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#379303 - 06/07/10 06:52 PM Underwater Seismology
Marty Offline
by Mike Campbell

For those who do not know Seismology is the science that studies earthquakes and related phenomena and is used in oil exploration. In the old days, on land, holes were drilled at intervals and pots (geophones) were placed at specific intervals and then you would shoot the holes. That meant blasting the holes with small explosives in a precise sequence and recording the seismic wave that is reflected through and by the substrata. These waves were recorded on LONG rolls of paper (logs) and studied in detail by geologists to predict the presence of formations likely to produce oil. Most likely the source of the seismic wave has been upgraded and there are certainly no longer long rolls of paper. Instead there are computers and programs to aid the geologist is his search for oil.

Seismic data acquisition of sedimentary layers in a seabed beneath a large body of water such as an ocean has traditionally been used to acquire images of underlying oil fields to facilitate the recovery of oil reserves. Such data acquisition enables offshore drilling sites to be established by indicating possible locations in which to extract oil. Seismic data acquisition involves generating seismic waves from a source and receiving or "listening" to a reflected or returning wave that carries information about the medium through which it has passed.

Conventional seismic sound sources for underwater seismic data acquisition have typically operated by mechanically generating sound from the rapid release of compressed air using an air gun, or from the mechanical impact of metal on metal for some other applications.

Air guns operate near the ocean surface, often approximately 7-10 m below sea level, and operate by firing a pulse that, though partially directed downward, is essentially omni-directional. A great deal of the air gun's energy is reflected off the seabed and remains trapped in the water column. This causes two immediate problems. First, to image properly deep reservoir targets, a large energy pulse needs to be generated and since the pulse length is short for acceptable seismic resolution (i.e., the ability to image thin layers), the sound levels need to be high. This large energy pulse is central to environmental concerns for marine life.

The second problem is the high sound level trapped in the water column. As noted above, most of the energy bounces off the seabed and is reflected back toward the surface. However, the sea surface is also reflective and sends the energy back down. This echo bounces off the seabed and the process repeats itself. These water bottom "multiples" are typically very large in amplitude and tend to mask the desired reflection data from the deep sedimentary layers. Although techniques have been developed for removing these water bottom multiples, this inherently requires additional processing and risks disrupting the actual data that is desired from the original reflection.

In addition to the environmental concerns and the high sound level trapped in the water column, the use of an air gun is relatively primitive in the type and amount of data that can be carried in a reflected signal. Moreover, the air guns typically need to be dragged along behind a vessel or attached in some way near the surface of the water body, which requires additional equipment, time, and effort and could get in the way of fishing nets or any other equipment that operate in the few meters below the surface where the air gun operates.

The type of seismic wave generation that would be used in Belize waters has yet to be established but probably the cheapest available method would be used. A call to Mr. Pedro Cho revealed that there are no current plans to conduct seismic activities in Belize waters. There is one company doing desk geology concerning their PSA but at present they have no plans to start a seismic survey. This would be their next step after completing the desk geology.

Although offshore drilling has proven safe in most cases the impacts of a seismic survey in the shallow waters of Belize on the fish population, breeding habits and other poorly understood factors must be considered and studied before seismic exploration is allowed. Studies have shown relationships between species behavioral changes and seismic activities that are not fully understood. Shallow water in drilling lingo is less than 2,000’. Most of the major species including the Nassau Grouper have spawning aggregations in certain areas outside the reef in little understood processes. It would seem that seismic wave generation unless using the most advanced equipment would likely affect these aggregations and if carried out inside the reef probably have a higher impact. Fish seem to be particularly sensitive to noise and sound travels several times faster underwater than in the air. As we all know throwing dynamite into the water kills surrounding fish due the the sonic wave created.

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#379334 - 06/07/10 11:18 PM Re: Underwater Seismology [Re: Marty]
elbert Offline
Thanks for bring this up Mike and thanks for all the watchdog work you've been doing lately.
http://www.richardmoorenature.com/Article/962123
http://protectfloridasbeaches.org/pdf/Impacts_of_Seismic_Surveys_AMCC.pdf
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#379368 - 06/08/10 01:50 PM Re: Underwater Seismology [Re: elbert]
Rigrat Offline
This however is as old as the hills and would not be the method employed in Belize.

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#379374 - 06/08/10 02:20 PM Re: Underwater Seismology [Re: Rigrat]
Mike Campbell Offline
Unfortunately that is something that is outside your ability to know as it is mine. Whatever is used in Belize will be cheap. Bet on that.

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#379398 - 06/08/10 05:56 PM Re: Underwater Seismology [Re: Mike Campbell]
Katie Valk Offline
Thats right Mike, the plan is for cheap. One of the people who has a license for Glovers (yeah, Glovers atoll) plans to set a drill rig on a huge barge, tow it down from TX, sink it in the shallow shoals within the atoll itself and drill. This family is in the oil biz in the US and has land investments in Belize going back 40 yrs or more. Nice postcard, Greetings from Glovers Atoll, with Love and Oil.
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#379488 - 06/09/10 03:49 AM Re: Underwater Seismology [Re: Katie Valk]
Mike Campbell Offline
Sounds like it would make an excellent artificial reef. Wonder how many boats it would take to pull it over? Just in theory you understand.

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#379593 - 06/10/10 03:45 AM Re: Underwater Seismology [Re: Mike Campbell]
Rigrat Offline
Originally Posted By: Mike Campbell
Unfortunately that is something that is outside your ability to know as it is mine. Whatever is used in Belize will be cheap. Bet on that.


Actually Mike, you are wrong (again). The method that you posted would not be used in Belize. It hasn't been used in the past when most of the seismic data was obtained, and as it is not suitable for Belizean waters wouldn't be used here.
The most likely method would be Seismic While Drilling, which gives a far more accurate picture of what is going on.

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#379594 - 06/10/10 03:55 AM Re: Underwater Seismology [Re: Katie Valk]
Rigrat Offline
Originally Posted By: Katie Valk
Thats right Mike, the plan is for cheap. One of the people who has a license for Glovers (yeah, Glovers atoll) plans to set a drill rig on a huge barge, tow it down from TX, sink it in the shallow shoals within the atoll itself and drill. This family is in the oil biz in the US and has land investments in Belize going back 40 yrs or more. Nice postcard, Greetings from Glovers Atoll, with Love and Oil.

Then after drilling the well and plugging and abandoning it, the barge would be refloated, and towed away. There would be nothing left to see.

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#379609 - 06/10/10 05:33 AM Re: Underwater Seismology [Re: Rigrat]
Mike Campbell Offline
You mean Katie is right! First time for everything I guess. Come on Rig this plan cannt be good. You specifically told me that no PSA had been signed for outside the reef. Whats with that? Did you forget Glovers Reef and off shore North Ambergris Caye? Ever hear of world heritage site. I said it was not known what type of seismic survey would be used here and that is true. You may pretend to know and it may prove you made a good estimate but you dont know, me neither nor did petroleum when I called. Please explain drilled plugged and abandoned. Are we talking about a system of underwater pipes through the atoll?

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#379617 - 06/10/10 06:38 AM Re: Underwater Seismology [Re: Mike Campbell]
Rigrat Offline
Originally Posted By: Mike Campbell
You mean Katie is right! First time for everything I guess. Come on Rig this plan cannt be good. You specifically told me that no PSA had been signed for outside the reef. Whats with that? Did you forget Glovers Reef and off shore North Ambergris Caye? Ever hear of world heritage site. I said it was not known what type of seismic survey would be used here and that is true. You may pretend to know and it may prove you made a good estimate but you dont know, me neither nor did petroleum when I called. Please explain drilled plugged and abandoned. Are we talking about a system of underwater pipes through the atoll?

Mike.

I am pretty sure that I will know the exact technique to be used before most people in Belize. We regularly get updates from the seismic companies about what equipment is where and also what contracts have been signed. No seismic company has signed a contract for a trawled array vessel as you have suggested to go to Belize for the foreseeable future (several years)
then, no, it doesn't mean Katie was right. The chances of that are very slim, but that is how a lot of barge drilling works.

At the moment, there are no drilling PSA's in force. Only PSA's that cover gathering or interpreting seismic. Once the seismic has been done and Belize gets to the second or third renewal period, then a request to drill will be submitted and may or may not be approved.
At the moment drilling is at least two years away. There are no plans by the government to allow drilling inside the reef, or on the reef, nor as I see it in any national park. Drilling may be carried out in shallow, medium or deepwater outside the reef, as per the block areas outlined in various PSA's (get the block areas off the geology department) but as yet there are no plans submitted by oil companies to do so.
When you drill an exploratory well, you do it for the purpose of seeing what is there, and how well any hydrocarbons will move within the rock. Then you fill up all the bottom of the well with bridge plugs and cement, cut and pull the upper casing, and fill the void with more cement. Then you remove the wellhead. The top hole will collapse and fill the void. Nobody will know anything had been there except for a dot on the map in the geology department.
Then after all data has been analysed, the company may elect to take a further renewal of their PSA. They can then submit a request to drill and if granted, they can drill appraisal wells. These are again exploratory, designed to find out the extent of the reservoir. These are often plugged and abandoned as well, but may be temporarily suspended for re-entry at a later date to turn them into producers or injectors. Temporary suspension means the bridge plugs and cement, but you don't cut and pull the casing.
In this case in shallow water you will have a drilling conductor sticking up out of the water with a capped wellhead on top. (No Christmas tree). In deep water there will be nothing to see at all as all of the equipment is down below on the seabed and is covered with an over-trawlable structure.

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