Geophysical imaging of geological and archaeological targets in the Chiang Mai Basin A field-based approach to applied geophysical education A 2011 GWB project

Presentation Outline

14:30 - Introduction/Overview

– Geophysical Methods – Electromagnetic methods (Aom/Toy) – Gravity/Magnetic methods (Made) – GPR methods (Jern/Meow) – Resistivity methods (Boy/Ngan) – Seismic methods (Eko/Aoy)

Break 15:40 Overview of Mae Jo

– Electromagnetic methods (M/Toy) – Gravity/Magnetics (Ohm) – Resistivity (CK) – Seismic reflection (Ae) – Seismic refraction/MASW (Ing/Praew)

Overview of Wiang Kum Kam – (Praew)

– Resistivity (Atta) – GPR/Integration (Tanad/Jar)

Concluding Remarks

Geoscientists Without Borders (GWB) Program

• SEG Foundation GWB program: connect university/industry with communities to benefit people and the environment through geophysics • Our project  student-directed trainings in applied geophysics for engineering and environmental projects .

Sponsorship

• SEG Foundation GWB program – Boise State University – Chiang Mai University – Thai Department of Mineral Resources – University of Western Australia – Colorado School of Mines – Geonics Limited – Siam Tone Co., Ltd.

Participants/Instructors 17 institutes/11 countries

Geophysical Methods

• 14:40 – Electromagnetic methods – Aom/Toy • 14:50 – Gravity/Magnetic methods - Made • 15:00 – GPR methods - Jern/Meow • 15:10 – Resistivity methods –Boy/Ngan • 15:20 – Seismic methods – Eko/Aoy • 15:30 - Break

Electromagnetic methods

Resistivity model

Electromagnetic methods

E

  

B



t

TEM: Tx and Rx waveform

Primary field Induced EMF Secondary field

Result after switch-off the primary field Secondary field Changing of magnetic field voltage voltage on function of time Apparent conductivity as function of depth

L125N Loop5 L100N L2 Loop11 L3 loop6 L4 loop7 Loop9 Loop10 Loop8

TEM survey at Mae Jo

Data acquisition

• Measurement:

dB/dt

as voltage decay • Transfer/input data to computer (

DATEM software

) Receiver

Data processing

Set polarity inside/outside loop • Inside loop →

+

• Outside loop → Profiles and Decay plot • Select signal gate • Eliminate Noise

Data processing

TEM data > Universal sounding format (USF)

Data processing & Result

TEM data > Universal sounding format (USF) 1D-Sounding / Profiles and cross section after inversion

Gravity and Magnetic Method

At Chiang Mai 02 Jan 2011-14 Jan 2011

Basic Gravity Theory Gravity is a potential field. it is a force that acts at a distance. r = distance between 2 masses G = constant of universal gravitation

m 1

F R F

m 2 Figure 1. Newton’ Law of Gravity

Instruments

The Scintrex Autograv has been used in this gravity survey. The Scintrex Autograv is semi automated, it has been shown to have a higher stability and experience less tares (a sudden jump in a gravity reading) over long periods of time than non automated machines.

CG-3 gravimeter

Raw Data Data Processing Drift Correction (g reading g obs ) Position Variation Correction • Free-Air Correction • Latitude Correction • Bouguer Correction Terain Correction Microsoft the Oasis Montaj

Drift correction In order to remove instrument effects if base station measurements are repeated every few hours. The drift correction is defined as

Δg D =g b + (t - t b ) * [(g e - g b ) / (t c – t b )] – g 1 mGal

.

7241.401

7241.400

7241.399

7241.398

7241.397

7241.396

7241.395

7241.394

0 1000 2000 3000 4000 5000 y = 7E-07x + 7241.4

6000 7000 8000

Correction to g obs

Correction

Latitude Correction Free Air correction Bouguer Correction

Formula

dgL dg dg F B

=0.812sin(2φ)dx

= -0.3086 dR ϕ = 0.0419 ρ dR ϕ

Description

Latitude variation Elevation above sea level the extra mass underlying observation points at higher elevations.

Subtracting the corrections from g observed isolates the signal from our target

Magnetics

• A magnetic field is created by a magnetic dipole. A dipole is composed of a positive and negative magnetic charge separated by a finite distance.

Figure 1: Earth's magnetic field with influence of solar wind. Note that the field is mostly dipolar.

• The magnetic properties of materials are described by two components – the induced and the remanent magnetization.

• Induced magnetization is a net magnetization developed in ferromagnetic materials when they are exposed to an ambient magnetic field, such as that of the Earth.

Remanent magnetization is acquired at the time when a material cools below the Curie point and is maintained as a “memory” in the material.

Figure 2: Inclination, declination, and magnetic field

Instruments Field Work

Pre-Processing step

- combining all of field data - Resampling - Despike - Destripe

Processing

• High Pass Filter/ Low Pass Filter • Upward continuation • Reduction to pole

Geophysical Methods

• 14:40 – Electromagnetic methods – Aom/Toy • 14:50 – Gravity/Magnetic methods - Made • 15:00 – GPR methods - Jern/Meow • 15:10 – Resistivity methods –Boy/Ngan • 15:20 – Seismic methods – Eko/Aoy • 15:30 - Break

Theory

Propagation of an electromagnetic wave (EM) consisting of cojoined electrical and magnetic waves

GPR applications has frequencies ranging between10 and 1500 megahertz, that often overlap radio, television, cellular phone transmissions.

GPR SYSTEM Propagation of EM wave depend on -Electrical properties of the subsurface materials -Relative Dielectric permittivity -Electrical conductivity

Relative Dielectric Permittivities (RDP)

The ability of a material to store a charge from an applied electromagnetic field and then transmit that energy

EM wave velocity :

v



c

 

r r



c



r

- Dielectric permittivity - Magnetic permeability,assumed = 1

Relative dielectric permittivity is inversely related to EM wave travel velocity

Relative Dielectric Permittivities (RDP) of some geological materials

Material Air Dry sand Dry silt Ice Asphalt Volcanic ash/pumice Limestone Granite 3-5 4-7 4-8 4-6 1 3-5 3-30 3-4 RDP

Example of GPR profile

GPR

• Easy-to-use, non-destructive technique • Good for shallow studies with high resolution • Might be not suitable in highly conductive areas • Various fields: Geological mapping, Archaeology, Environmental studies, Engineering

GPR DATA PROCESSING

WIANG KUM KAM

Processing Steps

Processing Steps

Processing Steps

Data with wow signal

Dewow Data

Processing Steps

Processing Steps

Processing Steps

2D Time Slices from 3D Cube

BOISE STATE UNIVERSITY APPLIED FIELD GEOPHYSICS WORKSHOP 2011

RESISTIVITY SURVEY IN WIANG KUM KAM AND MAE JO

January, 2011

EQUIPMENT INTRODUCTION The connection of the IRIS with the cables (left) and the cable with the electrodes (right).

Wiang Khum Kham: 2m Electrode spacing Mae Jo: 5m

INTRODUCTION

Shallow Geophysics Exploration

MAGNETIC MASW RESISTIVITY SEISMIC REFRACT GPR

BASIC THEORY



V



I

 2     1

C

1

P

1  1

C

1

P

2  1

C

2

P

1  1

C

2

P

2      2  

V I

1

K

Wenner

BASIC THEORY

C1 a P1 a P2 a C2

k

 2 

a

Schlumberger C1 na

k

  Dipole - dipole a

k

C1   C2 P1 a P2 na  1)

a

C2 na a P2 P1  1)(

n

 2)

a

BASIC THEORY (continue)

DATA PROCESSING PROSYSII (IRIS Acquisition Software)

Upload Data to Laptop Data Filtering Merge Survey Spread Edit Geometric Factor Export to RES2DINV Dat File

DATA PROCESSING

P2 C1 P1

EDIT GEOMETRIC FACTOR

C2

DATA PROCESSING RES2DINV

Noise Despiking Input Elevation Inversion Enhance Image

DATA PROCESSING SPIKE CORECTION

DATA PROCESSING TOPOGRAPHY CORRECTION

CONCLUSION

RESISTIVITY IMAGING SURVEY

High solution for intermediate depth Comparable with all other shallow geophysical instrument Reveal better resolution of shallow depth for deep geophysical instrument like Seismic Reflection and TDEM

Seismic Theory: Reflections, Refractions and Surface Waves

Seismic Wave Propagation

Displacements from EQs or man-made sources propagate in the subsurface

Wave Characterization

Homogeneous Acoustic Wave Equation, with a solution of the form:

u(x, t) = Ae

−i(kx-ωt)

Ae −iωt The objective is to find the subsurface structure, or impedance contrasts.

e

= reflection from a flat reflector Cartoon shot gather with many arrivals

SEISMIC REFRACTION METHOD

(Online : http://www.geologicresources.com/seismic_refraction_method.html)

1. Evaluation of the thickness of layers in subsurface 2. Estimation of the seismic velocity of each layer

1

v

Distance (m)

f(H, V) = t

2

i + v v

2

v

1 2 2 

v

1 2

v

2 Travel time curve

SURFACE WAVE A wave that can travel along an interface between two different mediums without radiation.

Low frequency Rayleigh waves generated are used in seismology to characterize the Earth's interior

v

Dispersion curve Frequency (Hz) Dispersion relation

ρ

MASW/Refraction METHOD

The purpose of this study: - Refraction: To estimate the depth of shallow earth layers (~10s m) - MASW: To estimate S-wave velocity of the shallow surface

Geophysical Methods

• 14:40 – Electromagnetic methods – Aom/Toy • 14:50 – Gravity/Magnetic methods - Made • 15:00 – GPR methods - Jern/Meow • 15:10 – Resistivity methods –Boy/Ngan • 15:20 – Seismic methods – Eko/Aoy • 15:30 - Break