2 Seismic Interpretation

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    CHAPTER 2 SEISMIC INTERPRETATIONS

    2.1 INTRODUCTION

    The first Chapter of this training material was on the topic of geology. In it we stressed the importance of understanding geological principles because our end product - the seismic section - is

    a representation in time of the geology.

    This course would not be complete if we did not have a look at the interpretation of seismic data.

    No matter what our particular job is in the industry, we should all have an understanding of the problems forced by the interpreter, whose job it is ultimately to identify a feature that is a possible

    drilling creation.

    In this chapter, then, we will look first at the tools used by the interpreter, and then do a couple of

    interpretation exercises on data from two completely different tectonic regimes. Hopefully these exercises will bring the course to a satisfactory conclusion, and help you to understand how the

    seismic tool images the geology.

    2.2 WELL LOGGING TOOLS FOR THE GEOPHYSICIST

    Geologists make use of a number of logging tools - tools that are lowered into a well in order to

    make in-situ measurements of various parameters within the formations. Such parameters as

    interval velocities, density, electrical resistance and gamma ray activity all provide useful information about the rocks at depth. As the source - receiver distance in a logging tool tends to be

    quite short, the information gained has a high frequency content and is hence high resolution.

    2.2.1The Sonic And Density Logs

    The most important tools for the geophysicist are those used to obtain interval velocities and

    formation densities (why?).

    For obtaining interval velocities we use the sonic tool. The sonic tool consists of a source of energy and a receiver, separated by some distance (typically a few metres), mounted in a tool that is

    clamped to the side wall of the well bore so that there is good contact with the formation. The

    standard tool in use today is the Borehole Compensated tool - this has two transmitters and four receivers (see Figure 2.1) in such a configuration that borehole irregularities, and tilt of the tool are compensated for. Measurements are made as the tool is drawn up from the bottom of the well. The data is presented as a continuous trace (Figure 2.2), calibrated in depth and in transit time - that is, in units of microseconds per foot, or per metre. From these measurements can be derived interval velocities within the formations.

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    The other parameter important to the geophysicist is formation density. Densities are measured

    using a gamma-ray source and a detector shielded so that it records only back-scattered gamma radiation from the formation (Figure 2.3). (The figure is that of a compensated detector; it employs a second, short range detector, which responds more to the mudcake and small amplitude bore-

    hole irregularities; these reading are used to correct the readings from the main detector.) The intensity of the back-scattered radiation depends upon the electron density of the formation, which

    is roughly proportional to the bulk density. The form of presentation of the density log is shown in Figure 2.4. So we have two parameters measured within the well. What do we do with them?

    Figure 2.1 THE SONIC TOOL 1

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    Figure 2.2 THE SONIC LOG

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    Figure 2.3 THE DENSITY TOOL

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    Figure 2.4 THE DENSITY LOG

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    2.2.2 The Synthetic Seismogram

    The reflection coefficient series (which we also refer to as the earth impulse response) derived at a

    well location represents an important calibration point for our surface seismic measurements. It is only at a well that we can make real measurements at depth; measurements that, if we are lucky,

    we can calibrate against actual rocks pulled from the well as core samples. Even without core samples, however, well logging is vital to our understanding of the formations that we "see" on a

    seismic section.

    Having obtained our reflection coefficient series, what can we do with it? The clue lies in the

    operation of convolution - recall that the seismic section is a result of convolving the seismic wavelet with the earth impulse response. In collecting our surface seismic data, we know little

    about the source wavelet, and nothing about the impulse response. All our measurements are made after the fact, so to speak. However at the well, we have actually measured the real impulse

    response (within the limitations of the logging technique; nothing is ever perfect). Why not design a

    reasonable wavelet (i.e. design a filter) and convolve it with the measured impulse response?

    The result is the synthetic seismogram - basically a model of what we would expect a seismic trace to look like at this location given a similar source wavelet (see Figure 2.5). The choice of which wavelet to use for generating a synthetic seismogram is a judgment call for the

    interpreter to make based on a knowledge of the area. It is often useful to test different wavelets,

    and to output a number of synthetics using different frequency wavelets, then testing to see which model fits the seismic section the closest. For southern Alberta, the Orsmby wavelet is often the

    best to use.

    2.2.3 The Checkshot Survey

    In order to obtain better information about seismic velocities at a well location, a check-shot survey

    will be carried out. The idea is relatively simple: lower a geophone into the well, then trigger a seismic source close to the well location and measure the first arrival time; this will give a direct

    measurement of the average seismic velocity to that geophone level at the well, and this

    information can be used to convert seismic time to seismic depth.

    An actual check-shot survey will take measurements at a number of depths within the well. The geophone spacing within the well will be in the order of hundreds of metres, and only the first

    arrival information is used (see Figure 2.6). Included in this figure is the most important information that the check shot survey gives the interpreter - the time-depth curve at the well.

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    Figure 2.6 CHECK SHOT SURVEY - SCHEMATIC

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    2.2.4The Vertical Seismic Profile (VSP)

    A natural and most useful extension of the check-shot survey is the VSP. The recording technique is

    essentially the same except that many more levels are recorded, and the recording time is extended to 3 or 4 seconds. A VSP will typically be recorded every 25 metres within the well. This means that

    a VSP is much more expensive to record (more rig time is required), and hence they are not done

    often. Depending on the TD of the well, and the number of levels recorded, a VSP can run from $50,000 to $150,000. The big advantage of the VSP is the amount of information it gives the

    geophysicist. The principle of VSP acquisition, and a VSP plot are given in Figure 2.7 and Figure 2.8 respectively.

    Figure 2.7 VSP - SCHEMATIC

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    Figure 2.8 VSP - PLOT

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    2.3 POLARITY AND ITS IMPORTANCE

    The problem of polarity is as old as the seismic method itself. The geophysicist needs to know the

    polarity of the section in order to help determine the stratigraphy of the section. Polarity has been defined by the SEG (see definitions) and is important for the following reason: we define a

    reflection coefficient as positive at an interface where a compressive wave is reflected as a compressive wave. This implies an increase in seismic velocity from the first medium to the second,

    and the geophysicist (or geologist) will then know the kind of stratigraphic change taking place.

    Hence it is vital to know what a peak or a trough is truly representing on the seismic section.

    The main tool the interpreter has to determine polarity is the synthetic seismogram, in which the reflection coefficient series is convolved with a known wavelet, and then displayed at both polarities

    for that wavelet. The seismic section that truly (or most closely) matches the synthetic derived with the wavelet of positive polarity will be designated as the "log normal" section. Polarity becomes

    especially critical in stratigraphic plays. Basing one's interpretation on the wrong polarity can mean

    the difference between drilling success and failure (why should this be?).

    2.4 THE STACK VS. THE MIGRATED SECTION

    The final interpretation done by a geophysicist will almost always be done with the migrated

    section; it is vital to have any structural elements and faults placed in their correct spatial location relative to each other and relative to known geographical coordinates. The final objective of the

    interpretation is a drilling location; basing a location on non-migrated data would be foolish at best, disastrous at worst.

    However the stack section is very useful in two situations that come to mind immediately; in highly structured and faulted areas and in reef plays.

    In both types of plays, it is vital to pinpoint exactly the edge of a fault, or the edge of a reef. These

    are often marked by the apex of diffraction patterns, whi