How does multiple layer seismic reflection work? How do we distinguish arrivals from different layers and their velocities?

Principles of Seismic Reflection

Seismic reflection is a powerful technique used in geophysics to study the subsurface structure of the Earth. It is based on the interaction of seismic waves with different layers of rock below the surface. By analyzing the reflected waves, geophysicists can gain valuable insight into the composition, thickness and geometry of subsurface layers.

The process begins with the generation of seismic waves at the surface. This is typically accomplished by the controlled release of energy, such as the use of a seismic vibrator or the detonation of explosives. The seismic waves then propagate through the earth, penetrating different layers of rock.

When these waves encounter an interface between two different rock layers, some of the energy is reflected back to the surface. This reflection occurs because of differences in acoustic impedance, which is a measure of the density and seismic velocity of the rocks. The reflected waves are picked up by sensors called geophones or hydrophones, which are strategically placed at specific locations.

Distinguishing arrivals from different strata

The challenge in seismic reflection is to distinguish the arrival of reflected waves from different subsurface layers. This is achieved through a process known as seismic data processing, which involves several steps.

One of the main techniques used is called “common midpoint” (CMP) stacking. In CMP stacking, seismic traces recorded at different geophone locations are sorted based on the midpoint between the source and receiver positions. By aligning and summing these traces, the signal-to-noise ratio is improved and reflections from different layers become more prominent.

Another technique is called velocity analysis. Seismic waves travel at different speeds through different types of rock. By analyzing the travel times of the reflected waves, geophysicists can estimate the velocities of the subsurface layers. This is critical to accurately locating and correlating the reflections with the corresponding subsurface interfaces.
Velocity analysis involves the construction of velocity-depth models, which are representations of subsurface velocity variations. These models are iteratively updated and refined to match the observed travel times of the reflected waves. The process typically involves the use of computer algorithms and advanced mathematical techniques to optimize the model parameters to achieve the best fit between the observed and predicted data.

Determination of subsurface layer velocities

Estimating the velocities of subsurface layers is a key step in seismic reflection processing, as it enables accurate interpretation of reflection data. There are several methods used to determine these velocities.

One common approach is called first-break picking. In this method, the arrival times of the first significant reflections from different interfaces are manually picked from the seismic data. These arrival times are then used to calculate the corresponding travel times. By dividing the travel distances by the corresponding travel times, the average velocities of the subsurface layers can be obtained.
Another technique is called “pre-stack depth migration”. This method takes into account the complexity of the subsurface by accounting for velocity variations with depth. Pre-stack depth migration involves a computational process that iteratively migrates the seismic data into a depth domain, taking into account the estimated velocity models. This technique provides a more accurate representation of subsurface structure, especially in areas with complex geological features.

The importance of multi-layer seismic reflection

Multilayer seismic reflection is a fundamental tool in geophysics and has many applications. It is widely used in oil and gas exploration to identify potential reservoirs, evaluate their characteristics and plan drilling operations. It is also used in geotechnical investigations to assess the stability of the ground for construction projects and to locate underground water resources.
By distinguishing arrivals from different layers and accurately estimating their velocities, geophysicists can create detailed subsurface models that help understand geological formations, map faults and fractures, and locate potential mineral deposits. These insights are critical for making informed decisions in various industries and scientific studies related to the earth sciences.

In summary, multi-layer seismic reflection is a powerful technique that allows us to explore the Earth’s subsurface. By analyzing the reflections of seismic waves, we can distinguish arrivals from different layers and estimate their velocities. This information provides valuable insights into the composition, structure and properties of the Earth’s subsurface, leading to a better understanding of our planet’s geological processes and resources.

FAQs

How does multiple layer seismic reflection work? How do we distinguish arrivals from different layers and their velocities?

Multiple layer seismic reflection is a technique used in geophysics to study the subsurface structure of the Earth. It involves sending seismic waves into the ground and recording the reflections that bounce back from different layers of rock or sediment. By analyzing the characteristics of these reflections, we can determine the properties of the subsurface layers, such as their depths, thicknesses, and velocities.

What are the key components of a multiple layer seismic reflection survey?

A multiple layer seismic reflection survey typically involves three main components: a seismic source, receivers, and data processing equipment. The seismic source generates energy, usually in the form of vibrations, which is transmitted into the ground. Receivers, also known as geophones, are placed at various locations to detect and record the reflected waves. The data processing equipment is used to analyze and interpret the recorded data, allowing us to create subsurface images.

How do we distinguish arrivals from different layers in a seismic reflection survey?

In a seismic reflection survey, the arrivals from different layers can be distinguished based on their travel times. Each layer has a specific velocity through which seismic waves travel, and this velocity is related to the properties of the layer, such as its density and stiffness. By measuring the time it takes for the seismic waves to travel from the source to the layer and back to the receivers, we can calculate the depth and thickness of the layer.

What techniques are used to determine the velocities of different layers in a seismic reflection survey?

Several techniques are employed to determine the velocities of different layers in a seismic reflection survey. One common method is called “velocity analysis,” which involves analyzing the recorded data to identify the patterns and characteristics of the reflections. By comparing the travel times and amplitudes of the reflections from different layers, we can estimate their velocities. Additionally, advanced processing techniques, such as pre-stack depth migration and tomography, can provide more accurate velocity information.

How do we account for variations in seismic velocities caused by complex subsurface structures?

Complex subsurface structures, such as faults, fractures, and variations in rock types, can cause variations in seismic velocities. To account for these variations, geophysicists use advanced processing techniques and modeling approaches. For example, they may employ velocity modeling algorithms to create a detailed model of the subsurface, taking into account the known geological information and the observed seismic data. This allows for a more accurate estimation of the velocities and helps in distinguishing arrivals from different layers.

What are the limitations of multiple layer seismic reflection?

Multiple layer seismic reflection has some limitations. One limitation is that it requires access to the subsurface, which can be challenging in certain environments, such as deep oceans or densely populated urban areas. Additionally, the accuracy of the results depends on the quality and density of the recorded data, as well as the assumptions made during the data processing and interpretation. Complex subsurface structures can also pose challenges in accurately determining the velocities and distinguishing arrivals from different layers.