Survey Design - Resistivity

BACKGROUND

Unlike the gravity and magnetic surveys we have already conducted, this will be your first experience with a controlled source experiment. Until now, you have been mapping anomalous variations in the earth's natural fields. For this survey, you will create the electrical field (control the source). This approach has both advantages (better control over signal and noise) and disadvantages (including cost and complexity).

Resistivity also differs from gravity and magnetics surveys in that the phenomenon being measured (resistivity) is primarily controlled by the water content rather than by the characteristics of the rock itself. This makes it difficult to design surveys for areas where you have no prior information about ground water conditions. This survey is a good illustration of that problem. You already have some understanding of the subsurface geology based on both the geologic setting relative to the Front Range and on your previous surveys, but you have no way of knowing, however, whether ground water even exists at the site, much less any evidence for the depth to or gradient of the water table. For that reason, ground water projects often start with reconnaissance surveys to provide enough information to determine whether more expensive, detailed surveys are warranted and to design those surveys more intelligently. This exercise represents one such reconnaissance survey, and it will be followed with a request to design another survey if the results of this are favorable.

OBJECTIVES

There are five learning objectives for this project:

  • Gain additional experience in developing predicted geologic models and estimating geophysical parameters,
  • Continue to practice and refine your structured decision making and survey design skills,
  • Develop an understanding of the use of controlled source geophysical methods and how they differ from natural source methods,
  • Gain a feel for the source, magnitude, and character of each of the principle forms of noise that affect a resistivity survey, and
  • Improve your ability to select optimum survey parameters based on a comparison of expected signal strength and noise characteristics.

PROCEDURE

You are to submit a bid for a reconnaissance resistivity survey to determine whether the igneous dike described by previous surveys might be interrupting groundwater flow and creating a subsurface pool of water on the up-gradient side as described in the Request for Bid. You will follow much the same procedure that you used in designing the gravity and magnetic surveys (estimate amplitude and frequency of signal and noise, design a survey to enhance the signal and minimize the noise at the lowest overall cost, and estimate the cost of the survey). The steps to be followed should include the following:

  • Assuming the dike is acting as a subsurface dam, predict a likely range of depths to water table for several locations on each side of the dike. Given your general knowledge of the geologic structure at the base of the mountains, estimate possible depths to bedrock. Determine the range of resistivities that might be found in the alluvium above water table, alluvium below water table, and bedrock,
  • Tabulate the sources of noise that will affect the resistivity survey and estimate the amplitude of each of these noise sources,
  • Discuss techniques that can be used to minimize the noise from each source, and
  • Using one of the resistivity modeling packages pointed to below, produce synthetic apparent resistivity curves for the range of depths and resistivities expected. For this survey, you can use one of either two types of DC resistivity array geometries typically used: Schlumberger or Wenner. Each has its own advantages and disadvantages. Choose one array type, and do all of your modeling with that type of array. In the USA, Schlumberger arrays are the most common type of array used for problems of this type.
  • Develop a detailed plan for a survey that can be expected to acquire data sufficient for interpretation. Some technical issues to consider include:
    • Choice of Wenner or Schlumberger array configurations,
    • Midpoint locations and line orientations for the survey and the range of resistivity curves that are likely to be produced at each location. You may collect as many soundings at different locations and with different orientations as you likes,.
    • Shortest and longest electrode spacing and the number of electrode spacings at which to acquire data per decade in electrode offset. Remember, when planning your survey, the client has not obtained permission for you to operate off of his property. Thus, your surveys must be confined to the site under consideration,
    • Standard deviation of the random noise and the need for multiple readings. Resistivity meters typically used for near-surface exploration are capable of producing about 0.5 amps of current and reading voltages to about 0.01 volts, and
    • Likelihood of reduction or elimination of noise.
  • Estimate the cost of the survey you have designed. The economic factors governing the survey include:
    • Field operations require three crew members,
    • It takes 30 minutes to lay out the wires and tapes for each sounding,
    • For Schlumberger soundings, it takes 3 minutes to set out the electrodes and 5 seconds to make a single voltage measurement,
    • For Wenner soundings, it takes 6 minutes to set out the electrodes and 5 seconds to make a single voltage measurement,
    • 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 three are required at all times in the field with the survey 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 resistivity equipment depreciates at the rate of 1%/day (original cost = $10,000),
    • 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---.
  • To proceed to the data acquisition portion of this exercise, you must have the following survey parameters defined.
    • Electrode configuration type
    • Number of soundings to be performed
    • Location of the center point of each sounding using the standard survey coordinates
    • Orientation of each sounding: east-west or north-south
    • Minimum electrode spacing, in meters, to be used (AB/2 for Schlumberger, A for Wenner)
    • Number of decades in electrode spacing over which to move the current electrodes
    • Number of electrode locations at which to acquire data per decade of electrode spacing
  • Discuss the differences, advantages, and disadvantages of using a controlled source for geophysical data as compared to natural sources.

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), a brief discussion of the advantages and disadvantages of a controlled source experiment, 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.