Two New Applications of Time Reversal Mirrors: Seismic Radio and Seismic Radar Sherif M. Hanafy September 2010

Outline Introduction Seismic Radio Seismic Radar Methodology Field Examples Seismic Radar Summary and Conclusions

Finding trapped miner(s) using seismic data is a three steps process Introduction Finding trapped miner(s) using seismic data is a three steps process First step; before the collapse. Record a group of calibration records (Band-limited Green’s Functions) Second step; after the collapse. Record a SOS call from the trapped miner(s). Third step. Use the ‘one’ SOS call and the ‘many’ calibration records (GF) to find the location of the trapped miner(s).

First Step; before the collapse Introduction First Step; before the collapse Geophones are planted on the ground surface above the mine. Select some communication points inside the mine Receiver Line Ground Surface Subsurface Mine ……………....... G1 G2 G3 Gn

First Step; before the collapse Introduction First Step; before the collapse From each communication point (Gx) we will record the earth’s natural band-limited Green’s function Direct Waves Receiver Line Ground Surface Subsurface Mine Gx

Second step; get the SOS call Introduction Second step; get the SOS call After a collapse occurs, trapped miners should go to the nearest communication point and hit the mine wall at this point This (SOS) call will be recorded by the geophones on the ground surface Receiver Line Ground Surface Subsurface Mine G1 G2 G3

Introduction Now, the problem is: We have many reference GFs (calibration records) We have only one SOS (with low signal-to-noise ratio) Where are the trapped miners? Where did the SOS come from?

We use a gather-to-gather no-shift Intorduction We use a gather-to-gather no-shift crosscorrelation Crosscorrelation results Recorded SOS call Band-limited Green’s function

Third step; where are the trapped miner(s)? Introduction Third step; where are the trapped miner(s)? Does the recorded SOS looks like one of our previously recorded band-limited calibration Green’s functions? NO NO NO Yes G1 G2 G3 ………. Gn The location of the trapped miners is the location of the calibration Green’s functions that best match the recorded SOS Recorded SOS

Results of Trapped Miners (U of U) Steam-Tunnel Test Results of Trapped Miners (U of U) Normalized m(x,0) X (m) X (m) X (m)

Examples of the Results Tucson, Arizona Test Examples of the Results Tunnel 2 Tunnel 3 Normalized m(x,0) Normalized m(x,0) X (m) X (m)

Results with Random Noise Results without adding noise Results with adding noise Normalized m(x,0) X (m) Normalized m(x,0) X (m) New S/N = 1:1738 Normalized m(x,0) X (m) Normalized m(x,0) X (m) New S/N = 1:2670

Unknown SOS Excitation Time 1 Excitation Time Location of Trapped Miner Normalized Amplitude -1 -0.25 45 Time Shift (ms) X (m) 0.25

Re-phrase the problem Find trapped miners in collapsed mines Find seismic source with unknown location and excitation time

For more details, see Hanafy et al.: Using super-stack and super-resolution properties of time reversal mirrors to locate trapped miners. TLE, March 2009, P. 302-307.

Part 1 Seismic Radio

Coding Data Binary coding Telegrams Seismic TRM S = (1010011)Bin = (83)Dec Morse Code None zero stat Seismic TRM Seismic Radio Zero state

Unknown SOS Excitation Time 1 Excitation Time Location of Trapped Miner Normalized Amplitude -1 -0.25 45 Time Shift (ms) X (m) 0.25

Cross Section

More Than One Shot

Sending Seismic Coded message Step 1: Calibration record Record a Green’s function with high signal-to-noise ratio. Step 2: Coded Message Turn on all receivers at the receiving line. Listen to any seismic signal coming from the transmitting station. Step 3: Decode the message Use GF from step 1 to decode message recorded in step 2. Transmitting station Receiving line

Decode Principles Six elements are used to decode the message: Signal is interpreted as "1“. No-signal is interpreted as "0“. (111111) for the beginning of the message. (000000) for spaces between words. (110110) for end of message. Each group of 6-successive bits will represent one letter.

Suggested Code Book Beginning 111111 7 000111 H 010001 R 011011 Space 000000 8 001000 I 010010 S 011100 End 110110 9 001001 J 010011 T 011101 101010 A 001010 K 010100 U 011110 1 000001 B 001011 L 010101 V 011111 2 000010 C 001100 M 010110 W 100001 3 000011 D 001101 N 010111 X 100010 4 000100 E 001110 O 011000 Y 100011 5 000101 F 001111 P 011001 Z 100100 6 000110 G 010000 Q 011010

Part 1 Seismic Radio Field Example

Field Example Data are collected at Tucson, AZ., USA. One Green’s function is recorded with 30 stacks. Couple of messages were send using a sledge hammer. Line of receivers consisted of 72 receivers with 5 m receiver interval

Field Example

Site Sketch

Data Samples Reference Green’s Function Coded Message – No Noises are Added Same Coded Message – Noise are Added X (m) 355 Time (s) 0.5 X (m) 355 Time (s) 15 X (m) 355 Time (s) 15 Signal-to-noise ration lowered by a factor of 340

Decoding Messages Decoding Original Message Decoding Message with Noise

Another Example

Part 2 Seismic Radar

Unknown SOS Excitation Time 1 Excitation Time Location of Trapped Miner Normalized Amplitude -1 -0.25 45 Time Shift (ms) X (m) 0.25

Sources/Receivers Layout If the source location is known, we can find excitation time (Radio application). For many possible source locations distributed in 1D, finding the right location is easy (trapped miners application). What about 2D source matrix? It is also possible (radar application). Transmitting station Receiving line Using 2D receiver matrix will improve the results, since the extra receivers will help enhancing the final results.

Field Example Data are collected at Tucson, AZ., USA. 120 Green’s functions are recorded 5 receiver/shot lines are used with inline offset = 5 m and crossline offset = 15 m. Each line contains 24 shots/receivers.

Field Example

Field Example

Four Different Routes

Four Different Routes

Four Different Routes

Four Different Routes

Field Example Green’s Function Passive Data Receiver No. 120 Time (s) 120 Time (s) 0.5 Receiver No. 120 Time (s) 5

Results of Route 2

Results of Route 2

Results of Route 2 (Velocity) Stations 2 to 11 is 45 m Time is 17.7 s Velocity = 45/17.7 = 2.5 m/s. Stations 108 to 98 is 50 m Time is 17.5 s Velocity = 50/17.5 = 2.9 m/s.

Results of Route 3

Results of Route 3 (Velocity) Stations 25 to 46 is 105 m Time is 21.1 s Velocity = 105/21.2 = 5 m/s. Stations 87 to 74 is 65 m Time is 11.9 s Velocity = 65/11.9 = 5.5 m/s.

Sensitivity Test

Implication This concept can be used to track moving objects, find movement speed, and get an idea about the moving object (human, cars, etc.) Secure important areas and buildings is the main applications. Remotely monitoring activity in an area of interest is another application

When it does not work? If the area of interest has many activities, such as in cities, near road; simply where many moving objects are expected.

Summary and Conclusion Time reversal mirrors can be used to send coded messages with seismic waves. Time reversal mirrors can also be used to track moving objects for location and speed. Two field tests are made to test the accuracy of each proposed method.

Thank You