Tuning

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Tuning is a common phenomenon associated with thin beds in seismic data. It refers to the brightening or dampening of seismic amplitude because of constructive and destructive interference from overlapping seismic reflectors.[1]

This article will attempt to define the seismic phenomenon of tuning. It will explain thin-bed tuning and address the confusion that can arise when thin beds occur in hydrocarbon prone areas. It will also briefly touch on a method of detuning, which attempts to remove the tuning effect from seismic amplitude data.

Tuning Thickness

Tuning is most commonly explained via the use of a wedge model, such as a sand bed surrounded by shale (figure 1). The wedge model allows interpreters to study how the seismic response changes as a function of layer thickness.

Figure 1: A typical wedge model. As the bed thins to the left, the side lobes and main reflections interfere constructively to produce brighter reflections.

The boundaries of the wedge will produce reflections with different polarities due to the impedance contrast between the different lithologies, such as a peak at the top and a trough at the base.  When reflections have enough space between them, that is, when the bed thickness is greater than one quarter of a wavelength (λ/4), you have distinct reflections. However, as the bed thickness decreases the top reflectors get closer to the base reflectors and constructive interference begins to take place. The thickness at which this interference starts to occur is termed the tuning thickness and it is defined as one quarter of the wavelength(λ/4).

                                                           

In zero-phase data, tuning occurs when the peak of a positive reflection aligns with the side lobe peak of the negative reflection. At one quarter wavelength (λ/4), constructive interference occurs, causing the amplitude to become much brighter. It is this increase in the amplitude that is termed the tuning effect.

Tuning Effect and Direct Hydrocarbon Indicators

Direct Hydrocarbon Indicators (DHIs) are anomalies that aid in identifying hydrocarbons in the seismic data.[2] Common DHIs include :

  1. Bright Spots
  2. Dim Spots
  3. Flat spots
  4. Phase Changes

Gas in a sandstone will lower the bulk modulus of the rock, since gas is highly compressible.[2] As such, this reduces the P-wave velocity and the impedance, leading to high amplitude seismic anomalies called bright spots.[2] Flat spots, which represent reflections from the hydrocarbon-water contact, are also a DHI closely related to bright spots. [3] They typically have positive reflection coefficients. Polarity reversals, another type of DHI, occur where the seal rock has a higher impedance than the hydrocarbon filled sand, but a lower impedance than the water filled sand.[3]

Where hydrocarbons exist and the target lithologies have thin beds, the tuning effect can lead to high, bright amplitudes that can be confused with DHI's. In exploration it is important to distinguish between these two phenomena. If tuning is thought to be a issue in the seismic it may become necessary to detune the data.

Case Study: Block F3, Dutch North Sea

Seismic data within this area revealed bright spots punctuated by a noticeable flatness. It was suspected that this flatness may have been due to a tuning effect that was taking place.[3] The possible tuning effect was investigated using 3 models. The only parameter that changed in each model was the impedance of the shale.[3] The general parameters are seen in figure 2 below.

Figure 2: General set up of the models used.

Two of the three models created are shown below:

  • Polarity reversal : gas sand impedance < shale impedance < water sand impedance

At one quarter wavelength, there is some constructive interference from the main lobe of the top of the reservoir and the side lobe of the base of the wedge. This creates a brightened negative amplitude (Figure 3).[3] As the wedge pinches out, there is a polarity change where an interpreter should switch from picking the negative reflections, to picking the positive reflections of the flat spot, however; as the wedge pinches out, the negative side lobe of the flat spot is seen above the top of the reservoir (Figure 3).[3] This side lobe has a negative amplitude, which may cause an interpreter to continue picking along the negative reflector which would cause the polarity reversal to be overlooked (Figure 3).[3] The effect of tuning in this case can make interpretation quite difficult, potentially causing the interpreter to miss important indicators such as polarity reversals.

Figure 3: At λ/4 there is constructive interference (red line) from the main lobe of the top of the reservoir (blue line) and the side lobe of the base of the wedge. Where the top and base pinch out, the negative side lobe of the flat spot appears above the top of the reservoir. The polarity reversal can be disguised as a flat bright event, making interpretation difficult.
  • Dim spot over a flat spot : Shale impedance > water sand impedance ; Shale impedance > gas sand impedance

When the wedge is thick (> λ/4), there is constructive interference between the side lobes of the top and bottom main reflectors, leading to a brighter negative reflection (Figure 4).[3] At λ/4, destructive interference begins to take place, leading to smaller amplitudes. Where the wedge pinches out there is constructive interference between the main lobes of the top and bottom main reflectors (Figure 4).[3] This leads to brighter reflections. An interpreter who may be looking for bright spots may follow the negative side lobe of the flat spot rather than the gas zone.[3]

Figure 4: When thickness is >λ/4, constructive interference occurs between the side lobes of the top (blue line) and bottom main reflectors creating a bright negative reflection. At λ/4, destructive interference begins to take place, leading to smaller amplitudes (red line). Where the wedge pinches out there is constructive interference between the main lobes of the top and bottom main reflectors creating brighter relfections.

Overall, tuning effects can cause flat negative bright events to overlie strong positive reflections, which has the possibility of making a dim spot appear as a bright spot.

Detuning

As seen above, thickness related tuning effects can have a large effect on the interpretation of seismic. Amplitude changes due to tuning can subdue the changes due to fluid effects and rock type, so to get accurate interpretations it will be best to remove the effect of tuning.

To correct for tuning you have to:

  1. Define a tuning curve empirically from seismic picks (Figure 5).
    Figure 5: Amplitude tuning curve generated from the relative acoustic impedance from an inverted seismic dataset.
    This is usually obtained using seismic that has been inverted to show relative acoustic impedances. [4] A reference seismic response is then chosen from the curve.[4] It is usually half of the peak tuning amplitude.
  2. Scale the data. At any given thickness, the seismic amplitude will be multiplied by a factor, F, where F is the ratio of the reference amplitude to the value on the tuning curve at that given thickness. [4]

Figure 6 shows the detuning of a hard sand turbidite using the particular method outlined above. [4] Figure 6a shows the time thickness, and the extracted amplitude is shown in figure 6b. Using the tuning curve in figure 5 and the thickness from the time map, the data was detuned. Figure 6c shows the final results. Some of the bright amplitudes seen before have been subdued or even removed. The amplitudes seen are now less variable and more geologically significant.

Figure 6: Detuning of a hard sand turbidite. 6a is a time thickness map. 6b shows amplitude extracted onto the horizon. 6c shows the detuned data obtained from scaling calculations using the F factor.

References

  1. Tuning effect,https://www.glossary.oilfield.slb.com/en/Terms/t/tuning_effect.aspx , accessed October 01, 2018.
  2. 2.0 2.1 2.2 Nanda, N.C., 2016, Direct Hydrocarbon Indicators (DHI), in Springer, Cham, eds., Seismic Data Interpretation and Evaluation for Hydrocarbon Exploration and Production, 103-113.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 Guo, Q., Islam, N. and Pennington, W.D., 2015, Tuning of flat spots with overlying bright spots, dim spots, and polarity reversals: Interpretation, 3.3, SS37-SS48.
  4. 4.0 4.1 4.2 4.3 Francis, A., 2015, A Simple Guide to Seismic Amplitudes and Detuning: Geo ExPro Magazine, 12, no. 5, 68-72

See Also

A simple guide to seismic amplitudes and detuning

Detuning

Thin Bed Characterization video

Tuning Effects' Impact on Seismic