Survey Design - Magnetic

BACKGROUND

Like gravity surveying, magnetic surveys are based on the premise that a target is limited in space and has a different physical property, in this case magnetic susceptibility, from the surrounding geology. Unlike gravity surveying, however, the variation in magnetic susceptibility for various rock types is orders of magnitude greater than the variation in density for the same rock types. Thus, even knowing the types of rocks in a specific area does not provide sufficient information to constrain susceptibilities. Like density contrast, variations in susceptibility trade-off strongly with other model parameters. Therefore, if susceptibility, or other model parameters, can not be constrained from different observations, it is difficult to make quantitative estimates of the geologic structure based on magnetic observations alone.

In this particular survey, we do have the additional constraints that allow us to use the magnetic observations in a quantitative fashion. This information is derived from two separate data sets: geologic and gravity. Therefore, the procedure we will use in designing the magnetic survey is to first constrain our geologic model using the gravity observations collected previously. Once we have constrained the range of plausible geologic models from the gravity observations along the line corresponding to y=0, we will design a magnetic survey to esimate the spatial extent of the structure producing the observed gravity anomaly and estimate its susceptibility.

In planning the magnetic survey, we will follow the same procedure used in planning the gravity survey. We will predict the noise from sources not of interest in the survey, estimate the standard deviation of the random (operator and instrument) noise, calculate the shape of the signal (the theoretical anomaly produced by the assumed source), then decide whether the noise can be reduced to the point where the signal will be interpretable. If the answer is affirmative, then we determine the survey parameters that will produce the best compromise between cost and data quality.

OBJECTIVES

There are four learning objectives for this project:

  • Begin to understand the power of using multiple geophysical methods in an integrated geophysical survey,
  • Develop a conceptual understanding of the shape of the magnetic anomaly associated with a particular geologic target and the sources of noise that mask that anomaly,
  • Learn to codify the decision-making process and to quantify conclusions,
  • Reinforce the fact that economics is a part of all engineering practices.

Given the Request for Bid , the objective is to verify that magnetics is the appropriate technique to use and then to design a survey that is likely to produce the best possible data at the lowest possible cost. There are two milestones in the process of accomplishing this objective:

  • Reaching a conclusion on survey parameters (for our use in creating synthetic data) and
  • Submitting a bid that takes into account both economic and technical factors.

PROCEDURE

Using information provided in the Request for Bid and the geologic overview, do the following:

  • Create a suite of geologic models that may be responsible for the gravity anomaly observed on west end of the line,
  • For each geologic model in this suite, construct a geophysical model of the structure. That is, given the geology, how do geophysically relevant parameters, in this case density since we will be attempting to model a gravity anomaly, change with depth or position. For example, vertical faults and sills could be represented geo-physically by thin horizontal slabs of more dense material. Notice that a simple geophysical model could represent multiple geologic structures. Thus, it is not possible from the geophysical model alone to infer geologic structure. Also, notice that most, if not all, of these geological models can be represented to first order by relatively simple geophysical models: slabs, cylinders, spheres, etc., and
  • To test whether any of these geophysical models can explain the gravity observations, use the appropriate modeling programs (below) and attempt to model the gravity anomaly observed on the western end of the line.

  • These programs should be adequate for you to model the reduced gravity observations in terms of a variety of geologic models. From these models, determine which could produce the observed gravity anomaly with geologically plausible parameters. Using the geologic information, you can constrain the range of plausible density contrasts for each model. Is it possible to model the gravity anomaly within this range of density contrasts? If not, then the geologic model can not be a plausible explanation for the geophysical observations.
  • Once you have limited the range of plausible geologic models, use the gravity observations to constrain the geometry (depth, width, position, etc.) of the density anomaly producing the observed gravity anomaly.
  • Now that you have constrained the geophysical model, use the "Magnetic Dike" program and produce a series of plots of the magnetic anomaly associated with your geophysical model. These plots should include not only variations in geometry but should also allow susceptibility to vary. Note that for the magnetic anomaly, the geometry includes its direction of trend.
  • Develop a detailed plan for a survey that can be expected to acquire data sufficient for interpretation of the actual geological target. Some technical issues to consider include:
    • Amplitude of the magnetic anomaly,
    • Width of the magnetic anomaly,
    • Standard deviation of the random noise, and
    • Elimination of temporally and spatially coherent noise.
  • Estimate the cost of the survey as designed above, then consider whether the survey design can be modified to reduce cost without causing significant degradation of data quality. Economic factors governing the survey include:
    • It takes 30 seconds to take a reading,
    • Mobilization and demobilization will require 1/2 day each,
    • Total person-hours required for processing, interpretation, and report preparation is the same as total person-hours in the field,
    • Estimate the diurnal component of the field by continuously monitoring the field at a base station. Thus, you will need to rent two instruments and supply field crews for both,
    • Field hands make $10/hour, and two are required at all times in the field with the survey instrument. One is required at all times to monitor the base station instrument,
    • Field hands will only work 8 hours per day,
    • Processors, interpreters, and report writers make $20/hour,
    • Subsistence and travel expenses are $100/person/8-hour day while doing the field work,
    • The magnetometers depreciate at the rate of 1%/day (original cost = $7,500),
    • Vehicle depreciation is $50/day,
    • Fringe benefits for employees are 25% of salary,
    • Overhead is 100% of total direct cost excluding equipment depreciation, and
    • Profit is ---your choice---.

OUTCOMES

The final report should be in the form of a bid. The heading can be in standard memo format. The bid must include survey-design parameters, a summary of the decision-making process that led to that design (including an estimate of the likelihood that the survey will work), and a firm statement of total cost. The report must be no longer than two pages, but details (flow-chart of the survey design process, tabulation of survey- design parameters, breakdown of costs, etc.) can be included as appendices. Be sure to look at the Request for Bid to ensure that you have included in your bid everything that the client has requested. Remember that the bid is a sales document; it should communicate quickly and effectively and should focus on those issues that will be of most interest to the client.