MULTIPLE LOW FREQUENCY (MLF) ANTENNA Authors: Almelu Mangamma V. Hebsur M.Tech (Remote Sensing) ‏ Department of Civil Engineering Indian Institute of Technology Bombay MLF antenna is one of the antennae used in GPR survey. As name suggests the frequency of MLF can be tuned to various frequencies. This animation shows the efficiency of MLF in GPR data collection on an undulating terrain using point mode method.

Learning Objectives: After interacting with this Learning Object, the user will be able to:  explain the point mode method of GPR data collection  explain the relationship between arm length, frequency, and equivalent depth of penetration (i.e. two way travel time) ‏

Definitions of the components: Transmitter Antenna: It is the electronics which sends the Radar pulses into the earth at particular central frequency 2.Receiver Antenna: It is the electronics which collects the reflected radar pulses from the subsurface. 3.Arm-Lengths: Arms are a part of antenna electronics. Their lengths decide the frequencies and can be adjusted to obtain desired central frequencies. 4.Two-way Travel Time: It is the time taken by the radar pulse to go into the ground and come back. It is measured in terms of nanoseconds(ns). It indirectly indicates the Depth of penetration.

Definitions of the components: Point Mode Method: In Point mode at every point the data is collected manually. 6. Radargram: GPR data in an imagery form is called radargram. It is the graph plot between time(ns) and distance (m).

Diagram 1: GPR data collection using Multiple Low Frequency (MLF) antenna One of the arms of the receiver One of the arms of the transmitter

Set up of Multiple Low Frequency Antenna (MLF) – side view of transmitter and receiver Diagram 2:

These arms can be extended for both receiver and transmitter Receiver or Transmitter Antenna Front view of the transmitter/ receiver and extendable arms Diagram 3:

Arm lengthFrequencyMinimum spacing between the receiver and transmitter Equivalent Depth of penetration = two way travel time 60 cm + 60 cm80 MHz1 m400 ns 120 cm cm40 MHz2 m800 ns 180 cm cm35 MHz2 m1200 ns 240 cm cm20 MHz2.5 m1500 ns 300 cm cm15 MHz3 m2000 ns Diagram 3: (contd..) ‏ 1) The table indicates that the frequency decreases with increase in arm length. 2) The equivalent depth of penetration (i.e.) two way travel time is the time taken for the signal to be transmitted to the ground and reflected back. For example if the two way travel time is 400 ns it means that the signal took 200 ns to reach beneath the ground surface and 200 ns to reflect back.

Want to know more… (Further Reading) ‏ Diagram Animation Area Test your understanding (questionnaire) ‏ Lets Learn! Definitions Lets Sum up (summary) ‏ Instructions/ Working area Radio buttons (if any)/Drop down (if any) ‏ Interactivity options Sliders(IO1) ‏ / Input Boxes(IO2) ‏ /Drop down(IO3) ‏ (if any) ‏ Play/pauseRestart Output result of interactivity (if any) ‏ What will you learn Credits

Undulating Terrain with Four Layers Layer1 Layer2 Layer3 Layer4 1m Layer 1 Layer 2 Layer 3 Layer 4 MLF Antenna in Point Mode Method Master Layout: 1m

Undulating Terrain with Four Layers Layer1 Layer2 Layer3 Layer4 1m Layer1 Layer2 Layer3 Layer 4 Animation: 1m

Description of the actionAudio narration Show the undulating terrain with four layers. The MLF appears at first green point. The pulse trigger button is pressed to transmit signals. Show the radargram appearing left to right. Observe the radargram getting generated. Refer to master layout Step 1: The MLF Antenna is collecting data at first point

Description of the actionAudio narration The MLF appears at second green point. The MLF will now collect data at he second point. This point is 1 m away from the first point. The pulse trigger button is pressed to transmit signals. Show the radargram growing left to right from the previous one. Observe the radargram generated. Refer to master layout Step 2: The MLF Antenna is collecting data at second point

Description of the actionAudio narration The MLF appears at third green point. The MLF will now collect data at he third point. This point is 1 m away from the second point. The pulse trigger button is pressed to transmit signals. Show the radargram growing left to right from the previous one. Observe the radargram generated. Refer to master layout Step 3: The MLF Antenna is collecting data at third point

Description of the actionAudio narration The MLF appears at fourth green point. The MLF will now collect data at he fourth point. This point is 1 m away from the third point. The pulse trigger button is pressed to transmit signals. Show the radargram growing left to right from the previous one. Observe the radargram generated. Refer to master layout Step 4: The MLF Antenna is collecting data at fourth point

Description of the actionAudio narration The MLF appears at fifth green point. The MLF will now collect data at the fifth point. This point is 1 m away from the fourth point. The pulse trigger button is pressed to transmit signals. Show the radargram growing left to right from the previous one. Observe the radargram generated. This method of GPR data collection is called the point mode as the data is collected by going to each point manually. This method is very useful to obtain the GPR data for undulating terrain. Refer to master layout Step 5: The MLF Antenna is collecting data at fifth point

) What are the components of MLF Antenna? a)Transmitter b)Receiver c)Antenna Arms d) All of them 2) What is the relation between Length of antenna arms and frequency of antenna? a) As the length of antenna arms increases, Frequency of antenna decreases. b) As the length of antenna arms increases, Frequency of antenna increases. c) As the length of antenna arms increases, Frequency of antenna remains same. d) As the length of antenna arms decreases, Frequency of antenna increases. Questionnaire

) What is the relation between Frequency and Depth of penetration? a) As frequency decreases, Depth of penetration increases. b) As frequency decreases, Depth of penetration decreases. c) As frequency decreases, Depth of penetration remains same. d) As frequency increases, Depth of penetration increases. 4) What is the advantage of MLF over other antennae? a) Point mode data collection on Undulating Terrain. b) Time mode data collection c) Distance mode data collection d) All Questionnaire

Summary The frequency decreases with increase in arm length. The equivalent depth of penetration (i.e.) two way travel time is the time taken for the signal to be transmitted to the ground and reflected back. For example if the two way travel time is 400 ns it means that the signal took 200 ns to reach beneath the ground surface and 200 ns to reflect back. As frequency decreases, the equivalent depth of penetration increases. In point mode method, the GPR data is collected at each point manually. This method is useful for collecting GPR data for an undulating terrain.

Reference websites: Books: Jol, Harry. M., (2009), “Ground Penetrating Radar : Theory and Applications”, 1 st Ed., Elsevier Science. Links for further reading contd..

Research papers: Yelf. R.J. (2007). Application of Ground Penetrating Radar to Civil and Geotechnical Engineering. Electromagnetic Phenomena,Vol- 7,No-18 Sato, M. (2001). Fundamentals of GPR data interpretation. Toholur University, Japan Davis, J.A. (1989). Ground-penetrating radar for high-resolution mapping of soil and rock stratigraphy, Geophysical Prospecting, 37,