Introduction
Seismic Mapping refer to technique, exploration and geophysics methods that utilizes principles of seismology to estimate contents of the Earth’s subsurface from recorded seismic waves. This paper seeks to discuss Reflection Seismology as one of the methods used in seismic mapping. According to Blewett and Kennett (2014) this method uses seismic source of energy like dynamite or seismic vibrator to estimate components of Earth subsurface. The search for hydrocarbons and other Earth subsurface resources have seen researchers come up with ways through which these can be detected (Blewett and Kennett, 2014; Wencai, 2013). As these substances are found deep underground, Seismic reflection has helped in revealing substances that are present beneath the ground (Blewett and Kennett, 2014; Wencai, 2013). Seismic Mapping apart from locating available hydrocarbons and Erath subsurface resources is sometimes used to monitor Earthquakes. Seismic Mapping therefore has several applications in human activities today.
Steps in Reflection Seismology
The general principle that underpins seismic reflection is sending elastic waves, usually done using strong energy source such as vibroseis or dynamite explosion, into the Earth. Each layer of the Earth according to Blewett and Kennett (2014) reflects part of the waves but allows the rest to refract through. This reflected energy is recorded over a period of time using receivers that sense motion on the ground. There are four basic steps that are used in Seismic Mapping. In these four Steps, specialized equipment and experts are used to come up with accurate data. Seismic Reflection method according to Blewett and Kennett (2014) is the most common Seismic Mapping used today. Apart from Seismic Reflection Blewett and Kennett (2014) state that there are other methods too used in Seismic Mapping. The use of Seismic Reflection is because it has been used for a long time and has yielded positive results. The following are the majoy steps that are followed during Seismic Reflection.
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Step 1: the first step in reflection seismology involves identifying a location in which the study is to take place. According to Blewett and Kennett (2014) because of the expensive nature of Seismic Mapping, it required of experts to identify potential locations that may yield positive results. This is to avoid possible waste of resources. Apart from just identifying location, those involved in the study set up equipment that will be needed in the whole process. That includes points where the waves will be sent into the Earth Subsurface and other points where the data will be recorded (Blewett and Kennett, 2014; Wencai, 2013). This whole process requires accuracy as the experts will have to identify the exact points where the ray from the source will be reflected before the data can be recorded. Blewett and Kennett (2014) assert that identifying location is often guided by suspicion of presence of hydrocarbons or any other subsurface resources. However Blewett and Kennett (2014) categorically state that a pre-study is usually conducted to determine the prospects before finally embarking on Reflection Seismology. Blewett and Kennett (2014) add that is some places the studies have gone without mapping any substantial findings.
Step 2: After identifying the location, in order to come up with a seismic image of the Earth’s Subsurface, a Seismic source using high energy wave source such as dynamite is used after which the reflected wave is recorded. The energy source used must be strong in order to penetrate the Earth’s surface. This explains why a dynamite explosion or vibroseis is preferred. Using a weak energy wave source may not yield any results since it may not be able to penetrate the Erath surface. There is a line that separates the lower velocity Earth surface and higher velocity Earth Surface. Not all the energy wave from dynamite is reflected. According to Blewett and Kennett (2014) some of the waves are refracted. The waves reflected hits the line separating the, lower and higher velocity to bounce back to the Earth surface (Blewett and Kennett, 2014; Wencai, 2013). The remaining energy because of traveling from low too high density surface is refracted. In Seismology reflection, the point of interest is in the reflected ray. It is the reflected ray that is recorded to come with seismic images (Blewett and Kennett, 2014; Wencai, 2013). Figure two below shows how wave from the source is reflected by to the Earth surface for recording while some are refracted in the Earth subsurface. Blewett and Kennett (2014) argue that the reflected ray provides an image of what is present in Earth Subsurface.
Step 3: This step involves acquisition of data that come from the reflected energy wave. By using equipment such as geophones, the reflected wave is converted and recorded at the set stations (Figure 3) (Blewett and Kennett, 2014; Wencai, 2013). The deviation of the measured movements from the baseline is referred to as seismic response (Blewett and Kennett, 2014; Wencai, 2013). This is what is analyzed to understand the subsurface of the earth (Blewett and Kennett, 2014; Wencai, 2013). In order to assist in recording data, complex computer systems as shown in are used. Usually, the distance between the Seismic Source and Recording Staion is calculated accurately in order to identify the exact location where the reflected ray will be collected. Recording or data are read by geophones. According to Wencai (2013), geophones record the data and convert them into computer readable information. Sometimes the data may come from different points depending on the number of reflected energy waves (Wencai, 2013).
Step 4: This is the last step and involves analyzing, processing and interpreting seismic images recorded. The data collected is fed into computers to produce Seismic images and sections (Blewett and Kennett, 2014; Wencai, 2013). From the images produces, geological structures that exist in Earths subsurface are identified, mapped and interpreted. Analyzing, processing and interpreting seismic images are done by people aided by computers. Wencai (2013) assert that in cases where there maybe point of interest, then it is possible that the recorded images will show the mappings. Often, experts use Seismic Mapping of other places to compare the findings and identify some of the features that may not be present in other places. Seismic Mapping is a process that takes over a period of time and is often done repetitively. That is, upon recording and interpreting data, the whole process is done to confirm and record any changes that may happen over some period. The whole process once is complete then all the readings and data recorded over the whole period is compiled for a final report.
Seismic Mapping in UK region
Interest has grown over the years in the UK for onshore oil and gas deposits. According to Selley (2012) Shale gas in the UK has attracted attention since 2007. This has seen according to Selley (2012) increased seismic exploration activities. Of main interest has been Bowland Basin which as of June 2013 had seen many wells drilled. Selley (2012) argues that Curadrilla Resources started the exploration activities in August of 2010. After a series of tests and studies, in 2011 September, the exploration company announced what it termed as a huge discovery in Fylde Coast in Lancashire. Another Company IGas Energy in 2011 and 2012 found gas, Bowland Shale which according to Selley (2012) was found to be thermally mature. Seismic Mapping in the UK in the past seven years has been restricted more to oil and gas mapping.
Blewett and Kennett (2014); Wencai (2013) argues that Seismic Mapping is used in modern societies to identifies oil and gas trapped in Earth Subsurface. It is usually a long process because sometimes it involves trial and era. By using the technique however, experts from IGas Energy and Resources have been able to identify huge deposits of gas in Lancashire County in the UK. As argued by Wencai (2013), seismic mapping is a process that is done over a period of time before finally presenting a complete data. The Lancashire shale gas finding according to Selley (2012) took Curadrilla Resources 11 months to finally announce its findings. Seismic Mapping since the debate on shale gas started in 2007 in the UK has increased significantly (Selley, 2012). The government has awarded licenses to several companies to Map onshore oil and gas deposits that may have commercial value.
British Geological Survey has been at the center of all this Seismic Mapping activities (Selley, 2012). The body is mandated to supervise and monitor the exploration activities of licensed companies and ensure they are within specified law (Selley, 2012). The Figure 6 provided below show regions where Seismic Mapping activities are very active. The regions marked in Red indicate where Seismic Mapping activities currently taking place in such of oil and gas deposits. Some of the regions such as Lancashire had already been confirmed. There are however still active explorations according to Selley (2012) to discover more deposits. The deposits at Lancashire were estimated at 1,329 trillion cubic feet by the British Geological survey with a possibility of having more deposits in other regions within the County. The geological survey body has recently intensified its research and Seismic Mapping activities in Lancashire. This is with an aim of providing the exact estimates of shale gas deposits in the county. According to Selley (2012) this was prompted by an over sensationalized findings that were announced by Curadrilla Resources. BGS has also stepped its Seismic Mapping activities to establish how safe it would be to extract the deposits without creating seismic tensions that may result into earthquakes. This came especially after some activities of IGas Energy and Curadrilla Resources triggered a minor Earthquake in areas of explorations.
Conclusion
Seismic Mapping is techniques and methods that estimate the contents of Earth subsurface. These methods and techniques are commonly used to map deposits of hydrocarbons and other fossil fuels. Reflection Seismology is a common method that has over the years been used extensive in geological and geophysics surveys. It involves sending a strong energy wave into the Earth subsurface and recorded data from the reflected wave. It is the reflected wave that is used to create seismic images for interpretation. Reflection Seismology is a long process that requires extensive research and time in order to come up with accurate readings. The principles used in Seismic Mapping have been used in the UK significantly to establish the presence of shake gas. This has been a point of interest in the UK and formed public opinions since 2007. After a series of tests and experiments, Curadrilla Resources and IGas Energy, in 2011 and 2012 announced finding of husge deposits of shale gas in the Lancashire County in UK.
