Survey Design - Seismology

BACKGROUND

Like the resistivity method, the refraction seismic method is employed (usually) as an active method. A source of seismic energy is detonated, and the ground motion produced by this source is recorded at various distances from the source. As you could see from your experience with the resistivity method, the application of active geophysical methods generally requires more logistical forethought than the passive methods. By this, we simply mean that there is more equipment involved and more survey parameters that must be chosen. Because of this, however, we usually have more control over noise reduction and the regions of the earth that perturb our signals than we would with a passive method.

With the seismic method, we can control even more variables related to survey design than we did with the resistivity method. In addition to the configuration of the receivers (including spacing and orientation), in the seismic method, the source can be moved independently of the receivers and different types of sources can be employed. With all of these variables and the expense of the equipment required to collect and interpret the observations, seismic methods tend to be more expensive than the other geophysical methods. Thus, reconnaissance surveys, like resistivity surveys, are often employed before acquiring seismic data in an effort to determine the necessity of doing seismic exploration and to determine the best survey design parameters to use when acquiring the seismic observations.

OBJECTIVES

There are five learning objectives for this project:

  • Begin to appreciate the logistical demands of employing seismic techniques,
  • Continue to develop an understanding of the various physical parameters and structures that can be constrained seismic methods,
  • Continue to practice and refine your structured decision making and survey design skills,
  • Continue to develop an understanding of the use of controlled-source geophysical methods and how they differ from natural-source methods, 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 refraction seismic survey to determine the geometry of water table and the saturated alluvium/bedrock interface beneath the site. As defined in the Request for Bid , special interest should be given to determining 1) whether a sufficient groundwater reservoir exists beneath the site to be used for supplying water to the site, and 2) whether the ground under the site is part of the Clear Creek watershed. You will follow the same procedure we have used for all of the surveys up to this point (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 following are the steps that should be taken to complete the bid:

  • Using the results of the surveys performed to date, determine the range of depths and seismic velocities (P-wave) 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 those noise sources,
  • Discuss techniques that can be used to minimize the noise from each source,
  • Using the refraction modeling packages pointed to below, produce synthetic refraction profiles for the range of depths and velocities expected,
  • Develop a detailed plan for a survey that can be expected to acquire data sufficient for interpretation. Some technical issues to consider include:
    • Number of profiles to be collected,
    • Choice of seismic source (sledgehammer, shotgun, or explosives),
    • The number of active channels you need in a recording system (12,24,48,60,96,120,etc.),
    • The location of the first active geophone in the spread in
      • standard survey coordinates,
    • Orientation of the geophone spread (east/west or north/south),
    • Number of locations in the spread to be occupied by geophones,
    • Distance between adjacent geophones in the spread,
    • Number and location of each source to be shot into the spread, and
    • Like the resistivity survey, remember that you have permission to occupy the client's site only,
  • Estimate the cost of the survey you have designed. The economic factors governing the survey include:
    • Recording Costs
      • Recording Equipment - Recording equipment depreciates at 1%/day. The cost of the recording system is dependent on the number of channels it can record. A general estimate is $1000 per channel. Thus, a 24 channel system costs $24,000, and a 48 channel system costs $48,000. Systems are available that record 12,24,48,60,96, and 120 channels.
      • Geophones - Geophones depreciate at 1%/day. Geophones cost $100/geophone.
      • Cable - Seismic cables depreciate at 1%/day. Cables cost $50/channel. You need at least as much cable as the number of channels that you have in your recording system. If you choose to rent more geophones and cable than channels in your recording system, you can deploy all of the geophones and cables, hook up a portion of these to your recording system, and then move the recording system to hook up the next portion, etc. If you choose to do this, it only takes 10 additional minutes to move the spread (as opposed to the 30 minutes described below).
      • It takes 90 minutes to initially lay out the cable, geophones, and test the electronics, and
      • If your spread and recording equipment requires geophones to be moved (that is, if your instrument records fewer channels than the ground stations you would like to occupy), it takes an additional 30 minutes every time you have to move the geophones and cables.
    • Source Costs
      • Sledgehammer: No additional equipment costs; it takes 5 minutes to set up at the source and 5 minutes to acquire the data for each 10 swings of the sledgehammer.
      • Shotgun: Shotgun sources depreciate at 1%/day. A shotgun source costs $5000. It takes 5 minutes to set up at the source and acquire the data. Each shell, consisting of a 9-gauge steel slug, costs $2.
      • Explosives: Explosives must be detonated in holes approximately five feet deep. Sources are buried to enhance source coupling, that is to get as much energy from the explosive into the ground, and to prevent surface damage. A depth of five feet is appropriate for charges up to 1/3 pound. You will not need explosives any larger than this on this particular job. Portable drilling rigs for drilling shot holes depreciate at 1%/day and cost $7,000. Explosives cost $100 per charge. It takes 30 minutes to drill and load each shot hole. Once drilled and loaded it requires 5 minutes to acquire data from the shot. The drilling crew consists of two people - a field hand and a licensed explosives handler. The explosives handler makes twice that of the regular field hand. This crew is deployed in addition to the acquisition crew described below.

    You can drill and load shot holes either while you are collecting the seismic data, or before you collect the seismic data. Each has its advantages and disadvantages. If you choose to drill and load shot holes while you are acquiring the seismic observations, remember that the acquisition crew can not acquire seismic data while the drilling crew is drilling. Thus, you must shut down the drilling crew to acquire seismic data and the seismic crew to drill and load shot holes. If you choose to drill and load all of the shot holes prior to the acquisition of the seismic data, you must hire an additional field hand to occupy the site during the off hours for security and liability purposes.

    Finally, the use of explosives involves inherent risks, both personal and property, beyond those normally found in more routine geophysical field work. Thus, if you choose to use explosives you must carry additional liability insurance. You should carry a minimum of $500,000 of additional liability insurance. The cost of this is $100 per field day.

    • Mobilization and demobilization requires 1/2 day each,
    • Total person-hours required for processing, interpretation and report preparation is the same as total acquisition 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,
    • 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 for each shot in your survey (you may record multiple shots on the same profile):
    • Location of the first geophone in the profile in standard survey coordinates. The first geophone in the profile is specified as the western most for an east-west directed line or the southern most for a north-south directed line,
    • Number of ground stations in the profile at which to collect observations,
    • Distance between adjacent geophones in the profile in meters,
    • Orientation of the profile (east-west or north-south),
    • Source type to use (sledgehammer, shotgun, explosive), and
    • Location of the source relative to the location of the first geophone in the spread in meters. Positive locations are east or north of the first geophone in an east/west or north/south oriented line, respectively. Negative locations are west or south of the first geophone in an east/west or north/south oriented line, respectively.

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 type-written 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.