b'Seismic window Seismic windowAfter spherical divergence and a number of reflections the signal strength is close to one billionth of the input source energy yet despite this we can still produce an image of the subsurface. To illustrate this, if the source was a one kilogram charge at the surface it would be equivalent to a microgram of the same explosive at the reflection point. Im trying to visualise a microgram of explosiveits not very big, about 1/50th of a grain of rice if my kitchen calculations Michael MicenkoFigure 1.Illustration of spherical divergence are accurate. Associate Editor for Petroleumas the wavefront expands the energy / unit area icenko@bigpond.com decreases proportionately with the square ofQdistance travelled.The quantity Q is a measure of the Reflections and scattering loss of energy caused by internal Reflections. friction when one grain slides The downward propagating seismicagainst another and creates heat. In the June issue I made the commentenergy is partially reflected eachSome typical values (Table1) show it is incredible how a seismic sourcetime a reflector (a change in acousticQ can vary between 20 and 100 that can be barely feltcan provideimpedance) is encountered. In thefor sandstones and shales while details of the geology 4 km or moreexample of Figure 2 a significantmore consolidated limestones below In the late 1970s - 80s aboundary reflects 16% of the incidentand quartzites can have a Q value team of technical gurus questionedenergy and 84% is transmitted. Supposeup to 600. The inverse, 1/Q is why the seismic reflection methodwe have 10 similar reflectors everythe percentage of energy lost in worked at all. I dont recall the detail of100 ms two-way time. After travelling 1each cycle and is independent of their conclusion but by rights seismicsecond (TWT) the down going energyamplitude and frequencythe same reflection shouldnt work, certainly notis (0.84)10 of the original energy. Apercentage of energy is lost per cycle as well as it does because the seismicreflection from this 10th layer has passedirrespective of frequency. Because source energy is rapidly dissipatedthrough 9 layers before being reflectedhigh frequencies have more cycles and becomes overwhelmed by noise.and would be (0.84)9 x 0.16 x (0.84)9 orthan low frequencies in a given time Described below are three factors that0.0069 of the original strength when itthere is preferential attenuation of affect the strength of the seismic waveis recorded at the surface. This examplehigh frequency information.as it propagates through the earth, butassumes vertical ray paths and flat planar there are more. reflectors. Of course if there is someA sandstone with Q = 20 loses 1/20th rugosity on the reflector the reflectionor 5% of its energy every cycle while Spherical divergence may be scattered and never return to thea much more consolidated quartzite The largest effect on seismic wavereceiver array. And if the ray path is notwith a Q = 300 loses only 0.33 % per strength is the decrease associatedvertical there will be mode changes atcycle because relative movement with geometric spreading. As thethe interface as some energy is converted seismic wavefront travels away from itsto shear waves at the expense of the p wave we are trying to record. Table 1. source the energy per unit area acrossSome measured values of the wavefront must decrease as theQtaken from Reflection Seismology area of the wavefront expands. In aby Kenneth H Waters, 1978homogeneous medium the wavefront will be spherical and its energy perRock sample Q 1/QNo of unit area is inversely proportional to(%) cycles to reach 10% the square of the distance from theof originalsource (Figure 1). After travelling a kilometre the energy per unit surfaceGranite 311 0.32 700area is one million times less thanBasalt 561 0.18 1300that at only 1m. Interestingly, surfaceMarble 547 0.18 1250waves are confined to a boundary Quartzite 392 0.26 900and hence, being a 2D phenomenon Figure 2.The down-going ray loses energy atLimestone 203 0.49 950their strength decreases linearly with distance travelled. So after 1km theireach interface as a certain amount is reflected. InAmherst sandstone 24 4.17 55this example 16% of the incident energy is reflected energy per unit area is diminished toand 84% continues downward (up-going andOld Red sandstone 93 1.08 210only 1000th. This partially explains whydown-going ray paths are drawn separated forPierre shale 17 5.88 38surface waves appear much strongerclarity but for vertical rays and horizontal reflectors than reflected waves. they are coincident.) Sylvan shale 72.5 1.38 160DECEMBER 2020 PREVIEW 44'