Which has nothing to do with the argument. In order to demonstrate the spherical nature of the earth via seismic waves all you would need is: three people, three seismographs, and a decent amount of explosives. What you do is;
1) place explosives and seismograph#1 and one person at any continental location on the earth (place the explosives far enough away from the seismograph so that it will not be destroyed by the inevitable blast, and the seismograph on continental bedrock)
2) place one person and seismograph#2 1.5 million meters away in any direction, as long as the destination point is on a continental bedrock.
3) place one person and seismograph#3 3.15 million meters away from the original location in any direction, as long as the destination is on continental bedrock.
4) set off explosives at location 1
5) each of the three people records the activity, or lack of activity identified on each seismograph.
6) congratulations, assuming you didn't blow yourself up, you just proved that the earth is a sphere. Why you might ask? you will, upon the completion of this activity notice that the results are as follows:
location one: both Primary and Secondary waves will be identified by the seismograph.
location two: both Primary and Secondary waves will be identified by the seismograph.
location three: only Primary waves will be identified by the seismograph.
How does this prove a round earth?
Primary waves and secondary waves are both produced by the initial release of energy from the explosion, however secondary waves are incapable of passing through liquid material, while primary waves are able to pass through virtually any material. You can prove this for yourselves by testing this activity at a smaller scale in a solid material and in the middle of a pool of water.
This means that at location#3, which was 3.15 million meters away (the opposite side of the spherical earth), the waves must have passed through liquid material (Ie: the core of the earth) which prevented the secondary waves from reaching that location. However, Secondary waves did reach the location 1.5 million meters from the original location (one quarter of the way around the spherical earth), which means that there was no liquid material between that point and the original location through the earths interior.
Were the earth flat, even with a liquid bottom, or core, or whatever garbage you would prefer to call it, you would identify secondary waves at ANY location on the earth, because they would simply pass above the liquid bottom/core/what ever you want to term it along the plane of the planet. This is not the case, and this simple activity proves just that.
Happy testing gentleman!