Electromagnetic downlink while drilling

A method for downhole communication, comprising:

• providing an oil-based fluid to a bottomhole assembly;
• selecting a frequency of at least 8 Hz for an electromagnetic signal based in part on the oil-based fluid;
• transmitting the electromagnetic signal from a surface location;
• and
• receiving the electromagnetic signal downhole at an uphole location of a length of drill pipe of the bottomhole assembly;
• insulating the length of drill pipe of the bottomhole assembly from the formation with the oil-based fluid;
• and
• transmitting the electromagnetic signal to a gap sub of the bottomhole assembly.

A method for downhole communication, comprising:

• performing drilling activities comprising drilling, reaming, or milling using an oil-based fluid;
• selecting a frequency of at least 8 Hz for an electromagnetic signal based, in part, on the oil-based fluid
• while performing the drilling activities, transmitting the electromagnetic signal from a surface location to a bottomhole assembly;
• while performing the drilling activities, receiving the electromagnetic signal downhole at the bottomhole assembly;
• and
• demodulating the electromagnetic signal at the bottomhole assembly.