بسم الله الرحمن الرحيم

هذا الموضوع ان شاء الله سوف أشارك به في المجلة يا رب يعجبك يا صالحين

هو عن 4D seismic technology

name: hosny hassan moussa

4th year geophysics

faculty of science

alexandria university

hosny_diab@yahoo.com

موضوع الاخ والمشرف العزيز darsh

موضوع اكثر من رائع وفكره جديده

Seismic Data Applied to Subsurface Interpretation
Introduction
On a fundamental level, seismic data can provide two major benefits. First, seismic data can be acquired over areas that do not have any well data. The scond benefit is that seismic data can provide explicity 2D data as opposed to the 1D nature of wellbore.The data on the seismic line (seismic section) may actually represent a very complex 3D subsurface geologic world.
Assumptions and Limitationsg
to make a clear interpretation for an area, data used must be properly acquired and processed appropriately. some possible major problems include severe horizontal velocity gradient, high noise area, or an extremely complex geologic subsurface. all of which may invalidate many assumptions necessary for the acquisition of good data. If these problems are present, they may present a challenge for even the best geophysict. In such instances, the expertise of geophysict may be absolutely necessary to solve the complxities. Before totally discarding the use of geophysics in these areas, you should recognize that if geology is complex enough to cause these severe problems, it it is not going to be a trivial exercise to find the solution on well data alone. Even in these complex areas, seismic data may contain valuable information that can be used in creating a reasonable subsurface interpretation.
The process
The procedure for making subsurface maps using seismic data is very similar to the sequence of steps used in constructing interpretations from subsurface well log information. The first step is one of the DATA VALIDATION, i.e.. analysis of what the seismic data represent. Do the seismic data actually have some relationship to the geology in the subsurface? This procedure is similar to the checking one does when a log is first used. In the case of log data, desisions about the validity and meaning of the log response must be made before the data can be used to form an interpretation.
The second step is the actual INTERPRETATION of the seismic section. This step is analogous to the correlation of well logs when using just subsurface well log information. Because the validity of the remaining work rests on having an accurate and geologically correct interpretation of the seismic data. this is the most important part of the process.
The third step involves EXTRACTING THE INFORMATION from the seismic data and transferring it onto the map so that it can be used effectively. Usually, transferring the data to a map is referred to as posting. This procedure is practically indentical to that used when recording subsurface well log data. Since seismic sections have 2D aspect that well log data do not possess, there are some unique aspects to this step when using seismic data. This step also represents the merger of the subsurface and the seismic information. Both types of data should be posted and used to construct the final interpretation.
An appeal is made to geophysicts as well as geologists. Very often, there are two sets of maps: the geological map and the geophysical map. A subsurface map should accurately represent the geology and both well logs and seismic data represent physical measurements of the subsurface. There is only one configuration of tdhe subsurface, and it is the job of the mapper to create an integrated and reconciled interpretation using all the data available.
The last step is the CONSTRUCTION OF THE SUBSURFACE GEOLOGICAL MPAS. This step represents the culmination of the previously work, and in many instances will be the result by which your work is measured. It is only as good as the information it contains, so do not rush to begin this step before th previous steps are completely finished.
In practice, though, this is never the last step. Several iterations of validation, interpretation, posting, and mapping are typically necessary before a satisfactory subsurface map is completed.
At some point it will become apparent that most of the major questions have been resolved and a satisfactory map has been made. While pride and satisfaction in the result is deserved, always keep in mind that additional data from either drilling or addiltional seismic shooting will almost always change some of your ideas. These changes may be small, but subsurface mappping can never perfectly represent the structural configuration of the earth. The more your ideas are actually tested, the more obvious it becomes that mapping is both art and science. Ideally, you will asymptotically approch the truth as more data become available. The measure of an interpreter is his or her ability to approch the truth quickly with the limited data available.

Novarupta



The Most Powerful Volcanic Eruption of the 20th Century








Location of the 1912 Eruption.

June 6th, 1912



The morning of June 6th arrived on the Alaska peninsula to find the area which is now Katmai National Monument being shaken by numerous strong, shallow earthquakes. The most powerful volcanic eruption of the 20th Century was about to begin – but very few people knew about it. The Alaska peninsula has a low population density today but it 1912 it was even lower. Beyond the land shaken by the earthquake activity the beginnings of this event were almost unnoticed.



Volcanic Monitoring - 1912 vs. Today



Today the stirring of an important volcano draws enormous global attention. Weeks or even months before most large eruptions a buzz circulates through an electronically-connected community of volcano scientists as clusters of small earthquakes are detected by a global array of seismographs. Many scientists working at diverse global locations interpret this data and begin to collaborate about an awakening volcano and the eruption that might follow. Reports are posted on the internet and news stories communicate the volcano's activity to millions of people. Often it is a false alarm – the volcano is simply stirring.










People in Juneau, Alaska, about 750 miles from the volcano, heard the sound of the blast – over one hour after it occurred.
If the earthquakes strengthen and begin moving upwards, many of these scientists will travel to the area of potential eruption to make observations and set up a local network of data-gathering instruments.

However, in 1912, Alaska was not a US state, very few scientists were supported to do volcanic studies and a worldwide network of seismic monitoring was not in place. Scientists were just starting to understand the mechanics of volcanic eruptions.









The relative size of the Novarupta eruption compared to others
on the basis of cubic miles of magma ejected. USGS Image.






Forty years after the eruption, investigators finally realized that Novarupta - and not Katmai - was the source of the tremendous blast.

Novarupta Volcano Erupts!



On June 6th, 1912 a tremendous blast sent a large cloud of ash skyward and the eruption of the century was underway. People in Juneau, Alaska, about 750 miles from the volcano, heard the sound of the blast – over one hour after it occurred.

For the next 60 hours the eruption sent tall dark columns of tephra and gas high into the atmosphere. By the time the eruption ended the surrounding land was devastated and about 30 cubic kilometers of ejecta blanketed the entire region. This is more ejecta than all of the other historic Alaska eruptions combined. It was also thirty times more than the 1980 eruption of Mount St. Helens and three times more than the 1991 eruption of Mount Pinatubo, the second largest in the 20th Century.









USGS Topographic Map of the Novarupta/Katmai Area
Provided by MyTopo.com.


Impact of the Eruption



The inhabitants of Kodiak, Alaska, on Kodiak Island, about 100 miles away, were among the first people to realize the severity of this eruption. The noise from the blast would have commanded their attention and the visual impact of seeing an ash cloud rise quickly to an elevation of 20 miles then drift towards them would have been terrifying.

Within just a few hours after the eruption a thick blanket of ash began falling upon the town - and ash continued falling for the next three days, covering the town up to one foot deep. The residents of Kodiak were forced to take shelter indoors. Many buildings collapsed from the weight of heavy ash on their roofs.

Outside, the ash made breathing difficult, stuck to moist eyes and completely blocked the light of the sun at mid-day. Any animal or person who was caught outside probably died from suffocation, blindness or an inability to find food and water.









Satellite image of the Novarupta / Katmai area showing
the geographic extent of the pyroclastic flow (yellow) and
ash deposit contours (red). Image by J. Allen (NASA)
using data from University of Maryland’s Global Land
Cover Facility. Cartography by B. Cole, Geology.com
Medium Resolution 164 KB. High Resolution 1330 KB.


Pyroclastic Flow



Back on the peninsula, heavy pyroclastic flows swept over 20 kilometers down the valley of Knife Creek and the upper Ukak River. (A pyroclastic flow is a mixture of superheated gas, dust, and ash that is heavier than the surrounding air and flows down the flank of the volcano with great speed and force.)

These flows completely filled the valley of Knife Creek with ash, converting it from a V-shaped valley into a broad flat plain. By the time the eruption was over the world’s most extensive historic ignimbrite (solidified pyroclastic flow deposit) would be formed. It covered a surface area of over 120 square kilometers to depths of over 200 meters thick near its source. (The satellite image at right shows the original geographic extent of pyroclastic flow deposits as a yellow line.)



Volcanic Ash



Immediately after the June 6th blast, an ash cloud rose to an elevation of about 20 miles. It was then carried by the wind in a westerly direction, dropping ash as it moved. The ash deposits were thickest near the source of the eruption and decreased in thickness downwind. (The satellite image above has red contour lines showing the thickness of the ash deposits in the area of the eruption. Measurable thickness of ash fell hundreds of miles beyond the one meter contour line.)








Valley of Ten Thousand Smokes. Photo taken in 1991
by R. McGimsey, U.S. Geological Survey.

When the eruption stopped on June 9th, the ash cloud had spread across southern Alaska, most of western Canada and several U.S. states. Winds then carried it across North America. It reached Africa on June 17th.

Although the eruption had these far-reaching effects, most people outside of Alaska did not know that a volcano had erupted. More surprising is that no one knew for sure which of the many volcanoes on the Alaska peninsula was responsible. Most assumed that Mount Katmai had erupted but they were wrong.



Valley of Ten Thousand Smokes



After the eruption, the National Geographic Society began sending expeditions to Alaska to survey the results of the eruption and to inventory the volcanoes of the Alaskan peninsula. Robert Griggs led four of these expeditions. During his 1916 expedition, Griggs and three others traveled inland to the eruption area. What they found exceeded their imagination.








Katmai Caldera. Photo taken in 1990. NOAA Corps.

First, the valley of Knife Creek was now barren, level and filled with a loose, sandy ash which was still hot at depth. Thousands of jets of steam were roaring from the ground. Griggs was so impressed that he called it the “Valley of 10,000 Smokes”.

James Hine, a zoologist on the expedition described the location: “Having reached the summit of Katmai Pass, the Valley of Ten Thousand Smokes spreads out before one with no part of the view obstructed. My first thought was: We have reached the modern inferno. I was horrified, and yet, curiosity to see all at close range captivated me. Although sure that at almost every step I would sink beneath the earth's crust into a chasm intensely hot, I pushed on as soon as I found myself safely over a particularly dangerous-appearing area. I didn't like it, and yet I did.”









Novarupta lava dome. Photo taken in 2003
by S. Smith, U.S. Geological Survey.


Katmai Caldera & Novarupta Dome



During the eruption a large amount of magma was drained from magma chambers below. The result was a removal of support from beneath Mount Katmai which is six miles from Novarupta. The top several hundred feet of Katmai - about one cubic mile of material - collapsed into a magma chamber below. This collapse produced a crater about two miles in diameter and over 800 feet deep.

Early investigators assumed that Katmai was responsible for the eruption. This assumption was based upon Katmai being near the center of the impact area, Katmai was visibly reduced in height, and early witness accounts thought that the eruption cloud ascended from the Katmai area. Closer observation was not possible and expeditions into the impact zone would be very difficult to accomplish.








Novarupta was a very high latitude eruption.

The first scientific investigation to get an up-close look at the eruption area did not occur until 1916 when Robert Griggs found a 2-mile-wide caldera where Mount Katmai once stood. He also found a lava dome at the Novarupta vent. These observations convinced Griggs that Katmai was the source of the eruption.

It was not until the 1950s - over forty years after the eruption - that investigators finally realized that ash and pyroclastic flow thicknesses were greatest in the Novarupta area. This discovery produced a revelation that Novarupta - and not Katmai - was the volcano responsible for the eruption (see satellite image medium resolution, 164 KB or higher resolution, 1330 KB). This is possibly the most important false accusation in the history of volcanic study.



Could Novarupta Erupt Again?










Valley of Ten Thousand Smokes. Photo taken in 1991
by R. McGimsey, U.S. Geological Survey.

Other large eruptions on the Alaska peninsula are certain to happen in the future. Within the last 4000 years there have been at least seven Novarupta-scale eruptions within 500 miles of where Anchorage is located today. Future activity is expected because the Alaska peninsula is on an active convergent boundary.

These large eruptions will have enormous local and global impact. Local impact will include the lahars, pyroclastic flows, lava flows and ash falls that are expected from a volcanic eruption. These can result in a significant loss of life and financial impact. The activity of these volcanoes is monitored by the United States Geological Survey and others so that eruptions can be predicted and their events mitigated.

Large eruptions of Novarupta's scale at high latitudes can have a significant impact upon global climate. Recent studies have linked high latitude volcanic eruptions with altered surface temperature patterns and low rainfall levels in many parts of the world. The 1912 eruption of Novarupta and other Alaska eruptions volcanoes have been linked with drought and temperature changes in northern Africa.








Landsat Image of the Novarupta / Katmai Area - 1990
Higher Resolution - Landsat Tutorial






An eruption the size of Novarupta would ground commercial jet traffic across the North American continent.


Another significant impact is the distribution of volcanic ash. The image at right shows the ash fall impact areas for five important volcanic eruptions of the 20th century. Augustine (1976), St. Helens (1980), Redoubt (1990) and Spurr (1992) all produced ash falls of significant regional impact. However, Novarupta's ash fall was far greater than any other Alaska eruption in recorded history and contained a greater volume than all of the Alaska eruptions which have been recorded combined.

One of the most important reasons to monitor volcanic eruptions is the potential danger that they present to commercial air traffic. Jet engines process enormous amounts of air and flying through finely dispersed ash can cause engine failure. Impacting the tiny ash particles at high speed is very similar to sandblasting. This can "frost" the jet's windshield and damage external parts of the plane. Before the danger of flying through finely dispersed ash was appreciated several commercial jets were forced to land after sustaining serious damage while in the air. Eruptions the size of Spurr, Augustine, Redoubt and St. Helens can damage jets flying over 1000 miles away. An eruption the size of Novarupta would ground commercial jet traffic across the North American continent.



What Can We Do About It?



People can not prevent this type of eruption. They can assess the potential impact, develop with the possibility of loss in mind, plan a response, educate the public and key decision makers, and monitor the region where it might occur.

The more you know about a natural hazard, the greater your chances of avoiding injury or loss. We are lucky to have this record of the past.

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بصوا يا جماعه

والله انا بعمل اللي اقدر عليه عشان انجز فيها ونخلصها

بس للاسف مقابلاني مشاكل كتير

كان اهمها ان مينفعش حجم المواضيع دي تتحط كلها

كان لازم الخص كل موضوع لصفحه واحده بس

وطبعا مع مراعاه الحفاظ علي فكره الموضوع الاساسيه

وحاجات تانيه كتير

والله يا جماعه انا مش مقصر

بس ان شاء الله الحوار ممكن يكون

اقصاه اسبوع بالكتير وتكون متعلقه كمان

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