Exploration and Environmental Geophysics

Slides:



Advertisements
Similar presentations
Weighing Earth. Announcements & Reminders 1. ES 123 Essay: Due Friday November 24, 3:00 p.m. (B&GS 10) 2. Lab final: Tuesday December 5, 10: :20.
Advertisements

GRAVITY SURVEY (i) -Introduction- June, Gravity Survey Measurements of the gravitational field at a series of different locations over an area of.
The general equation for gravity anomaly is: where:  is the gravitational constant  is the density contrast r is the distance to the observation point.
Instruments used in gravity prospecting
GG 450 Lecture 2 GRAVITY BASICS
LaCoste-Romberg.
Applied Geophysics What is it
Applied Geophysics An Introduction
3 feet 6 feet Sun (flashlight) Moon Earth Moon walks around Earth and stops at each quarter turn. Earth records what it sees. Each student should take.
Gravity in the Fernley & Hazen Flat Areas Geol 492/692 Spring 2005 Melissa Edwards, Harmony Farnsworth, and Rich Redd.
Nyack - Geophysical Characterization Problem - determine subsurface parameters, relevant to fluid flow and basin evolution, from non-invasive observations.
Gravity Theory Measurement practices –Relative –Closed loops Corrections –Drift –Free Air –Bouguer –Terrain Interpretation –Modeling.
Airborne Gravity Processing 101 Sandra Preaux
GEOL 3000 With Assistance from Nigel Wattrus.  Seismic Reflection  Seismic Reflection – subhorizontal geologic structures  Siesmic Refraction  Siesmic.
GRAVITY SURVEY (ii) -Gravity Data Correction-
Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and.
Model of the theoretical gravity Normal gravity Elevation effect The effect of material beneath the station - the plate effect Topographic or terrain effect.
By: Hamizah Hanim Hamzah, Helen Ellwood, and Zoltán Erdős.
Outline  Construction of gravity and magnetic models  Principle of superposition (mentioned on week 1 )  Anomalies  Reference models  Geoid  Figure.
Practical issues (This lecture is based largely on: The shape of the gravity anomaly depends not on the absolute.
Gravity Methods Gravity is not a “constant” 9.78 m/s 2 Responds to local changes in rock density Widely used in oil and gas, mineral exploration, engineering.
Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV.
Sundermeyer MAR 550 Spring Laboratory in Oceanography: Data and Methods MAR550, Spring 2013 Miles A. Sundermeyer Observations vs. Models.
GG 450 Lecture 3 1/22/08 Gravity Corrections Potential Geoid.
Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics II tom.h. wilson Department of Geology.
GRAV-D Part II : Examining airborne gravity processing assumptions with an aim towards producing a better gravimetric geoid Theresa Diehl*, Sandra Preaux,
Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and.
Period 2 Question 1. As a pendulum swings back and forth, the forces acting on the suspended object are: gravity, the tension in the support cord, and.
Environmental and Exploration Geophysics II tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV.
Gravity surveys Annie, Sue, Betsy. Regional location Bango Road canal bank V-line canal bank Reno Highway Carson Highway.
Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV.
Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and.
Environmental and Exploration Geophysics II tom.h. wilson Department of Geology and Geography West Virginia University Morgantown,
Environmental and Exploration Geophysics II tom.h. wilson Department of Geology and Geography West Virginia University Morgantown, WV.
Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and.
A Brief Introduction to Gravity UT Intro to Geophysics Class March 10, 2009 Austin-Bergstrom Airport Theresa Diehl, Ph.D. Research Geodesist NOAA National.
An E-W gravity profile across the La Bajada fault Zone in the Rio Grande Rift, North Central New Mexico Rajesh Goteti University of Rochester SAGE 2007.
Conservation of Mechanical Energy Mechanical Energy – The sum of Potential and Kinetic Energies ME=PE+KE The conservation of mechanical energy states that.
Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics II tom.h. wilson Department of Geology.
Field Geophysics – Lecture 3
Potential Fields Methods Potential Fields A potential field is  a field in which the magnitude and direction (vector) of the measurement depends on the.
Exploration Geophysics Indoors Rob Sternberg Dept. of Earth and Environment Franklin & Marshall College Teaching Structural Geology, Geophysics, and Tectonics.
Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics II tom.h.wilson Department of Geology.
Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and Geography West Virginia University Morgantown,
SIO 226: Introduction to Marine Geophysics Gravity
Gravity Summary In first approximation we can chose for the shape of the Earth an ellipsoid of rotation defined essentially by the degree n=2 m=0 of the.
Variation of G with latitude
Field Geophysics – Lecture 2 1)Brief introduction to earthquakes in ECSZ 2)Introduction to gravity – measurement and interpretation Reading: Chapter.
Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and.
Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and Geography West Virginia University Morgantown,
Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics II tom.h.wilson Department of Geology.
Environmental and Exploration Geophysics II tom.h. wilson Department of Geology and Geography West Virginia University Morgantown, WV.
Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and.
1 Geophysical Methods Data Acquisition, Analysis, Processing, Modelling, Interpretation.
Last Time: Gravity Measurements may be surface-based (i.e., taken with instruments placed on the ground surface) or space-based ; absolute or relative.
1 Introduction to Applied Geophysics & Geophysical Exploration Prof Jeannot Trampert (coordinator) Dr Fred Beekman
Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and.
Geology 5660/6660 Applied Geophysics 22 Mar 2016 Lab 6 © A.R. Lowry 2016 Gravity Start by discussing lab 3 assignment & your assignment for two weeks from.
Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV.
GRAVITY.
Gravity 3.
Gravity Methods (II) Environmental and Exploration Geophysics II
Final review sessions - II
GG 450 February 19, 2008 Magnetic Anomalies.
Exploration and Environmental Geophysics
Two major types of gravimeters
Laboratory in Oceanography: Data and Methods
Gravity Theory Measurement practices Corrections Interpretation
Outline Construction of gravity and magnetic models Reference models
Field Geophysics Instructors Tom Herring, Brad Hager Web: 01/20/05
Presentation transcript:

Exploration and Environmental Geophysics Free-Air Gravity Exercise Jeffrey A. Nunn Louisiana State University

Laboratory Exercises Gravity Acquisition Gravity/Magnetics Interpretation Ray Paths Refraction Acquisition Landmark/Refraction Interpretation Reflection Acquisition Reflection Processing Reflection Interpretation Resistivity Survey Resistivity Interpretation Well Logs (BakerHughes) GPR Acquisition GPR Interpretation

Context Audience: Prior Skills Graduate/Undergraduate; Some PE students 20 Students Weekly Laboratory Prior Skills Instrument Drift/Earth Tides Free-Air Correction

Instrument Drift and Earth Tides Data are from: Wolf, A. Tidal Force Observations, Geophysics, V, 317-320, 1940. Graph from galitzin.mines.edu/INTROGP

Free-Air Correction To correct for variations in elevation, the vertical gradient of gravity (0.3086 mGal·m-1) is multiplied by the elevation of the station and the result is added, producing the free-air anomaly. The free-air gravity anomaly is given by the formula: FA = go - gt + (δg/δz) h where: go = observed gravity (mGal) gt = theoretical gravity (mGal) δg/δz = vertical gradient of gravity (0.3086 mGal·m-1) h = elevation above mean sea level (m).

Goals Content/Concepts Estimate Height of a Platform in Howe-Russell Atrium using gravity measurements Instrument Drift/Earth Tides Corrections Free-Air Correction Noisy Data Precise Corrections to raw data

Instructions We will take a series of gravity measurements to estimate the height of the walkway near the top of the atrium in Howe-Russell. After taking a measurement at a base station, we will take turns taking measurements around the pendulum. At each station, we will record the time, location, temperature, and instrument reading. Periodically we will return to an earlier station to document any instrument drift and/or earth tide effect. We will then return to the base station in Howe-Russell to make a repeat measurement. The instrument readings must be corrected for instrument drift/Earth Tides and converted to mGals. We will not make a latitude correction. Station Location Time Temp. Dial Gravity Drift

LaCoste Romberg Gravimeter Appreciate the precision of a Gravimeter Learn about measurement error first hand Appreciate the effort involved in data acquisition Work in a group Things I learned the hard way – 20-30 minutes a measurement if they spin the dial.

Measurements Above: Base of Pendulum Opposite: Top of Attrium

Data Acquisition Measurement Location Time Temp., F Reading G1_1 Loading Dock 12:20 48.2 2926.405 G1_2 Base Pend 12:43 2926.53 G1_3 Top Pend 1:01 2922.32 G1_4 1:20 2926.503 G2_1 1:37 48.3 2926.715 G2_2 1:45 2926.565 G2_3 2:08 2922.187 G2_4 2:19 2926.685

Drift Correction (linear) Base Base Top Top

Conversion to mGals

Drift Correction Time Location Meter Corrected mGals 0.000 Loading Dock 2926.405 3010.641 23.000 Base Pend 2926.530 2926.492 3010.731 41.000 Top Pend 2922.320 2922.253 3006.367 60.000 2926.503 77.000 85.000 2926.565 2926.499 3010.738 108.000 2922.187 2922.213 3006.326 119.000 2926.335

Sensitivity of Gravity Interpretation (4 mGal) Base Base Top Top

Estimate of Height from Free-Air Correction Location mGals   Loading Dock 3010.641 Measured Height Base Pend 3010.731 del_g meters feet Top Pend 3006.367 4.364 14.141 46.384 3010.738 3006.326 4.412 14.297 46.894  Observed Meters 4.563 14.787 48.500

Laboratory Report Turn in a copy of data sheet plus an explanation of how you converted dial measurements to mgals plot of instrument drift/Earth tides correction estimate of elevation of the walkway using a Free-Air Correction a brief (paragraph or two) discussion of potential errors in the survey.