Conventional exploration for oil and gas reservoirs utilizes active seismic surveys. In these surveys, a pulse of seismic energy is generated by either a dynamite explosion or a vibrator truck, and the reflections of the seismic waves from underground structures are sensed by an array of seismic sensors (conventional geophones or newer MEMS sensors). By contrast, passive seismic sensing uses only naturally occurring seismic signals – from earthquakes and microtremors, ocean waves, and similar sources – to image subsurface structures. While conventional active seismic surveying utilizes the frequency range from ~10 – 300 Hz, passive seismic monitors lower frequency waves, which can travel long distances through the earth’s crust without attenuation. Passive seismic monitoring has been used for a long time, for earthquake monitoring as well as basic geophysical studies of the earth’s structure.
In recent years, however, intense interest has developed in using passive seismic sensing for oil and gas exploration due to the recognition that the very low end of the seismic spectrum – under 10 Hz – contains valuable information about the location of hydrocarbon reservoirs . Some have even postulated that this spectral region contains a universal direct hydrocarbon indicator (DHI). This offers the exciting possibility of more accurate reservoir location, reducing the number of dry holes drilled and potentially saving vast sums of money. Passive seismic sensing is necessary for the low frequency spectral region because man-made generators of seismic energy (explosions or sledgehammer-like vibrators) do not produce enough energy at the low frequencies needed.
Passive seismic sensing also requires a different type of sensor than conventional active surveying, because geophones and MEMS seismic sensors do not have sufficient sensitivity in the frequency spectrum below 10 Hz. Up to now, passive seismic has required relatively expensive broadband seismometers or special low frequency geophones, such as the I/O LF-24 and the Geospace HS-10 . Even though the sensors are spaced relatively far apart (~ 1 km vs. 10 m for geophones in conventional surveys), the cost of these instruments as well as their fragile nature makes passive seismic sensing an expensive proposition.
MET Tech’s Economical Low Frequency Seismic Sensing Solution
MET Tech has recently developed a low frequency sensor MTLF-1040 with outstanding performance in the low frequency spectral range. In our testing, the MTLF-1040 showed higher sensitivity than conventional 1 Hz and 4.5 Hz geophones, with less roll-off at low frequencies (Figure 1).
Figure 1. Amplitude vs. frequency for 2 MTLF-1040 sensors in the 0 to 10 Hz band (grey and red traces) compared to two conventional 4.5 Hz geophones (dark blue and light green) and a 1 Hz geophone (HS-10, dark green); cable shorted signal is purple (noise). A 40 Hz low-pass filter was used for all measurements. Note the MET sensors have much higher gain in the LF end of the spectrum than the other tested sensors.
Field Tests
We have conducted tests comparing the performance of the MTLF-1040 with both 4.5 Hz and 1 Hz geophones at a remote site in the Nevada desert. Figure 2-4 show the test location and experimental setup.
Figure 2. Aerial view of test site west of Boulder City, NV
Figure 3. Installation of sensors
Figure 4. Operation of weight-drop (left) and wireless data transmission from sensors (right)
Figure 5 shows the results of the field tests of the MTLF-1040 and both 1 Hz and 4.5 Hz geophones.
Figure 5. Plots of sensor data from the field test using a weight drop. Traces 1 & 2 are MTLF-1040 sensors; 3, 4, 5 & 8 are 4.5 Hz geophones; 6 is a 1 Hz geophone (Geospace HS-10-1); 7 is cable-shorted. Note the greatly increased sensitivity of the MET sensors compared to the geophones.
Unlike competing 1 Hz geophones whose high output impedance requires the use of high input impedance recording systems to avoid overdamping, the MTLF-1040 has low output impedance and can be connected to any recording system. Thus the MTLF-1040 outperforms competing low frequency seismic sensors, is more rugged and convenient to use; and is a very economical choice for passive seismic sensing.
http://en.wikipedia.org/wiki/Passive_seismic
“Is The Future of Seismic Passive?” http://www.aapg.org/explorer/geophysical_corner/2008/07gpc.cfm
http://www.aapg.org/explorer/2007/06jun/passive_seismic.cfm
http://www.geophone.com/techpapers/LF-24%20Brochure.pdf
http://www.geosys.co.jp/GEO/Sensor/hs-10.html
http://mettechnology.com/Products/MTLF1040.pdf
