Magnetics Survey Design
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 tradeoff 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: geology and gravity. Therefore, the
procedure we will use in designing the magnetic survey is to first constrain
our geological model using the gravity observations collected previously. Once
we have constrained the range of plausible geological models from the gravity
observations along the line corresponding to y=0, we will design a magnetic
survey to estimate 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 geological overview,
do the following:
- Create a suite of geological models that may be responsible for the gravity
anomaly observed on west end of the line,
- For each geological 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 geophysically by thin horizontal slabs of more dense material.
Notice that a simple geophysical model could represent multiple geological
structures. Thus, it is not possible from the geophysical model alone to infer
geological 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, download the appropriate modeling scripts pointed to below and
attempt to model the gravity anomaly observed on the western end of the line.
- These scripts should be adequate for you to model the reduced gravity observations
in terms of a variety of geological models. From these models, determine which
could produce the observed gravity anomaly with geologically plausible parameters.
Using the geological
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 geological model can not plausible
explain the geophysical observations.
- Once you have limited the range of plausible geological 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, run the script pointed
to below 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 also allow susceptibility to vary. Note that for the magnetic
anomaly, the geometry includes its trend direction.
- 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
- 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 depreciated 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
- Profit is ---your choice---
- My consulting fee is $200/hour
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, however details (flowchart 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.
Your browser must be Java-capable (and the Java features turned
on) to proceed with the modelling. If it is not, take the time now to upgrade
to Netscape 4 or Internet Explorer 4.
Modelling scripts which run in a Mathematica or Matlab
environment were previously available. If you have access to either of these
platforms, scripts may be available, but the Java modelling has continuing
support.
* The costs here are from the original US website, and do not
reflect Australian costs or practices closely. They are left in to give students
a level playing field (if no home ground advantage) to study the effects of
survey design.
To get some idea for survey costs, see the Manual for Field Geologists,
published by the Australian Institute of
Mining and Metallurgy.
