Category Archives: Earth Science

Also known as Geology, Geosciences, etc. It is a science that deal with Geologic materials and processes which made them. A multidisciplinary field in which also includes Geophysics, Hydrology, Structure and Civil Engineering and many more.

Overview of Seismic Survey

In the past (eg. 1950s), individuals with physics and mathematics degrees were trained to become geophysicists by companies and government institutions. Modern geoscience includes two major branches; geology and geophysics. Hundreds of universities across North America offer undergraduate BSc programs in geophysics. Geophysics can be defined as the mathematical approach to geology. It integrates both physics and natural geology into one subject matter. One of the most fundamental methods of geophysical data gathering is seismic survey.

Continue reading Overview of Seismic Survey

What is a well log?

A well log, also known as a petrophysical log is similar to a cardiogram, which records electrical activity of your heart. An electrocardiogram (EKG or ECG) does not provide answers to hearth problems. It provides an outlook into the state of the heart. The EKG is interpreted by a medical doctor to provide solutions to heart problems.

A well log records physical, chemical and other petrophysical properties of Continue reading What is a well log?

Certificate and the Ring

It is March already and graduating students from Engineering and Geoscience programs in several provinces across Canada are excited to receive their professional certificates and the infamous rings. Future Engineers get their Iron Rings and the future Geologists and Geophysicists get their Earth Rings1.

To some people receiving the certificate and the ring represent just another stage in their life. To others it represents absolutely nothing. In extreme cases students and professionals criticize this practice stating there is no reason to have a professional regulatory body. This is how the meaningfulness of having a ring ceremony can be lost in translation. Yes of course Canada is a free country and everyone is entitled to their own opinions. However, as a matter of principle I believe these professional qualification rings are not pieces of jewelry. The certificates are certainly not just meaningless papers you add to your pile of certificates.

earth_erings_certificateThe certificate and the ring represent an academic and professional achievement. They are also historically significant and indicate the wearers support for the ethical principles and governing laws of the profession2. Furthermore, the ring is a symbol of your hard work, dedication, knowledge and success in your academic life. Just like you and your friends who are graduating this year, thousands of others before you have went through the same process. The certificate represents that you have achieved the academic goals to be an Engineer or a Geoscientist. It shows that you are qualified to perform certain task under specific ethical and fundamental principles of the profession3. It also represent you are part of a wider professional organizations such as Engineers Canada, Geoscientists Canada and the Association of Professional Engineers and Geoscientists of Alberta (APEGA).

Majority of those who oppose regulations are in the geoscience sector. There are two popular arguments against regulatory bodies circulating in the opposition camp. First argument is that unlike engineers, geoscientists do not take critical enough decisions to be regulated. Respectfully, I completely disagree with the this statement because there are numerus engineering failures caused by the lack of geological input such as hydroelectric dam failures due to misinterpretation or rocks and soils. Geoscientists are as important as engineers but unlike engineers it seems we ourselves devalue our own profession.

The second argument is that geoscience certifications such as Geoscientist in Training (GIT) or Professional Geoscientist (P. Geo) is not widely recognized. This problem can also be fixed though regulatory bodies by enforcing rules and regulations to govern who can be qualified to work in the industry. The biggest problem in acceptance of geoscience professional certifications is that our own Geologists and Geophysicists insist on not recognizing them. I think this mentality has to change to protect our young professionals and future generations. It is disrespectful that some of the professional geoscientist who worked in the industry and research for decades wants to strike down policies that would improve acceptance of GIT and P. Geo certifications. It is a one thing if you personally do not want to apply for it; it is entirely different when you try to make a propaganda movement out of it. Companies should give priority for individuals with professional certifications over others. The hiring and career advancement practices should be overhauled to recognize and provide more bearing to professional certifications.

Finally if I have not convinced you of the importance of professional regulatory bodies and professional certificates, think about the general public. If you look from a general public’s perspective, ask yourself, if you are not proud about your achievement as a professional, then why should others care about what you do? Why should a company hire you if you have no respect for professional traditions? Would you go to a doctor who refused to respect Hippocratic Oath? In order to receive respect from others, you should respect yourself first. If you are unable to understand and appreciate the values of these traditional professional organizations, maybe you should not be a professional scientist at all.

Disclaimer:

I am a new graduate (2016) with B.Sc. in Petroleum Geology. I am not an expert of anything. However, I have been told repeatedly by veteran Geologists and Geophysicists why certifications such as Geoscientist in Training (GIT) or Professional Geoscientist (P. Geo) are “useless” and “meaning less”. Often GIT and P. Geo certifications have been described as “what’s the point” and “it’s just a ring with a paper”. I wrote this article not to disrespect educated, intelligent and experienced geoscientists, but to encourage positive optimistic discussion on why it is important to recognize certifications and professional regulatory bodies.

Footnotes:

1. Engineering graduates across Canada receives Iron Rings, but not all Geoscience graduates receive Earth Rings. Some Canadian provinces do not have an Earth Ring ceremony.
2. Association of Professional Engineers and Geoscientists of Alberta (APEGA)
3. Engineering and Geoscience Professions Act Alberta

Estimation of Q Factor

Estimation of Q Factor in Seismic Evaluations

Sanuja Senanayake
Geology Undergraduate Student: Winter 2015, University of Calgary.

Summary

The quality of seismic images varies with the several parameters. Fundamentally, the signal strength plays a major role in the clarity of seismic images. By analyzing the signal quality quantitatively as opposed to qualitatively, we can correct the loss of signal strength over a distance and time. The Q factor is a mathematical representation of signal degeneration. It can be used to evaluate the original seismic wavelet from a distorted wavelet. There are several methods to derive the value of Q. But currently there is no consensus among the geophysicists on which method is more accurate. In this particular study (Lupiancci, Andriano, Oliveira, 2015), researchers evaluated three methods; amplitude decay versus time decay, spectral ratio-based and Wang’s method. After several iterations of the data, they found the Wang method to be more accurate and provided the most consistent dataset. However, it should be highlighted the spectral ratio-based method also provided very accurate results.

Introduction

The seismic interpretations depend on both the knowledge and experience of the interpreter and the quality of seismic dataset itself. The seismic images are derived by collecting signal data from either seismic surveys or by measuring natural earthquakes. However, the measured data is almost always not the original seismic signal. As waves travel through a medium, they interact with the grains and fluids causing the waves to lose energy. Hence, the attributes of the original seismic signal is not detected by the geophones.

To obtain the original seismic signal from a distorted dataset, the rate or amount of energy loss should be quantified. This can be achieved by assigning a mathematical value to the quality of seismic images. The Q factor is one such mathematical parameter that can be used to quantify the energy loss of the seismic signal.

In physics, attenuation is described as the reduction in signal strength or the energy loss of signals over a distance or time. As the signal travels across a medium, it interacts with the particles within the medium. These interactions result in transfer of energy out of the signal. The Q factor measures how much of the original energy remains at the time of signal detection. Hence the Q factor is inversely proportional to the attenuation.

In this particular study, researchers focused on different methods of deriving the value of Q (Lupiancci, Andriano, Oliveira, 2015). They evaluated the errors in the Q factor obtained through amplitude decay versus time decay, spectral ratio-based and newly suggested Wang’s method.

Theory and Methodology

In order to analyze the seismic data, the signals must be decomposed using appropriate algorithms. Spectral decomposition is a commonly used method in which the change in frequency is analyzed. However, it is a non-unique process where depending on the algorithm used, the value of Q may vary widely for same dataset. Some possible methods for spectral decomposition includes, but not limited to, continuous wavelet transform, Gabor transform and S transform.

The seismic theory suggests that a wave propagating through an inelastic medium will result in exponential decay of amplitude. This relationship can be used to derive estimations for Q factor by regression applied to the decay function itself. However, the decay function itself is not clearly detected in both real world and in synthetic data. Decay function detection and analysis is especially difficult to obtain in short time windows due to less data values. In this study, researchers calculated Q factors in various time windows and verified the results with inverse Q filtering of a seismic section (Lupiancci, Andriano, Oliveira, 2015).

There are two parts to methodology: the modeling of seismic trace in a dissipative medium and the Q factor estimation approaches. The modelling was done using fundamental principles of wave propagation (1). The U0(ω) is the Fourier transform of the seismic pulse, ω is the angular frequency. V(ω) is the complex phase velocity and x and τ are the travel distance and time, respectively.

est_q_01

For weekly dispersive media, the Q factor is much higher than one (Q >> 1), hence the approximation can be reached using the following (Cravenly, 2001):

est_q_02

in which, VR(ω) is the real phase velocity and the Q(ω) is the medium Q factor. Hence V(ω) term in the first equation (1) can be replaced with the second equation (2). This new equation (3) is used to model the synthetic seismic trace for Q factor estimations.

est_q_03

In the above equation (3), the first exponent deals with the propagation and dispersion while the second exponent is responsible for the amplitude decay. Next, the equation (3) can be rewritten as:

est_q_04

In equation (4), τ = x/VR(ω)

The synthetic seismic data is modeled using frequency domain using the following relationship.

est_q_05

The equation (5) represents the Fourier transform of the trace and the wavelet with respect to the frequency domain. This function was derived using previous studies by numerous geophysics and physicists (Lupiancci, Andriano, Oliveira, 2015). Finally, by assuming transmission effect of the wave is negligible, finally the equation (6) was derived, which was used for calculating the Q factor for the nth layer.

est_q_06

The next part of this study is the analysis of Q factor itself. To obtain the values for Q, the researchers (Lupiancci, Andriano, Oliveira, 2015) relied on Gabor transform, a type of Fourier transform that uses a Gaussian window function to generate a time-frequency amplitude spectrum of a seismic trace.

Alternative methods of Q Estimation

Three alternative methods were briefly considered in the study. They all involve the amplitude spectrum of the time-frequency transform of the seismic trace. The first method was the amplitude decay versus time method. It is based on the amplitude curve of the time-frequency amplitude spectrum picked for a constant frequency value. For example, in this particular study it was 30 Hz (Lupiancci, Andriano, Oliveira, 2015).

The second method was the spectral ratio-based method. It is based on the measurement of the exponential decay along the frequency axes for a constant time.

The third method was coined as the “Wang’s method” by the authors because it was first introduced by Yanghua Wang in 2004. This involves measurement of the amplitude decay along the compound variable x = ωτ, where the variable x depends on the signal-to-noise ratio of the data. In this particular study researchers picked amplitudes along the curves defined by the equation, x = ωτ in the time-frequency domain. Then the average of the amplitudes was taken to define the exponential decay value s(x) for the following function (equation 7). (Lupiancci, Andriano, Oliveira, 2015)

est_q_07

Using the above three methods, the modelled seismic trace was used to evaluate the Q factor estimation approaches considering a medium formed by random spikes Lupiancci, Andriano, Oliveira, 2015). The estimations were measured at set time intervals; 0.5, 1.0, 2.0 and 3.0 seconds for the all three methods (Table 1-REMOVED due to Copyright).

Synthetic Data Results

The synthetic data were obtained through the use of equation (5) and (6). Three above mentioned Q estimation methods were then tested for the time windows of 0.5, 1.0, 2.0 and 3.0 seconds. For the spectral ratio-based method, the frequency band 5 to 70 Hz was used. For the Wang’s method, Xa = 23 and Xb = 180 were used for all time windows; equation (7).

The Q factor estimation reading were repeated for Q = 60 and Q = 90 and the results are analyzed (Table 1). The researchers observed that the Q factor estimation become more accurate and precise as the length of the analysis time window increase. Hence the time frame of the analysis directly related to the accuracy and precision of the Q factor. Longer the analysis time window, higher the accuracy of the Q factor. Another conclusion was that the exponential decay function also becomes more coherent as for longer time windows than shorter ones. This was expected by the researchers (Lupiancci, Andriano, Oliveira, 2015) because as the wave propagates through a medium, over time and distance the attenuation also increases. Attenuation modifies the signals through the loss of energy during the interaction between the propagating wave and the particles or atoms of the medium. The amplitude spectrum of the time-frequency transform of the signal is controlled by peaks and valleys due to interference from the reflected pluses. These resulted in oscillations that mask the exponential decay trend. This was highly reflected in analysis performed on short time windows. It should be highlighted that this issue occurred in all methods. However, the Wang’s method was the least affected by this problem. The amplitude decay versus time method showed fluctuations in the estimated Q value based on the chosen frequency. Therefore researchers have used a range of frequencies to reduce errors in estimating the value of Q.

Real World Data Results

The real world data were obtained using a seismic section from deep water Pelotas Basin, Brazil. The samples were processed using basic seismic processing methods. The processing flow can be summarized as: geometrical spread correction, deconvolution, velocity analysis, parabolic Radon demultiple, dip-moveout correction (DMO), common offset prestack migration (Stolt) and bandpass filter (5 – 55 Hz).

Once the data was processed, the Q factor estimations were obtained through all three (above mention) methods. The amplitude decay versus time method shows a progressive loss of energy through the degradation of the amplitude over a period of time. Compared to the synthetic data, it was found to have higher impedance. The researchers ( Lupiancci, Andriano, Oliveira, 2015) suggest that this is most likely caused by shallow gas within the formation. As the density of the material decreases, the attenuation also increases. The spectral ratio-based method on real world data made it almost impossible to detect the linear relationships between variables. The amplitude decay versus time method on the real world dataset also resulted in data without clear linear relationships. However, the Wang’s method produced a very clear data output for the real world dataset with very strong linear trend. This was predicted in both this study (Lupiancci, Andriano, Oliveira, 2015) as well as previous studies by Wang (2004).

Outcome of the Research

It was found that the amplitude versus time and spectral ratio-based methods did not work well in real world data. To obtained reasonable results, the spectral ratio-based method requires a careful choice of frequency intervals where the logarithm spectral amplitude ratio versus frequency curve is near liner. Additionally, this method is not robust for the estimation of Q factor due to its sensitivity to the size of the analysis window.

The amplitude decay versus time method performed well in the noise-free synthetic data, but the performance was poor in the real world dataset. This was most likely caused by the fact that this method requires a very good time-varying amplitude gain control in seismic processing. The processing should only correct the geometrical spread effect while preserving the exponential decay caused by attenuation. Hence it is difficult to achieve. Another problem is the presence of stronger reflections with anomalous amplitudes causing erroneous regression.

The Wang’s method was robust for real world datasets. The trace by trace analysis of Q estimation suggested by Wang (2005) provided the most consistent and statistically accurate Q factor estimations. In fact, the standard deviation was small in the two analysis window sizes (6.55 for the 0 – 2.0s window and 9.25 for the 1.0 – 3.0 window). The main reason for the effectiveness of the Wang’s method is that it effectively explores the time-frequency domain. This method shows that Q factor increased as the depth of the real world dataset increases. This is consistent with the petrophysics because once the wave propagates into deeper rock layers, which are more compact, the wave favors less seismic attenuation. Hence a higher Q factor in deeper and more compacted layers.

Other Studies

There are several different studies have been done on estimation of Q factor. In one particular study by Vassil Davidov (2012) on the earthquake seismology found that the Q factor estimation is very difficult to achieve in the real world scenarios. This is because the geologic materials in which the seismic waves propagate are almost always antistrophic, inelastic and heterogeneous. This results in variation in Q factor in almost every direction as the wave propagates through the medium. The data obtained through the geophones ground stations could not often properly calculate the Q factor due to these complications in physical geology itself.

Conclusions

While there are no set international standards on obtaining the value of Q factor, the estimations can be derived from variety of methods. In this particular study, researchers found that the Wang’s method proposed in 2004 to be the most accurate out of the three methods studied. Other studies on real world data such as the earthquake study by Davido (2012) also highlighted the need to alternative methods for obtaining Q factor estimations for real world datasets.

References

Lupinacci, Wagner Moreira, and Sérgio Adriano Moura Oliveira. “Q factor estimation from the amplitude spectrum of the time-frequency transform of stacked reflection seismic data.” Journal of Applied Geophysics 114 (2015): 202-209.

Wang, Y., 2004. Q analysis on reflection seismic data. Geophys. Res. Lett. 31, L17606.

Davidov, Vassil. Seismic quality factor (Q) of the mid-continental crust from regional earthquake seismograms. Diss. Northern Illinois University, 2012.

The research is done by respective authors of the listed papers under refrences. The resarch is not belong to Sanuja Senanayake. This is an online publication of the final Term Paper written by Sanuja Senanayake at the University of Calgary for Geophysics 559: Geophysical Interpretation in Winter 2015. The text is copyrighted to Sanuja Senanayake. You may follow the guidelines posted under the general Site Copyright Notice. Figures used in the original paper have been removed due to copyright laws.

No part of this publication may be reproduced or redistributed in any form or format without prior permission from the authors mentioned above. Please contact Sanuja Senanayake for requests for permission to reproduce and redistribution.

Careers Petroleum Geology

The terms “Petroleum Geology” and “Petroleum Engineering” have been around since the boom in the oil and gas industry. But you should ask yourself, is there any difference between a Geologist and a Petroleum Geologist? Can a Professional Geologist perform the same tasks as a specialized Professional Petroleum Geologist? To answer these questions, we need to understand the basics of petroleum geology and why it is important to our modern day energy needs.

Multidisciplinary Subject Matter

If you search the term “geology” you will come across several different definitions. One of which would be; Geology a science that studies the solid Earth and its life forms though past and current processes recorded in minerals and rocks. The key term in this particular definition is the processes. The idea of studying processes has been around even before the growth in petroleum industry. At least a basic understanding of geological and geophysical processes is very important to petroleum industry. Such information is valuable in predicting petroleum fluid accumulation zones which have economic significance. As natural resources become scarce, we need better methods for exploration and exploitation of sought after resources. We realized geology itself cannot find answers to these scientific and economic challenges. Hence to reduce costs associated with exploration and extraction of natural resources, specialization of Petroleum Geology was born. Specialization does not mean a student only learn one aspect of geology. For example, the petroleum geology program at the University of Calgary encompasses structural geology, geochemistry, physics, biology and many other disciplines with emphasis on their relation to petroleum industry.

Important Concepts

The petroleum Geologists are responsible for understanding the processes of petroleum source rock formation, fluid migration patterns, reservoir characteristics, hydrocarbon traps and structural or stratigraphic trap mechanisms. It is the job of the petroleum geologist to use such knowledge to advice oil and gas companies on where to find reservoirs and how to extract hydrocarbons economically.

Tools and Tricks of the Trade

There are many tools and tricks currently used by geoscientists (the term “geoscientist” is used as an umbrella for geologists, geophysicists, engineers, earth scientists, environmental scientists, etc) to understand the Earth.

The most fundamental knowledge comes from previous studies. Typically oil and gas companies keep large databases of previously published research projects, textbooks and other materials available to their geoscientists. The Government of Canada and the Government of Alberta also keep records of core samples and well logs from projects across the country. These resources are utmost important petroleum geologist because use of such information can minimize the possibility of making incorrect decisions.

Education, past experiences and the quality of past experiences also play a major role in success of a petroleum geologist. A geologist who is capable of understanding the basics of math, physics, chemistry, biology and structural systems trend to outshine his/her peers. This is because hydrocarbon reservoirs are dynamic in nature. Not only a petroleum geologist has to successfully predict deposit locations, but also has to be able to provide information on dynamic factors such as fluid migration during a hydrocarbon extraction project. You cannot simply find the reservoir and then walk out of it and if it is that the case, most petroleum geologists would be out of work in few years. This is why the specialization Petroleum Geology is important. While a typical geologist would be able to perform the same tasks, the specialization may help companies expedite the decision making process.

Technologically we use variety of tools to achieve our goals. Petroleum geologists and geophysics probably use more technologies than most other specializations in geosciences. For example, petrophyscial techniques on wireline well logs are used to interpret lithologies and fluid types in subsurface using software programs such as PowerLog, Petrel, AccuMap, etc. These software are also used for producing geological maps with interpreted data for exploration and development. Companies typically hire technical school graduates for software operations and maintenance. However, for advanced interpretations it is necessary to hire geology (or geoscience) graduates. One of the major problems with software use is that they often over or under predict multiple parameters resulting in naturally imposable interpretations. These can result in completely incorrect or incoherent simulations of reservoirs. This is where the education and experience of a professional geologist can make a difference. A professional geologist should be able to rectify such issues before it is too late by identifying these anomalies during the early stage of petroleum exploration and development.

We need Petroleum Geologists

Yes, a good geologist with no additional petroleum training may be able to perform the same task, but when you are working on a multimillion dollar project, you would want to hire geologists with specialized training and experience. This is because one mistake could cost a company millions in revenue loss. A petroleum geologist who has specialized training and/or experience in the oil and gas industry could make the difference between a successful profitable project and a complete disaster.

How would you become a Petroleum Geologist?

The term “Petroleum Geology” is very board. Anyone can identify themselves as petroleum geologist including those with engineering and other related degrees, certifications and professional experience. With proper training and experience and engineer or a technician can act as a petroleum geologist. However, there are several universities such as the University of Calgary currently offers petroleum geology classes and specialization degrees. In a competitive job market, having such courses and degrees definitely help you get into the industry much quicker than those who do not have such backgrounds.

It is important to highlight that having a degree in petroleum geology or by taking petroleum classes does not make a person a proficient professional petroleum geologist. Success comes down to the competency, professionalism and experience.