Twilight smiled at Maud Pie. Making conversation with Pinkie’s sister was always difficult. “So... what is it that you do exactly, on your rock science research trips?”
At Pinkie’s invitation, Twilight was visiting the Pie family rock farm, where her sister had returned home for a short break from her rocktorate study programme. Maud thought over the question before answering accurately: “We drill holes in the ground, and extract a core sample.”
Twilight smiled and stared back into Maud’s unblinking eyes.
“That sounds interesting...”
“It's boring.”
Pinkie Pie bounded over to provide further clarification. “But digging boreholes is time consuming, expensive, and it messes up your hair. That's why every rock farmer knows before you dig, you do a proper survey of the site to learn what's under the ground.”
“But won't you find rocks wherever you dig?” asked Twilight.
“But they might be the wrong sort of rocks.”
Twilight reflected that there was clearly a lot she still had to learn about the way of life of earth ponies.
“How can you find out what's under the ground without digging?”
“Let me demonstrate... Put your ear to the ground.”
Pinkie pushed Twilight’s head downwards until her ear was firmly embedded in the dirt. She then jumped up and down repeatedly.
“What do you hear?”
“You stomping on the ground.”
“Okay... Now try here.”
She lead Twilight a few metres to one side, then pushed her head back onto the ground and repeated the stomping exercise.
“Sounds sort of hollow... Is there an underground room there?”
“Actually it's our septic tank. You see how you can learn what's under the ground by listening to it.”
“Sort of,” said Twilight. “But you can't tell what sort of rocks are underground that way...”
“You can if you make a loud enough noise and listen very carefully.”
Pinkie proceeded to explain the principle of reflection seismology.
“First we lay a long line of geophones across the rock field. Then we bury sticks of dynamite at selected points. Playing with high power explosives is great fun – but you have to be very careful or somepony could get hurt. When we detonate the charge, the shock wave goes down under the ground, and bounces off the rocks and anything else. The geophones record the echoes. From the time the sound reaches different points, we can tell how deep the rocks are, and the speed of sound in the rock – from which we can tell what sort of rock it is.”
“How do you know all this?”
“I grew up on a rock farm Twilight. I did my first seismic sounding before I could walk. Then I learnt to fly a helicopter, in order to do airborne survey with ground penetrating radar and time domain electromagnetics.”
“What?”
“I'll tell you another time.”