Three (3) seismic Acquisition environments
Seismic Data Acquisition is the one amongst the three (3) major phases in a seismic project, while seismic Data processing and interpretation forms other two (2) phases. Seismic Data Acquisition can be defined as the systematic process of gathering seismic data about a particular geological terrain by using various seismic sensors.
Knowing the nature of a site where your seismic data have been collected is worthwhile as this may contribute to the success of a seismic project in one way or another. Despite there are variety of geological terrain, this post is tailored to seismic surveys aiming at searching for Petroleum deposits (Oil and gas). Through this post you will get familiarized with the basic environment at which seismic surveys are carried out.
What is a sedimentary Environment?
Sedimentary Environment can be defined as the part of the Earth's surface which is distinct from surrounding terrain in terms of physical, chemical and biological aspects.
I have used the term sedimentary environments and not either igneous or metamorphic as sedimentary covered most of the Earth's surface such as 75% of the whole environment. Also most petroleum deposits are formed within these environments. Also the biological aspects such as fossils are well preserved in these rocks. So most seismic projects are done in these environments.
The biggest factor to consider these environments is due to the presence of layers (bedding planes) that made it easy for seismic reflection imaging as layers (planes) creates boundaries at which when there is difference in density (acoustic impedance) will cause bouncing of seismic rays hence reflection occurs.
Most of us have thoughts that sedimentary environment occurs only due to deposition of sediments, However geologists confirmed that sedimentary environment can be either a site of erosion, non - deposition or deposition itself.
Although erosional environments are not preserved in stratigraphic records while non - depositional environments are those which are commonly preserved in the stratigraphic records as unconformities.
Depositional environments are those that are actually preserved in the stratigraphic records in the form of Sedimentary Facies.
Since this post is related to petroleum deposits which are the form of sedimentary deposits, First it will be very interesting if we will get familiarized with the term Sedimentary Facies.
What are Sedimentary Facies?
Sedimentary Facies are the bodies of sedimentary rocks distinguished from other rocks of the same geologic time by their Lithology, geometry, structures, Fossils and paleocurrent. Sedimentary Facies represent a particular depositional environment and depositional process.
Through Sedimentary Facies analysis at which several unit facies can be combined into a characteristic association of a given environment which in turn provide the basis of the Facies Modeling that can be used to reconstruct a particular sedimentary environment how it look at that past time as Palaeo-environment (Ancient environment).
This is related to seismic Facies Analysis in Seismic stratigraphy which is the one among the geologic approaches to interpretation of seismic data.
Reconstruction of Palaeo environment can be used as the predictive tools to assess the distribution of the subsurface geology and help for the searching of hydrocarbon (oil and gas deposits) with the final goal of attempting to define particular evolution of the Sedimentary basin throughout the geologic time.
What are the characters of Sedimentary Facies?
5 characters of Sedimentary Facies.
As described from the definition, here above any sedimentary Facies are characterized by five (5) parameters as briefly explained here below,
1. Lithology
Lithology came from the word Lithos means "Rock", so in lithology, it means the rock types including all rock characteristics such as color, texture, composition such as specific minerals contained. Each Sedimentary Facies are characterized by a particular rock.
Texture, this includes grain sizes, shapes and roundness that provide information regarding the energy in the deposition environment which was available for transportation and reworking of sediments.
Composition: Rocks composed of calcium carbonates (CaCO3) such as limestone, dolomites usually originated in shallow , warm areas where detrite mud is present. Another example is that sandstone rich in Feldspar minerals are often used as an indicator for an environment with little weathering since Feldspar are easily broken down into clay minerals by chemical weathering.
With evaporative minerals such as rock salt, gypsum is often indicative of an Arid environment. Regarding glauconite minerals are always deposited only in Marine environment of continental shelf.
Rock Color, red beds are used as indicative for sub aerial, terrestrial conditions. While black to gray color in mudrocks can be used to indicate greater amount of organic carbons buried at greater depth.
2. Sedimentary Structures
Here most of the structures such as those which are formed during the formation of Sedimentary rocks known as syn - depositional structures. These structures may give us clues regarding the medium such as wind or water and the direction at which sediments were transported, for example ripples and cross bedding structures.
Another example is that large scale (several 10 'm cross bedding is a characteristic of an Aeolian (desert) environment.
3. Fossils
These are either remains of actual body parts of an organism such as bones, horns known as Body Fossils or traces of their activities on the surface of the sediments such as swimming trails and tracks, footprints known as Trace Fossils.
With the body fossils can be subdivided into two (2) classes based on their sizes such as Macro (mega) fossils which can be seen by naked eyes and Micro Fossils with very small sizes that need microscope to visualize them example Foraminifera and Radiolarians fossils. These two are important microfossils that are utilized in the oil and gas exploration industry.
Fossils provide important clues on water depth such as bivalves can be found at depth around 20 - 50 m, Ammonites at depth around 80 - 100 m, while coral reefs at depth around 1 - 20 m.
4. Geometry
This includes the three dimensional shape of sedimentary Facies as a function of depositional topography, geomorphology of environment and post depositional history.
This means that determining the geometry is an important step when it comes to understanding the process governing the formation of a particular sedimentary Facies. Example River channels are characterized by tributary geometry while River delta have distributary geometry.
With beach environment having a blanket (sheet) geometry while Aeolian (desert) environment having lateral extensive sheets geometry
Note: Some geometry may be found in a particular different environment example Sandy sedimentary Facies can have both geometry described here above. This means that using only geometry itself to identify the sedimentary Facies should be used with much caution.
5. Paleocurrent
This includes the direction at which sediments were transported during past time. Direction of transportation of sediments can be either unimodal pattern which is single direction, or Bimodal pattern which is two (2) directions.
Example delta environments can be characterized with a unimodal pattern, Also Alluvial environments have a unimodal pattern of fan shape. While water ripple marks are characterized by Bimodal patterns..
I hope this is clear and you grasped a concept, let us move straight to the three (3) environment at which most seismic data Acquisition is done.
1. terrestrial environment
This includes part of the continent with dry land including river valleys, glacial (icy) and desert areas. However in terms of geology this term is not restricted only to cover dry land but also may include swamps, lakes, ponds. But in order to grasp the basic concept as applies to seismic surveys we have to restrict ourselves to dry part of the land.
So any seismic surveys that are carried on dry land at which seismic devices are directly plugged on the ground may be referred to as Land seismic survey.
The term Onshore is frequently used to refer to land-based areas away from shoreline where seismic surveys and other related operations are conducted.
2. marine environment
This includes areas that are covered by water such as sea and ocean. Seismic Data Acquisition is carried out at the surface of water by ship vessels or floating vessels equipped with a seismic energy source and streamer of seismic receivers such as hydrophones, hence the term Marine seismic surveys. See figure 1 below
Figure 1: Simple representation of Marine seismic survey
However geologically the term Marine environment is characterized by features such as Continental shape, continental slope, Abyssal plain, tidal flats just to mention a few.
The term Offshore is often used to refer to water-based areas away from shoreline at which seismic surveys and other related operations are carried out.
3. transition environment
This involves areas at intermediate between Marine and continental environments. So it can be regarded as areas at the boundary between terrestrial and Marine environments.
However geologically this area is characterized by features such as delta, lagoon, estuary.
What are the difference between Land and Marine seismic Acquisition?
Difference between Land and Marine seismic acquisition.
Land Seismic Acquisition is a land based seismic survey (data collection) which involves the sources typically vibroseis and dynamite that produced ground motion which is reflected in the subsurface and then recorded ground motion detectors known as geophones. See figure 2 below
Figure 2: Land based seismic acquisition.
While Marine seismic Acquisition is the Marine (water) based seismic survey ( data collection) that involves the sources typically compressed airgun that produces a pressure wave that is reflected in the subsurface and recorded by pressure detectors known as hydrophones. See figure 3 below.
Figure 3: Marine based seismic Acquisition.
All in all the above sedimentary environments give the basis at which seismic surveys can be implemented such as Land based seismic surveys and Marine based seismic surveys. However this tells us that in order to approach seismic sections competently and produce a reasonable interpretation, the basic knowledge of geology is required.
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