of the interpretation depends a lot on the quality and versatility of the software available for the data analysis. Just as seismic work stations have improved, advancements have been made in gravity and magnetics work stations. Interpretations are generally initially carried out independently by the PF geophysicist as a first pass at the data. This interpretation is then rationalized with the geologist and geophysicist working on the project.
Unfortunately, the synthesis of the results has often been taken rather lightly. Result synthesis is mostly the responsibility of the exploration team who have far more additional information at hand than is available to the PF interpreter. For maximum impact, the PF results need to be properly integrated with all information. Effective result synthesis will provide the basis for a well informed decision on the next exploration step.
The author thanks Mr. J. Brian Henry for his review and many suggestions for the article.