Millivolt Measurement and Control Circuit

Need a freelancer to design and build a circuit that measures millivolt-level voltage differences and uses the result to control an LED and buzzer. The system should calculate the voltage delta and trigger the LED to light and the buzzer to sound when the measured value reaches a specified threshold. The project includes selecting components, creating the circuit, and ensuring reliable operation. Please share relevant experience with similar sensor-based control projects. Currently I manually measure and log dc millivolt age values in the air in the environment, post them in a spread sheet, sum them and calculate the average value that I call my base line. With the base line established I then walk (scan) and sample dc millivoltages emitted from the earth. Then I compare the base line valve with the values emitted from the earth. Then when an (adjustable) user defined positive or negative dc millivolt delta between the air and the earth is measured, an LED is lit and a buzzer sounds. That's it. I want an Arduino sketch that will do this automatically without needing the spread sheet that will also log all values involved.