Survey Design - Gravity

A gravity survey is based on the premise that a target is limited in space and has a different density from the surrounding geology. The problem is to remove all of the other sources of gravity variations, or noise (tides, instrument drift, regional gravity contributions, etc.), so that you can interpret the anomaly in terms of the geological and geophysical parameters of the localized target.

A common mistake made by geologists and engineers -- and even many geophysicists -- is assuming that a gravity survey will work in any case where the target has an anomalous density. Geophysicists compound this mistake by choosing survey parameters such as station spacing based on intuition (they call it experience). Often the particular combination of size, depth, and density contrast produces a gravity anomaly smaller than the resolution of the survey. In other cases, the noise level masks the anomaly. A systematic approach to planning the survey and validating the design will overcome many of these failures.

Planning a gravity survey is done by reversing the interpretive process. We 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:

  • Becoming familiar with numerical modeling,
  • Developing a conceptual understanding of the shape of the gravity anomaly associated with a particular geologic target and the sources of noise that mask that anomaly,
  • Learning to codify decision-making processes and quantify conclusions,
  • Reinforcing the fact that economics is a part of all engineering practices.

Given the Request for Bid, the objective is to verify that gravity is the appropriate technique to use, then 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:

  • Create a range of likely geologic models and assign geophysical parameters: depth of tunnel, radius of tunnel, and the tunnel's density contrast, and
  • Use one of the gravity survey programs to produce a series of plots of the gravity anomaly associated with that range of geological models.
  • 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 gravity anomaly,
    • Width of the gravity anomaly,
    • Orientation of the gravity anomaly,
    • Standard deviation of the random noise (Experienced gravity crews can obtain observations with an error of between 0.01 and 0.02 mgals, far smaller than the errors observed in the data collected by the novice crews described in the previous exercise),
    • Elevation precision required,
    • Elimination of temporally and spatially coherent noise,
    • Frequency of reoccupation of a base station.
  • 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 5 minutes to take a reading,
    • It takes 15 minutes to return to the base station and 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,
    • Field hands make $10/hour, and two are required at all times in the field,
    • 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 gravity meter is depreciated at the rate of 1%/day (original cost = $20,000),
    • 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---.

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. Details (flow-chart of the survey design process, tabulation of survey- design parameters, breakdown of costs, etc.), however, can be included as appendices. Be sure to look at the Request for Bid so that you include everything in your bid 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.