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 so you need to have a measurable diff if you hope to “see” anything (maybe geophys not so diff from geology..have to see things (“measurable diffs”) with your eyes to make interpretations as a geol too…

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 contraints…no 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 (> 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

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 it’s not metal book example - Fig one tool gives you one boundary, another gives you the other boundary: Resis contrast Seismic contrast Clay Sand 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)