TEORIA DELLA SISMICA A RIFLESSIONE

•DIFFRAZIONE•SORGENTI e RIVELATORI•FILTRAGGIO•NORMAL MOVEOUT•MIGRAZIONE•DECONVOLUZIONE

Rif. Bibl. -Jones-Marine Geophysics-Robinson-Seismic Velocity Analysis and the Convolutional Model

SORGENTI E RIVELATORI

• La sorgente sismica ideale dovrebbe emettere un impulso di pressione il più breve possibile. Infatti sappiamo che il potere risolutivo di una qualunque radiazione e’ dell’ ordine della lunghezza d’ onda. Per le sorgenti in acqua si verifica sempre l’ effetto bolla (che vedremo nel seguito).

Sparkers• S.T. Knott and l.B. Hersey (1956) discovered

that when a spark is produced by the sudden discharge of a high voltage capacitor bank through an underwater electrode or as in EG&G model a triplet of tripod electrodes, the rapidly expanding bubble of ionized gas provides sufficient low-frequency seismic energy to penetrate several hundred metres of sediment the pulse length was 10-50 ms in function of energy employed.

• The sparker, consisting of an electrode array powered by a capacitor bank of a few hundred joules, has been widely employed as a source for studies of the continental shelf.

• Recently EG&G replaced the electrodes with a brush of tin cable.

• During 2000, Corradi,Giordano and Nicotra, realized SAM 96 a new electrodes linear array with 96 VHF coaxial cable tips with a pulse length of 1 ms at 300 Joule of energy.SAM-96 was employed in Antarctica expeditions starting from 2001 to present.

Direct wave from MULTI-TIP (SAM96 tips) sparker source (400 J) to geophones array (sampled at 8000 samples/sec)

Sorgente Sparker:SAM96

• SAM96:Sparker Array Multitip 96 electrodi• Sviluppato nell’ ambito del PNRA

BOOMERS

An electromagnetic source or boomer was developed during the early 1965 (Edgerton and Hayward, 1964). Seismic energy comes from the sudden separation of an aluminium plate from a flat copper coil embedded in a hard epoxy resin. The plate is initially held against the coil. On rapidly discharging an electrical capacitor bank trough the coil, eddy currents are induced in the aluminium plate causing a rapid repulsion and the creation of a cavitation volume in the water. The first pressure pulse is due to the rapid outward acceleration of the aluminium plate. This is followed by a pulse from the implosion of the cavitation volume.

• Boomers (EG&G Uniboom) with pulse lengths of 0.1-0.2ms have been developed for reflection profiling in water depths of a few metres. These are capable of resolving reflectors less than 0.3 m apart in near-shore sediments (Simpkin and Davis,1993).The two systems have different Radiation Diagram: perfect cylindrical symmetry for Boomer very asymmetric and with large angle in the transverse(respect to navigation) direction for Sparker .

• Boomer and Uniboom are energy limited to 500-1KJ, Sparker to 3-16KJ.

UNIBOOMAn electromagnetic source or boomer was developed during the early 1965 (Edgerton and Hayward, 1964). Seismic energy comes from the sudden separation of an aluminium plate from a flat copper coil embedded in a hard epoxy resin. The plate is initially held against the coil. On rapidly discharging an electrical capacitor bank trough the coil, eddy currents are induced in the aluminium plate causing a rapid repulsion and the creation of a cavitation volume in the water. The first pressure pulse is due to the rapid outward acceleration of the aluminium plate. This is followed by a pulse from the implosion of the cavitation volume.

La strumentazione acustica: La strumentazione acustica: UniboomUniboom

Raggio 0.5 m

 I SISTEMI ENERGIZZANTI A CONFRONTO

Energia

frequenza CANNONI MEAS SPARKER

UNIBOOM CHIRP SUBBOTTOM

8000 Hz

PARTHENOPE

AIR GUN

Forme d’ onda AIR-GUN (Firme)

28 metri (20 idrofoni x1.4m)

f=10 Hz ; lambda=150 m

f=100 Hz ; lambda=15 m