4 Components of Seismic reflection Imaging

There is no doubt that most of us know that seismic imaging techniques opened a new way for geoscientists to see the unseen part below the earth. And the method applied back for over 75 years especially in the oil industry, and is still giving the promised results. And with the recent advancement of technology the technique still innovates to give more depth of investigation and detailed images through techniques such as Seismic Tomography. However we have to be specific that this post is tailored to reflection tomography that applies artificial controlled active sources of seismic energy.

The basic concept of seismic reflection imaging seemed to be simple to grasp, such as sending seismic waves into the subsurface, then recording them on the surface using various seismic receivers after they bounced and modified into subsurface reflectors. It is that simple!

But when it come to reality this method is challenging why? First it is Logistically intensive in acquisition stage especially when the surveying site is large, also when heavy seismic sources such as vibroseis are utilized for land surveys (see figure 1), likewise when Long array hydrophones receivers are utilized for Marine surveys (see figure 3) and second it need suitable, competent skills and knowledge to attempt the stage of seismic processing and interpretation.

However, this post will describe to you four (4) basic components regarding seismic reflection methods.

The following are the four (4) basic components of the seismic reflection methods,

1. Seismic energy source

Seismic energy sources are the instruments, machines and tools that generate seismic energy by using various mechanisms. There are many sources ranging from simple sledge hammer to Vibroseis trucks vibrators with each source producing various seismic energy having a variety frequency ranges. So choosing the best source to use will depend on the primary objective of your survey on hand, the environment where the survey will be conducted, survey coverage area, your budget.

Figure 1: Array of vibroseis trucks ready in the field for land reflection survey

However as the rule of thumb it is true to say that an ideal seismic source should create a wavelet that has short duration and high energy and not otherwise. This is because short duration improves survey resolution and higher energy produces a higher amplitude wavelet so that it can detect the seismic signal within background noise easily. If you want a detailed description regarding various seismic energy sources and what to consider when choosing them, you can check the previous article regarding seismic energy sources.

Most land based seismic reflection surveys aimed at imaging the lithospheric crust utilizes the vibrators mounted on 3, 5 or more vibroseis trucks synchronized to +-0.001s or less. Vibrators are shot on various points while moving a single step after each shot. The seismic vibrator acquires traces at many offsets from every source as shown in the figure 2 below.

However the number of shots will depend on the size of the survey area such as for a site which is 500 km long, requiring about 10,000 or more shots.

Figure 2: A simple seismic reflection survey layout using vibroseis.

2. Lines of seismic receivers

Always a group of connected seismic detectors listen to the seismic vibration produced by the seismic source directly and after being reflected and modified in the subsurface by seismic reflectors such as discontinuities (see figure 2). There are two main types of Seismic detectors depending on the seismic acquisition environment and how they respond, which are geophones and hydrophones. If your survey is carried out on land then geophones should be used as they are sensitive to velocity changes due to ground motion (see figure 2) while for Marine surveys hydrophones have been designed for a such purpose and conditions of responding for pressure changes caused by water waves (see figure 3) .

Figure 3: Deployment of hydrophones streamer

Regarding the seismic reflection survey to image the lithospheric crust the lines of geophones of about thousands are set up on individual single lines of about 10 - 12 km long each.

However there are three (3) factors regarding seismic receivers that need careful consideration, the first factor is instrument response that gives the relation of input ground motion and output electrical signals. Also the natural frequency which produces the maximum amplitude output. And the last is the damping which reduces the amplitude of the natural frequency response and prevents the infinite oscillations that also need careful attention.

3. Recording system

The recording system should compose of digital recording media such as digital tapes plus the computer which are mounted on the recording truck. Signals are transmitted to the recording system for temporary storage before further processing. The recording system sometimes consists of a radio control unit that is used to control the source such as to initiate the signal vibration on vibrator truck.

4. Data processing system

This component consists of suitable computer software that can be operated on systematic steps to enhance the desired seismic signal and to attenuate unwanted noise, so that the results can be optimized.

However, when speaking, regarding seismic data processing, it is broadly subdivided into two processes, which are seismic data correction and seismic data enhancement. The term Seismic data correction involves the removal of the dependence of the data on seismic sampling variables, whereas seismic data enhancement involves both the improvement of the seismic data through processing and its conversion into a readily interpretable form.

The basic processing steps vary from seismic trace display, muting and editing, velocity corrections, stacking. See figure 4 below.

Figure 4: Basic simple steps for seismic data processing.

Example one among the processing step is Stacking first gathers traces from all available source - receiver offsets that reflect at a common midpoint (CMP) see figure above. But because arrivals from longer offsets have traveled farther, a time correction, called normal moveout (NMO) correction, should be applied to each gathering so as to flatten the arrivals. Then the flattened traces are averaged to produce one stacked trace that represents the normal-incidence (zero-offset) trace. See figure 5 below

Figure 5: Normal Move Out (NMO) correction and stacking

The final product of the 2D - seismic cross section of the Earth is obtained in terms of signal Transit time to offsets (see figure 5). Transit time is the two way time at which a seismic ray propagates from source into subsurface, then returns to the seismic receiver where it gets recorded (see figure 2).

This section is then converted to depth for comparison with the geology section. The conversion of time into depth require an estimate average seismic velocity, while applying simple relationship between time and depth, such as

V = 2d/t

Where, V = an estimated average velocity, d = depth while t = transit time.

Let us view an example using a good rule of thumb that 6 km/s can be used within a lithospheric crust. However this value should not be taken as standard for every survey, as to some extent when seismic waves propagate into the mantle then it may increase to about 8 km/s.

A good approach that will refute this dilemma regarding which average velocity to use in depth conversion is by carrying out a counterpart refraction survey to give this value of velocity either on surface or within borehole, but always it carried out within boreholes but this will depend on the geology of the area. Okay hope this is clear, then if we assume the transit time is 4 seconds, then using this rule the depth will be 12km.

Such as depth = transit time × average velocity/2

Depth (km) = 4s × 6(km/s)/2 = 12 km.

There are many requirements to consider in seismic reflection surveys, however the common and primary basic four (4) components that you will find in every reflection survey are seismic energy sources, Lines of seismic receivers, recording and data processing systems.