GEOPHYSICS (ELECTROMAGNETIC SKEMANG) Flashcards
based on the response f the ground to the propagation of EM fields (Alternating Electric intensity and Magnetizing force)
Electromagnetic Survey
an instrument use to primary or inducing field generated by passing an alternate current through a coil/loop or wire placed over the ground
Transmitter Coil
IN the presence of a conducting body, the MAGNETIC COMPONENT og an EM field penetrating the ground induces this type of secondary alternating currents to flow within the conductor
Eddy Currents
How can the anomaly be measured?
By meauring the difference between the Primary Field (Transmitted) and Secondary Field (received) induced by Eddy Current
What information ca the EM anomaly give about a conductor?
3D Perspectives of Mineral Bodies: Geometry and Electrical Properties
The depth of penetration in EM Surveys is inversely dependent on
Frequence of EM
Conducitivty of the medium
represent the maximum depth at w/c a conductor mau still produce an EM anomaly
Effective Depth of Penetration
Used in locating Massive sulfide Boies
Fixed Source Vertical Loop Method
EM Method with uses poweful RADIO Transmitters as used in long range communication in which Depth Penetration is low
Very Low Frequency (VLF)
This methos uses a natural EM field induced by thunderstorm and enables deeper pentration due to high frequency waves
Audio Frequence Magnetic Method (AFMAG)
What is the terms used to denote Ntural EM field caused by thunderstorms?
Sferics
Measurement of the secondary field in the absence of the primary field wherien the eddy currents defuse aroung the boundary of a highly conductive subsurfac body and decay slowly when the inducing field is removed.
Rate of decay of Eddy Current can be measured to locate anomalous conducting bodies
Primary Field is non-continuous
Time Domain EM (TDEM)
Uses magnetic components of the EM Field so that there is no need for physical contact with the ground for either transmitter and receviever and can be used at much faster speed
Non Contacting Conductivity measurement
Widely used during Reconnaissance for traget detection due to its high speed approach to cover vast areas in shortest time making it cost effective
Airborne EM Survey
airborne survey when only the receiver is airborne
Passive
Both the Transmitter and the receiver are mobile and use where areas of difficult ground access
Active Systems
A type of Active Airborne system that use a large transmitter sling between tail and wings and receiver is towed from a cable of about 150 m behind the airfraft. Due to uncotrolled height and direction during flight movement, only the PHASE difference between primary and resultant fields caused by a conductor can be measured
Quadrature System
A type of Active Airborne Systemw here the transmitter and coils are mounted either on thwe wings of an aircraft at ground clearance 100-200m or on a beam fixed beneath a helicopter flown at an elevation of 20m maintaining Fixed Separation and height
Fixed Separation System
Airborne TDEM
INPUT (Induced Pulse Transient)
Flight pattern
To and Fro
E-W/W-E
N-S/S-N
Mining and Econ Geol
?Metallic ore, Massive Sulfides
?VMS Deposits
?Locating abandoned mineshafts, crown pillars and subsidence features
?Identifying Bedrock an Mineralization Discontinuities
?Identifying sources of contamination in mining complexes
Structural
?Delineating faults, shear zones with thin metallic veins , Facture zones and contacts
Hydrogeol
?Hydrogeological Studies
Environmental and Engg
?Determination of past land fill sites
?Mapping of Soil types, Land utilziation and Land drainage pattern
Archaeology
Buried objects
large scale low frequency natural magnetic fields w/in and around earth (Natural EM Fields)
Magnetotelluric field
Natural Alternating Electric Currents flowing within and around earth
Telluric Currents
The onlye electrical technique capable of penetrating to depths of interest to the OIL INDUSTRY in which potential differences are monitored using NON Polarizing electrodes or plates
Telluric Methods
Used in the investigation of crust and upper mantle and is more complex than telluric since both electric and magnetic fields must be measured that provides info about the subsurface
Magnetotelluric Surveying
Physical Properties
All boies w/ high electrical conductivity have strong EM Fields
Some which contain insulartors may produce secondary EM Fields if conductive accessory minerals are suffieicient
Most conductive VMS
Chalcopyrite and Pyrrhotite
Weak Conductors
Sphalerite and barite
Profilinvs vs Sounding
Takig a series of reading ALONG A LINE using a fixed configuration of coils or electrodes (Moving but fixed set up ng electrodes)
Sounding only one lcocation with several coil spacing and orientations (Stationary but different setup)
Max depth for Groun surveys
500 m
Max depth for Airborne
50 m
