Applied Seismology: Introduction and Principles
4.5 Seismic energy sources
4.5.7 Animals
Squid 2000 at 1500 J
Delta Sparker at 6000 J
Voltage (V) 1.0 0.8
0.6
0.4
0.2 0.0 –0.2
–0.4 –0.6 –0.8
–1.0 4.5
4.0 3.5 3.0 2.5 Time (ms) 2.0
1.5 1.0 0.5
Figure 4.27 Typical pulses from (A) a squid sparker at 1.5 kJ and (B) a delta sparker at 6 kJ. Courtesy of Applied Acoustics.
advantages of this system over traditional airgun arrays are the minimal disturbance to marine life due to reduced cavitation; a smaller radius of interference away from the source so that it can be operated near production platforms without affecting diving activ- ities; and particularly, that the source characteristics are extremely well known and controlled. The vibrator plates are powered by a pneumatic-nitrogen supply and compressed air, and are deployed in a flotation gantry. Marine vibrators have been discussed in more detail by Baeten et al. (1988).
For smaller scale surveys, a system has been developed by GeoA- coustics Ltd, UK, (now part of Kongsberg Maritime, Norway), called the GeoChirp. The system can be deployed in either an over-the-side mounted assembly or as a towed fish (Figure 4.33). The instrument consists of up to four source transducers with frequency bandwidths of either 2–8 kHz or 1.5–11.5 kHz. An upsweep of frequencies (the
‘chirp’) is transmitted in a pulse lasting either 16 or 32 ms, and repeated 4 or 8 times a second.
Immediately behind the source mounting, which looks like the head of a tadpole, is located an 8-element streamer that is about 1 m long. As the source transducer characteristics have been measured by the manufacturer, the system is programmed with these charac- teristics in order to provide an autocorrelated ‘de-chirped’ analogue output. This can be printed directly on to a hard-copy device, such as a thermal linescan printer. Alternatively, the analogue signals can
be put through a sonar enhancement processing unit that provides facilities for real-time filtering, gain control, and so on, and then output as required.
Another Chirp-type system has been introduced by IXSEA (Echoes 1500), which uses a single transducer repeatable sound source. The single transducer is suspended from a buoy and towed as necessary. The manufacturer claims the system has a 27 cm res- olution.
A shear-wave source that is being developed uses the principle of a vibrating mass embedded within the ground (VibroPile). The source vibrates horizontally, thereby generating shear waves. For reservoir monitoring, a configuration would have 25 installed piles into the seabed with one moveable S-wave source. Each pile is
∼0.6 m in diameter and has a length of about 2 m. The vibrator itself weighs around 1.5 tonnes. The total weight with the submerged power unit is 6 tonnes. A linear sweep through 10 Hz to 80 Hz is used with a duration of 10 s. It is thought that by using shear waves it is possible to study reservoirs beneath gas clouds, to monitor fracture development and to help optimise production in oil fields.
ELECTRICAL
DRIVE SIGNAL SWEEP
GENERATOR
TORQUE MOTOR
PILOT SPOOL
OIL PUMP
MAIN SPOOL
PHASE COMPARE
REACTION MASS START
(A)
(B) RADIO
ACCELEROMETER
GROUND
BASEPLATE
HOLD- DOWN WEIGHT VEHICLE CHASSIS D.C.
AMP
LVDT’s
Figure 4.28 (A) A Vibroseis truck. The weight of the vehicle is transferred to the jack in the middle of the vehicle when operating.
Courtesy of ION Geophysics Inc. (B) Schematic of a Vibroseis truck. Courtesy of BP. [C]
FIELD RECORDING
CORRELATION PROCESS
Klauder wavelet
Single trace C
B
A+B+C
A B C*
(Phase inverted)
A
Pilot sweep shifted in
time
Time
–t = 0 R1
R2 R3 OUTPUT
CORRELOGRAM S* G = H G
S
Field recording Reflection
2 Reflection
1
Reflection 3 Pilot
sweep
Figure 4.29 Schematic to illustrate the process of generating a Vibroseis pilot sweep, acquiring field recordings and correlating the field record with the pilot sweep to obtain the output correlogram with Klauder wavelets.
(A) (B)
Figure 4.30 (A) Elvis land vibrator (box in direct contact with the ground) in use as a SH source using a Geode (box with screen, top left) as the recording seismograph. (B) Mini-Sosie land vibrator source. Courtesy of Geomatrix Earth Science Ltd. [C]
Mass-actuator assembly
±±45°
Figure 4.31 Land vibrator that can be used to generate both P- and S-waves. From Christensen (1992), by permission.
communication over large distances (O’Connell-Rodwell, 2007).
Elephants produce such signals in two ways: (a) by stomping on the ground, and (b) by vocalising at around 20 Hz at high amplitude. An elephant weighing 2720 kg, for example, can stomp on the ground hard enough to generate a signal that has been modelled to travel for 32 km. Vocalisations can be made with amplitudes of 103 dB Sound Pressure Level (SPL) and 90 dB SPL at 5 m, respectively. Such low- frequency vocalisations at high amplitude couple with the ground and form Rayleigh waves that propagate along the ground surface for large distances, and further than can be achieved by the sound waves in air.
Elephants have an additional seismic sophistication in that they can orientate themselves to the sources of low frequency seismic
Figure 4.32 Rayleigh wave generator (left), signal processing and recording equipment (right) and the first of a series of geophones is circled. [C]
signals by using their feet and trunk to detect the phase of the ground vibrations. Rayleigh waves travelling at a speed of 210–250 m/s, for example, in the case of some elephant habitats, have a wavelength of∼12.5 m. The distance between the front and rear feet of an adult elephant (2–2.5 m) makes the phase difference of an incident wave discernable. By lifting a front leg and using its trunk, an elephant can orientate itself to the direction of travel of the incoming ground wave. It it also thought that elephants use simultaneously the difference in the arrival times of the sound wave and the Rayleigh waves to determine the distance to the source of the signals. The animals sense these signals as part of their defence mechanisms. Elephants possess a cartilaginous fat pad in the heel of each foot that is thought to facilitate impedance matching of signals between the ground and the elephant’s body.
Asian elephants have been reported to respond vigorously to earthquakes (Jackson, 1918) or to trumpet loudly at the approach of an earthquake (Nicholls, 1955). Their inconsistent behaviour after the 2004 tsunami in southeast Asia suggests that there is need for further research in this area of earthquake response (Wikra- manayake et al., 2006).