Chap 1

1

• Chap 1 Burger Approaching the subsurface
• Options for approaching the subsurface
• diff reasons for studying subsurface
• economic
• contamination
• archaeological
• diff ways to study
• direct (outcrop, wells)
• indirect (geophys)
• geophys has value several ways
• large areal coverage (sq miles vs one hole in
  the ground)
• rapid time frame (seismic lines vs a well
  drilled)
• cost-effective manner
• but has some big limitations too
• can only msr contrasts in props, but sometimes
  contrasts small or masked by large anomalies
  nearby
• seis velocities
• elec, grav, mag contrasts

2

• Fundamental considerations - consider these
  throughout the course
• We always idealize reality to model it!
• So we change it before we ever start.planar
  beds, no dips, etc.
• Define your objectives
• We all live with contraintsno time, no money, no
  clue.
• Know what tools do what (this course will help)
• Know how to find prelim data to help guide your
  work (previous workers)
• Think ahead to what might go wrong, plan for
  it.
• Recognize limitations
• 1. must have sufficient contrasts in phys props
• 2. non-unique solutions (gt 1 possible right
  answer)
• 2 types of modeling performed
• direct - geology known, generate geophysical
  response
• inverse - pull the equation out of the geophys
  response, guess the geology (what we usually have
  to do)
• book includes gravity, mag, resistivity (but
  seis too)
• 3. Resolution (eg, how small a scale can you
  resolve?)
• 4. Noise, human and natural

3

• Why multiple methods?
• Diff tools see diff things, so comparison of
  responses can help narrow the interpretation
• look at resistivity and magnetic responses- with
  most metals, you see both low resistivity and
  high magnetic response. If you see low
  resistivity and NO magnetic response, then
  perhaps its not metal
• book example - Fig 1-1 - one tool gives you one
  boundary, another gives you the other boundary

Clay
Resis contrast
Sand
Seismic contrast
Granite
Multiple methods very popular in environmental
field (looking for multiple things), seismic
reflection still predominant in oil/gas
exploration (image subsurface structure and
stratigraphy)