Shallow seismic reflection

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Geralt
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Shallow seismic reflection

Post by Geralt »

Hello fellow geophysicists,

We are soon to do some seismic reflection with depth requirements up to 300 m in karst environment (Croatian mountains).
Our mission is to see geological structures up to 300 m depths. Hydrotunnel is planned to be built under mountain passage so our primary is to see faults, fractures and weak zones. We will combine seismic reflection with geoelectrical tomography (which can go to only 150 m with wenner array which is not enough)

Equipment and method we are going to use:
- Seismograph: 24 ch Abem Terraloc PRO 2
- Cables: 2 x 12 ch
- Geophones: 100 Hz vertical geophones
- Trigger: seismic gun digged into ground
- Method: roll-along with 12 active geophones
- Profile length: 595 m
Since this is new method for us, some advices would be welcome :)

Are 28 Hz geophones maybe better choice?
Would a 10 kg sledgehammer be sufficient source? (we dont have access to explosives at the moment)
What is optimal offset to acquire depths from 150 - 300 m? Is 30 m enough?
Doing this method in karst environment is like flashing light through broken glass, so we dont expect very clear reflexes but we dont have any other choice so if anyone has any kind of advice we would be thankfull.

We are going to process data with ReflexW. Its a rather complex software, so if anyone has any "homemade" guide/tutorial specifically for seismic reflection that would be marveollous. We also have WinSeis and VisualSunt but ReflexW seems more powerfull.

Michael_Seman
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Re: Shallow seismic reflection

Post by Michael_Seman »

No 30 m offset is inadequate for target 150-300 m. You would have little normal moveout. Why 30 m? Are the cable takeouts only 3 m apart? I would say you need some 24-channel cables and rollalong switch to use all 24 recording channels. A shotgun source would give you up to 150 Hz, so you would have to consider aliasing as well. I would probably start off with 24 channels at 5 or 6 m spacing and see how it looks. I don't envy you doing that sort of survey.

Geralt
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Re: Shallow seismic reflection

Post by Geralt »

We dont have much experience with reflection hence im looking for advices here. We already did survey.
Geophone takeouts were 5m and source was hammer afterall. We recorded 109 shots with roll along 12 active geopohones. We played in ReflexW, so far we managed to do all steps except topography/static correction. We know this is by far not the best way to survey with reflection like that but its our first time. We did 30m offset bcs we do the same on refraction survey offsets.

Mohamed
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Re: Shallow seismic reflection

Post by Mohamed »

Did you try to contact sandmeier? I'm sure he'll help you, plz keep us updated with the processings & results.

Regards.

Geralt
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Re: Shallow seismic reflection

Post by Geralt »

Yes, we have contacted mr. Sandmeier. Unfortunately he gave us very little info and help since we do survey in karst and with limited equipment. He told us only its gonna be difficult. Anyway, we did survey and results are no good. We did it with 12 roll allong geophones and hammer and we had to get results up to 300 m. In a month or two we are going to retry survey but this time with explosives and 36 active geophones. We hope we are going to get better records now.

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