Geophysics 325, Instructor: Dr. V. Kravchinsky
Assignment #1
Gravity methods
Due 5 pm on September 22nd, 2015.
Assignment can be submitted by
(1) handing in before the due time to the drop box for the course. The drop box is located in
the CCIS building o
325-2013-final
Selected answers
Question 1
The Bouguer anomaly should be only the effect of the cylinders. (This is correct, the
Bouguer anomaly should remove the effect of elevation and mass due to topography in
order to isolate gravity from density diff
MAGNETIC EXPLORATION = passive technique
Intrinsic magnetic field of Earth: H
Surface magnetic field: BE = 0(1+kavg)H
(magnetic flux density)
kavg is the magnetic susceptibility of average rock in survey
area
BE mostly dipole
geodynamo + solar wind
+ cr
Migration
Assumptions in seismic sections from
stacked CMP gathers:
reflectors occur directly below the
midpoint of each gather
horizontal planar layers (no
topography)
reflectors have an infinite extent (no
edges, corners, points)
But in the real Eart
Magnetic Exploration
Basic concepts
Magnetic force produced by moving electrical charges (current in a
wire or electron motions in an atom)
Magnetic monopole point where magnetic force appears to originate
- consider two magnetic monopoles
Coulombs law:
Amplitude of Non-vertical Rays
Body waves
SH waves: Simplest expressions of
reflection and transmission coef.
SV or P waves: More complex, but
the stress and displacement continuity
conditions hold.
SH-wave
String
1v1 2 v2
R=
1v1 + 2 v2
2 1v1
T=
1v1 + 2v
Single channel seismic profiling
one source, one
detector in approx. same
place
(mostly marine work -multiple shots expensive)
cannot uniquely determin
velocity and depth of an
interface
(depth, velocity tradeoff)
No effective noise
reduction, i.e., st
A brief Introduction
Chapter 1 in textbook
In a nutshell, exploration geophysical methods are
Remote Sensing techniques to measure subsurface
physical properties such as:
1. Density
2. Magnetic susceptibility
3. Seismic wave velocity
4. Electrical Resisti
Gradiometers (uses multiple mags to find
gradients/changes in magnetic field, concept
similar to use of base station)
Applications: Archeology studies
changes in magnetic susceptibility at archaeological sites
may also have remanent magnetization in bur
Seismic Sources
HOW TO GENERATE SEISMIC WAVES?
Exploration seismology mostly artificial sources
active technique
Natural sources can also be used (e.g. earthquakes) usually
for tectonic studies (passive seismic exploration)
!
What is a good source?
- econ
Measurement of Gravity
gravity anomalies are generally less than a few mgals
<0.01% of Earths gravity field (~981,000 mgal)
need very sensitive instruments to see these anomalies
instruments are called gravimeters
measure the vertical component of gr
A real shot gather:
- positive arrivals are shaded black
- first arrival is the direct P-wave
- slow arrivals are steeper
-ground roll (A) has high amplitude
and is long-lasting obscures
reflections at small offset
- Multiple reflections appear as
hyperbo
Geophysics 224 Mid-term exam
Instructor
Dr. Martyn Unsworth
Date
Tuesday February 14th 2006
Time allowed
8:00 a.m. to 9:20 a.m.
Name:_
Total = 50 points
Please attempt all three questions
Notes and books may NOT be used during the exam
Calculators are per
Before Weathering Correction
After Weathering Correction
Steeples et al. 1990, Geophysics 55
Synthetic CMP gathers
normal
With polarity
reversalsl
4.6.3 Time corrections Dynamic
1. Choose a stacking velocity (Vst)
Original CMP Gather
0
2. For each detec
Section 2.4 Gravity data reduction
REDUCTION: raw data gravity anomaly data
Temporal corrections tides and instrument drift
Spatial corrections latitude and elevation
GRS67 = gravity variation with latitude at sea level:
Elevation corrections
after remo
Gravity Exploration
GRAVITY AND GRAVITY ANOMALIES
Newtonian Gravitation
Gravity: force of attraction between objects with mass
Consider two objects with mass m1 and m2:
Newtons theory of gravitation:
m1
Fg
Fg
distance (r)
m2
Gm1 m 2
Fg =
2
r
G is the univ
x 2z 1 cos c
t=
+
v2
v1
Dipping interfaces
As x increases, CD becomes longer
arrival is delayed
steeper slope on a distance-time plot
Reciprocity: wave equation (that governs
propagations of deformation) obeys a reciprocal
relationship with respect to s
GRAVITY EXPLORATION
g at surface
(observation)
subsurface
density
subsurface
geology
(material property)
Gravity anomalies of some simple
shapes
Reminder: we are working with values about.
0.01-0.001 mgal 10-8 - 10-9 g
N
!
Applications of Gravity Surveys
CHAPTER 2 Seismic waves
2.1 Introduction to waves
A wave:
is a periodic disturbance
transmits energy through a material
no permanent deformation
!
Seismic waves:
transmit elastic strain energy
(stretching, tearing, bending, distortion across
some surf
Geophysics 224 Assignment 2 Seismic reflection
1. The figure shows a shot gather that was recorded in a location where the subsurface
consists of a series of horizontal layers. The travel times are listed on the attached table.
You may answer the followin
Geophysics 224 Assignment 3 Seismic reflection and refraction
Question 1
The figure above shows an unreversed seismic refraction profile.
(a) Compute the velocity at which the direct wave travels. Compute maximum and
minimum velocities consistent with the
x 2z 1 cos c
t=
+
v2
v1
Dipping interfaces
As x increases, CD becomes longer
arrival is delayed
steeper slope on a distance-time plot
Reciprocity: wave equation (that governs
propagations of deformation) obeys a reciprocal
relationship with respect to s
Name:
Geophysics 224 — Mid-Term Exam (Fall 2011]
Instructor: Dr. Claire Currie
Date: Thursday, October 20, 2011.
Time: 8:00 to 9:20 am
Exam total: 60 marks
0 Please attempt all questions.
0 Answers can be recorded directly on the exam. Extra exam booklets
Schedule
Week 13 Dec-04
Week 14 Dec-07
Week 15 Dec-15
Assignment #4 due (at the beginning of class)
Topic 11: Hydrology
Assignment #5- in class review activity
(to be handed in at the end of class- 1% of final grade)
Final Exam
Tuesday, December 15th, 3:3
Schedule
Week 12 Nov-27
Week 13 Nov-30
Dec-02
Dec-04
Week 14 Dec-07
Week 15 Dec-15
Intro to Assignment #4 (due date Dec 4th -1% of final grade)
Topic 10: Energy and Mineral Resources
Topic 10: Energy and Mineral Resources
Topic 11: Hydrology
No lecture
As
Large Scale Gravity and Isostasy
Chapter 9
Isostasy: The Concept of Floating Blocks
Isostasy = the concept that large topographic features
effectively float on the asthenosphere
Iso = same stasis = standstill
Involves a state of constant pressure at an