A method for determining resistivity distribution of formations below a bottom of a body of water, the method comprising: simulating an air wave response at each of a plurality of spaced apart positions (26A, 26B, 26C) in the body of water from a place of imparting a transient electromagnetic field into the water; imparting a transient electromagnetic field into the water at the place; detecting an electromagnetic response to the imparted transient electromagnetic field at the plurality of spaced apart positions; subtracting the simulated air wave response from the detected response to produce a subsurface impulse response at each of the plurality of positions; and using the subsurface impulse responses to determine the resistivity distribution; wherein the simulating the air wave response comprises: generating an initial model of resistivity distribution in the water and in the formations; calculating a response of a first part of the air wave corresponding to each spaced apart position, wherein the first part of the air wave is charaterized by a high amplitude, short duration peak when the time derivative of measured voltage is graphed over time; adjusting a resistivity distribution of a water part of the initial model and repeating the calculating response of the first part until differences between a measured first part and the calculated first part are minimized for each of the spaced apart positions; and calculating a second part of the air wave for each of the spaced apart positions, wherein the second part of the air wave is characterized by a decay over time when the time derivative of measured voltage is graphed over time, and combining the second part with the result of the adjusting and repeated calculating the first part to generate a complete air wave for each of the spaced apart positions. simulating an air wave response at each of a plurality of spaced apart positions (26A, 26B, 26C) in the body of water from a place of imparting a transient electromagnetic field into the water; imparting a transient electromagnetic field into the water at the place; detecting an electromagnetic response to the imparted transient electromagnetic field at the plurality of spaced apart positions; subtracting the simulated air wave response from the detected response to produce a subsurface impulse response at each of the plurality of positions; and using the subsurface impulse responses to determine the resistivity distribution; wherein the simulating the air wave response comprises: generating an initial model of resistivity distribution in the water and in the formations; calculating a response of a first part of the air wave corresponding to each spaced apart position, wherein the first part of the air wave is charaterized by a high amplitude, short duration peak when the time derivative of measured voltage is graphed over time; generating an initial model of resistivity distribution in the water and in the formations; calculating a response of a first part of the air wave corresponding to each spaced apart position, wherein the first part of the air wave is charaterized by a high amplitude, short duration peak when the time derivative of measured voltage is graphed over time; adjusting a resistivity distribution of a water part of the initial model and repeating the calculating response of the first part until differences between a measured first part and the calculated first part are minimized for each of the spaced apart positions; and calculating a second part of the air wave for each of the spaced apart positions, wherein the second part of the air wave is characterized by a decay over time when the time derivative of measured voltage is graphed over time, and combining the second part with the result of the adjusting and repeated calculating the first part to generate a complete air wave for each of the spaced apart positions.