Letter: Albin Kerekes

date: Mon, Oct 20, 2008 at 9:23 PM
subject: TLE Article October 2008

Chris,

With regard to vibroseis energy source: If one would re-plot Figure 1 not as particle displacement but rather as energy, would one get a more linear response?

I was often criticized for using surface dynamite where “most of the energy goes up in the air.” The truth is that only the particle motion is small to the ground and large to the air, but (seismic) energy which is particle motion times force, is the same in both direction.

Would the same be true for down stroke and up stroke in vibroseis?

My field experience points towards saturation of the elastic limits of the ground below the base plate as the source of harmonics. The greater such “decoupling” the more the harmonics. To reduce harmonics, I have always advocated reducing drive levels and using more units to get the same level of source energy.

Your thoughts?

Best regards,

Albin.

------------------------REPLY--------------------------

date: Wed, Oct 22, 2008 at 11:37 AM
subject: Re: TLE Article October 2008

Albin,

I can't really say much about the physics that leads to the compressed
wave and harmonics. In the hearing case, the wave propagation is
linear outside and inside the ear. it is just the interaction of
sound with the eardrum that is nonlinear. By analogy, I suspect (but
cannot prove) that elastic waves in the earth from a vibroseis source
are linear, even in the near field. That would mean the harmonics
arise from nonlinear interaction at the source point, rather than
nonlinear propagation.

I am thinking about rigging up a lab experiment here at UH to study this.

Thanks for the question and comments,

Chris